[censored], or a glass mirror that broke on them?

Ive got a few of the plastic mirrors, and the bags they come in or the plastic protective films they come with seem like mere figleafs - not much protection at all. Same with the glass mirrors - no much protection in the packaging at all.

I like glass for some applications (flight vest) and a Rescue Flash™ (LEXAN® polycarbonate) Signal Mirror in my backpack. They are stored such that they aren't given the opportunity to get scratched or broken. I wouldn't carry a metal mirror.

First, welcome in.
Second, you see the language sensors already caught something.

I have both types of mirrors, and use the commercially available protective sheets that are designed to protect smart phone screens (specifically iPhone) on all my mirrors.

I have yet to have one scratch, but I have broken a glass mirror in a fit of clumsiness.

I just went to look that the signal mirror in my EDC, its a Coghlans #9900 signal mirror. Cracked. Along the back with three veins. Mirror itself is fine. Probably put some duct tape or clear vinyl tape over the back till I replace it.

I'll weigh it out and look for a lighter replacement.

Have one in my hiking/ biking kit for 5 years in - will go take a look at it.

Is there any real difference for survival purposes in first-surface (reflective coating on the front) and second-surface mirrors (reflective coating behind the glass)?

I am surprised that Corning, with all its experience in glass and ceramics hasn't come out with something useful, highly reflective, and unbreakable (or nearly so). They do so much for optics and space materials that one might think they would have something, and we just haven't heard about it.

Sue

"vacuum desposited mineralization" means an aluminium coating evaporated onto it - since that's the standard way of making a mirror for about the last 80years that's not too surprising.

You can put a coating on the front of a surface - the advantage then is that the base material doesn't have to be transparent. But it is liable to get scratched.

Or you can coat the back of a transparent material. This is how almost all mirrors are made unless you work in an optics lab. The disadvantage is that a small amount of light (6-7%) reflects off the front surface - which doesn't matter for a rescue mirror since it's going in the same direction as the main light.
The advantage is that the mirror surface is sealed and so can't scratch. If the seal is airtight the reflectivity is higher than for a front surface mirror since aluminium drops to about 60-70% reflectivity on contact with air.

The advantage of a glass mirror is that it can be made stiffer and flatter than plastics. Any surface error or overall curvature spreads the beam out and so reduces the power that hits the eyeball of the observer.


ps. Yes that is how you spell aluminium

Buy 2, one to practice with and one that you keep safely stored and unopened in the package in your emergency kit.

[pedant]More accurately, that's ONE way, "aluminum" being the more common way, at least in the US. [/pedant]

There's also a tiny loss to transmission through whatever the substrate is, but it's truly insignificant in this situation. As you mentioned, a good quality seal between the coating and the substrate more than makes up for it.

I have kept an ACR signal mirror on my boat for at least 5 years, and it looks as clear as the day it came out of the package. It is kept in a cubby hole at the helm,just siting, no case. CountyComm offers a nice one that comes with a nice case.

I usually lose my mirrors long before they get significant damage.
The glass ones are brighter but the polycarbonate (lexan) ones are much lighter.
Polycarbonate is moderately scratch resistant. I usually stuff them inside my spare socks in my pack.

Hey thanks for all the comments and the welcome. Not sure what I said that was censored - weird - was perfectly friendly posting.

Seems that no-one has any significant durability complaints as far as plastic mirrors go, buy there are a few breakage issues with glass ones.

What about floatation?

Ive seen a lot of mirrors pushing the feature that their mirrors float, but it seems to me that if the mirror is floating and not attached to you, it will rapidly float away.

Is floatation important?


What about the aiming devices?

The retro reflective mesh-type aiming devices seem to vary from mirror to mirror - lots of people who have used mine find them a bit difficult to use and difficult to find the fireball.

The one unusual one seems to be the StarFlash, which has some kind of lens element in there. It gives a bright fireball, but visibility through the device is really bad.

What do people think about the aiming devices in their mirrors?

The CountyComm one doesn't have a retroreflective aiming device - I checked with them. Looks like a nice mirror though.

> Is floatation important?

Flotation has always seemed marginal to me. Your first line of defense should be the lanyard. If your mirror is tied to your life vest or body, you're good. If it isn't, the mirror can be left behind, or float away.

However, lanyards can break (though they are typically nylon), and I have had the square knot in a mil-spec lanyard come undone on me. Flotation provides a second chance if the mirror fails in deep water. It's not a sure thing, though - retrieving your mirror in rapid or choppy water is iffy at best, and who's to say you'll notice that you dropped it in the first place - in an emergency, you are likely to have a lot on your mind.

Most US Coast Guard approved life raft mirrors don't float, though the USCG standards seem to predate floating mirrors. The mirrors currently issued to US Coast Guard personnel do float, I believe. The U.S. Military buys both glass and plastic buoyant signal mirrors, so they aren't hard over on flotation. Malcolm Murray, who is probably the most dedicated signal mirror maker I know, makes both floating and non-floating mirrors.

> What about the aiming devices?

Hands down, you want a retroreflective aimer - you'll put far more flashes on the target than with other methods. This is backed up by rigorous test data. The U.S. military uses retroreflective aimers nearly exclusively.

Just make sure your mirror has a real retroreflective aimer, not a non-functional picture of one - see Doug Ritter's article here: http://www.equipped.org/phony_signal_mirrors.htm
[Disclosure: I provided two diagrams, and the patent and MilSpec for that article.]

Since retroreflective aimers don't work when the sun-mirror-target angle is more than about 135 degrees, you should also learn a backup method, like the "finger-vee" ( I have deep reservations about "finger-vee", but this post is going to be way too long as it is.)

In WWII, the US Bureau of Standards, in conjunction with the US Coast Guard, after testing many mirror aiming approaches, selected four for a rigorous scientific test. Six novices received brief training, then were put in a liferaft in choppy water, and asked to signal to a circling scout plane by each method. An observer on the scout plane counted the flashes they saw. The average results over the six subjects, in flashes seen per minute:

(1) Improvised foresight: 0.3
(2) British heliograph-style" mirror with paddle foresight: 8
(3) American GE "cross-in-glass" rearsight mirror: 14
(4) Early retroreflector tab: 35

Bottom line: the retroreflective aimer was seen 2.5 times more often than the runner-up.

The U.S. reaction was to switch production from (3) to (4), even though they had hundreds of thousands of (3) in the field.

An independent British test of (2) and (4), using two samples of each type of mirror, from a dinghy to a circling plane, similarly concluded that the U.S. retroreflective tab aimer produced visible flashes at a rate four times that of the British and Australian paddle foresight aimers.

While neither test included the "finger-vee" method, which is a "foresight" method, I would expect the "finger-vee" to be significantly worse than method (2): more than four times less effective than a retroreflective aimer. Lining up the 0.52 deg wide sunbeam shadow spot with the paddle foresight hole seems infinitely more reliable to me than using two fingertips to locate the center of a rectangular beam to that accuracy. However, a rigorous comparison test would be welcome.

Similarly, one can question the applicability of this test to the current mesh / solid / perforated retroreflective aimers, since the actual retroreflective aimer tested was a "canted tab" device. In U.S. service, the solid "bullseye" retroreflector replaced the retroreflective "tab" aimer, and was in turn replaced by the retroreflective "mesh" aimer.
You can see the three types on my site, in the first photo here: http://www.richard-fowell.fotopic.net/c1663439.html Since both innovations were invented by the same scientist who conducted the U.S. test, the inference is that he thought the current mesh aimer was even better than the test results above would indicate. However, a rigorous new test would be welcome.

Another thing that could be argued is that in these tests, the unstable platforms used (U.S. liferaft, British dinghy) tended to overemphasize the importance of an aimer compared to a land emergency with solid ground. That seems valid to me, and a strictly terrestrial test would be of interest. On the other hand, a real-life survival situation is much more stressing than the experiments above - adrenaline makes your hands shaky, as does hunger and pain from your injuries.

I had to pay to get the full U.S. and British reports, but a condensed version of the U.S. report (which omits the British foresight data) is freely available here: "Signaling with Mirrors: Reflex-Button Type of Mirror Adopted for Navy Survival", Naval Aviation News, 15 Sept 1944, pp. 32-33, which can be downloaded from the official government site here: http://www.history.navy.mil/nan/backissues/1940s/1944/15sep44.pdf

> The retro reflective mesh-type aiming devices seem to
> vary from mirror to mirror - lots of people who have
> used mine find them a bit difficult to use and
> difficult to find the fireball.

The retroreflective aimers do require training, and are a bit non-intuitive. However, the training need not take long, done properly, and a few minutes of practice should lock it in.

A good training aid is a patch of retroreflective material 100 yards away (the license plate of a (parked, unoccupied) car is good), and they should practice flashing in various directions relative to the sun - flashing towards the sun is easiest, and the retroreflector will only work out to about 135 deg from the line to the sun. I've made up portable retroreflective targets for my mirror testing and training using automotive reflective tape. Practicing with each hand is also a good idea - in an emergency, one arm may be pinned or broken.

The aiming methods that may seem more intuitive, as shown above, produce woefully worse results.

The way to mitigate the training issue is to provide better training material and methods, which I'll try to do below.

While most such mirrors come with written instructions on the back, the US Air Force does not regard those as sufficient - every pilot is explicitly trained to use a signal mirror.

Consumers don't get that training as a matter of course, but there are some pretty decent supplementary instructions available in printed, diagram, Web and video form. Here are some links:

Web links:

(1) Doug Ritter's instructions
http://www.equipped.org/psp/psp_rescueflash.htm
(2) My instructions
http://www.equipped.org/pp/pic2042.htm
(3) Cody Lundin's instructions from "98.6 Degrees, The
art of keeping your ass alive!" (pp. 187-190 and
C12-C13 cover retroreflective mesh signal mirrors -
the best coverage I've seen in a commercial survival book)
http://tinyurl.com/signal-mirror-drawing
(4) Gregory Davenport's instructions
http://books.google.com/books?id=7ZJjaNNgHhgC&pg=PA210

Video:

(1) Air Force Academy retired survival training director
and operator of outdoorsafe.com Peter Kummerfeldt
lectures/demonstrates signal mirrors for 4:34 of this
5:28 long video clip online:

Raw video: Signaling : News : KXRM FOX 21
http://www.coloradoconnection.com/news/video.aspx?id=374547

(2) Publicity video of May 2008 NASA signal mirror training
(short, note cap reversal to avoid shadow)
http://www.nasa.gov/centers/dryden/mov/228106main_mirror_640x360.mov

(3) (A) "Be Safe with Ed Viesturs - Survival Essentials"
Go to: http://www.adventuremedicalkits.com/
On the center of the page, click on:
"Be Safe with Ed Viesturs - Survival Essentials"

While the runtime on my machine is not strictly repeatable:

Signal mirrors are discussed at 00:35-01:50 and 03:40-04:08
The main tutorial is from about 00:45-01:50, and is reviewed
after Ed's farewell, from about 03:40-04:08

Notes:

(1) If you see a "slider bar" across the bottom of
the video, click on the vertical bar at the right
that says "AMK Video" to remove the slider bar.

(2) The video (probably unintentionally) illustrates
a common problem using signal mirrors - shading
the mirror with one's hat brim or cap. When the
subject wearing the baseball cap illustrates
finding the reflected light on his free hand,
you can see that the reflection on his palm is
not a complete rectangle - the upper right
quadrant is clipped off by the shadow of his
cap bill. In contrast, in the NASA training
video, the subject with the cap has his cap
reversed, with the bill over his neck, so that
the cap cannot shade the mirror.

> The one unusual one seems to be the StarFlash, which
> has some kind of lens element in there. It gives a
> bright fireball, but visibility through the device
> is really bad.
> What do people think about the aiming devices
> in their mirrors?

My favorite is the original 3M stainless steel mesh retroreflective aimer used in the old 3"x5" MIL-M-18371E glass mirrors - that's what is in my glove compartment, but 3M ceased production of that mesh in the 1970s. The "fireball" brightness was neither too dim nor too bright, and the apparent diameter of the "fireball" was just about exactly the 0.52 deg diameter of the sun.

The three issues I find with modern retroreflective aimers are:

(1) The brightness of the "fireball". Some are too bright in full sunlight (which I address by using sunglasses), and some are too dim in dim sunlight. This is a long recognized problem ( see this patent: http://www.google.com/patents?id=ys1DAAAAEBAJ ). Of the modern mirrors, I find the Starflash too bright, Rescue Flash, Coghlan's and Vector I brighter than I favor, the one Rescue Reflector I have a bit brighter, the 3"x5" S.I. Howard mirror aimer is "just right" and the 2"x3" S.I. Howard mirror on the dim side.

(2) The diameter of the "fireball" is about 3x the sunbeam diameter (1.5 degrees) in every modern mirror I've checked, unlike the old 3M mesh with the 0.5 diameter. This is also a long-recognized issue: it is mentioned in the cited patent above, and the USCG standard for "reflex type" (retroreflective) aimers calls out a maximum allowable diameter of 2 degrees: http://www.uscg.mil/hq/cg5/cg5214/survivalequip.asp#160.020 Basically this means that it does not suffice to get the "fireball" on target - you want the center of the fireball on the target.

(3) Clarity of the window - scattered light can be a problem -
another reason to keep the mirror clean and protect plastic mirrors from scratches. The USCG requires that cleaning cloths be provided with lifeboat mirrors. Another thing to check with the Rescue Flash mirror - make sure you peel back the transparent protective membrane when you use it (and sometimes it is on the back, for some reason) - the viewing is muddy with the membrane in place. It would seem that the size of the clear hole in the mesh would play into this, but I haven't done a careful study to determine my position on that.

I have used signal mirrors a fair amount. They are fabulous, at least in the western US, with lots of sunlight. I have broken at least one glass mirror, but it was still usable, since the interior cementing layer still held everything together (and I had an extra cutting edge!) I just looked at a plastic mirror I had stashed away in a kit, and it was quite a bit hazy, just from sitting around.

Nowadays, I don't routinely carry a dedicated signal mirror, but I make sure that my compass is the mirrored type. The surface area of the compass I use is about the same as some of the smaller dedicated signal mirrors.

I am not too concerned about the somewhat lessened accuracy of aiming of a compass mirror. If I am using one of those, I will be painting the sky, the hills, and the whole visible universe to get someone's attention. I have used the compass mirror for real a few times, and it got the job done.

Ive spoken with him - a genuinely nice man.



I use the tail-lights on a car, which are retroreflective, as an aiming target.

I find the AMK aimer works over nearly the full 180 degrees. Havent tried the others over that range.



My guess is that the 2 degree fireball comes from using road safety retroreflectors, which are optimised for a 4 degree viewing angle.

Thanks for your most informative and excellent posting.

I havent found any of the mirrors I have to be "too" bright - the main complaint i have is that some of them are too dim, and some of them require a certain amount of hunting around to find an eye-mirror alignment that has a bright fireball - i.e. the reflective meshes vary in retroreflectivity across the mesh. The 2x3" Howard mirror is especially like this.



The StarFlash is particularly bad in this regard.

I have most of the well-known mirrors. I carry the small glass countycom.com one when weight doesn't matter so much, and the Rescue Flash mirror when weight matters more.

To be safe, makes sure you know the two-handed method shown on equipped.org:

http://www.equipped.org/pp/pic236.htm

When it come to using a Heliograph, practice makes perfect. You need to used something that is far away, but not to far. I prefer to take clients to the edge of a small lake and have them use the trees on the far side. They then can tell very quickly the effectiveness of all types of mirrors and practace enough to they have a chance.

That is because a square knot is a "binding knot", not a "bend". You need to use a bend to tie two ropes together. Many people misuse a square knot as a bend. With results as you stated ... the knot comes untied. Those who tie ropes together with square knots and then place their life in that knot's hands are often times Darwin Award winners. A square knot is a good knot, if used as intended - for binding things. It can be a terribly dangerous knot if used incorrectly - as a bend.

Now back to the subject of this thread. My StarFlash mirror is alive and well after maybe 7 (?) years in my survival kit. But after reading rafowell's very informative posts and links to signal mirror information, I'm wanting to go look for a glass one. I will probably buy/make some kind of hard case with padding inside to protect it. Maybe some kind of dual-use padding/fire tinder to wrap it in and then placed inside my Esbit stove.

A lot of the lanyards I use now are spliced by melting the ends of the nylon cord and making a butt joint with them.
You melt them just about the same as if you were doing a flame whipping and then just shove them together end to end.
I usually roll the joint between wet finger tips before it hardens just to smooth it out.
You end up with a short hard joint about the same diameter as your cord.

No question that people have often been successful with improvised signal mirrors, though one's chances are greatly enhanced by being well practiced in the technique. I've practiced a lot with improvised signal mirror aiming methods.

The "somewhat lessened accuracy" was determined independently by the Yanks and the Brits to be more than a factor of four in tests that were far more thorough and scientific than I've had the resources to duplicate {Tests detailed in my prior post.}

The beam of sunlight is very narrow, and the average passerby is inattentive, and not really looking for you in the first place.

I figure I'm going to be lucky to get them to respond to the first half-dozen flashes they do see. I'm not inclined to give myself a handicap unnecessarily when the AMK Rescue Flash mirror in my neck pouch cost under $10 and weighs 0.7 oz.

CASE STUDY I

One of the authors of "The Complete Walker IV" learned the hard way that getting people's attention with an improvised signal mirror can be harder than you like, even in the very favorable circumstance that they are: looking for you, expect you to be using a signal mirror, and are in the neighborhood for a long time.

On pages 706-708 of that book, the author describes his experience in trying to get the attention of an airdrop plane with the pre-arranged signal: a 2" diameter mirror pointed using the "thumb" method taught him by the pilot, a former Air Force survival instructor. He practiced before the trip until he thought he had the technique down pat.

The plane arrived overhead, and started circling 2 miles away. There were at least two observers on the plane expecting and looking for his mirror flash. He signaled for a full hour without success. He finally got their attention with a smoke plume as they flew over him heading back to base.

He now carries a dedicated signal mirror.

CASE STUDY II

I interviewed by email one survivor who was finally successful in flashing a helicopter with a plain camp mirror, a gambit he was aware of because he was a dedicated reader of the survival articles in Backpacker magazine. He had been unsuccessfully flashing passing planes for days.

He asked me to recommend a signal mirror.

My everyday targets are made from retroreflective tape, which works fine at 100 meters, but that's closer than I'd prefer, considering that SAR practice counts on seeing mirror flashes at 5 mile range.

One thing that is superb for a signal mirror practice target is an optical corner cube. At a range of 0.5 miles, the return from a 1.5" diameter corner cube is bright enough to "burn through" the fireball from a retroreflective aimer and let you know you hit the target. I wouldn't try signaling that cube at a range of less than 1/4 mile - too much like looking directly into the sun.

Without a "fireball" in the way (e.g., foresight or rearsight aiming) the return from the cube is visible at much longer range. I saw the return from the 2"x3" stainless steel mirror from a cube I'd mounted 3 miles away. I figure this approach drops off with at least the 4th power of distance, but even so, an array of 16 such cubes should work fine at 5 mile range (I tried it at 11.1 miles - no joy).

Alas, optical corner cubes are generally pricy, but occasionally you can get deals on Ebay. I've seen do-it-yourself plans for making a corner reflector from 3 mirror tiles, but I haven't tried that approach.

In a way, corner cubes and other retroreflective targets are too easy - after you hit them once, the feedback helps you hit them again. They are good for initial familiarization and drill, though.

For scoring, a spotter or video camera is better. At middling ranges (e.g., 1-10 miles) a star filter on the video camera makes it easier to analyze the video results later.

Two instructors and two groups, at range, is ideal.
For long range mirror signaling as part of On Target activity, we use cell phones and amateur radios for coordination.

You make a very persuasive case. I suppose I could shoehorn an extra 0.7 oz into my pack.....

I'd like to emphasize that you need to practice on a distant target such that you can't see the reflection hitting the background and guide it onto the target. Otherwise you are not really using the aiming system.

As mentioned before, a retro-reflective target such as special tape, automobile tail lights, bike reflectors or road signs work well for this.

My mirror is a Vietnam-era glass job w/o retrorelective mesh. From as stationary platform I can hit a stationary target very easily. A moving target would be more difficuly. From a moving platform, it would be very tough.

So, what features or capabilities would the ultimate signal mirror have? If you could have it all, what would you have in a signal mirror?

e.g. would some kind of telescope functionality be usefull? Even something like a 4x optic might increase the accuracy of your signalling. Not saying that's possible, but just to kick off some wild and crazy thoughts.

Case Study III - another real-life example where mirror signaling accuracy was not so great ...

An article in a US Coast Guard Auxillary publication from Oct 2009
(October 2009 issue of "Northwind", page 10, downloadable here)
http://resource.d11nuscgaux.info/northwin/2009/09OctNW_Web_View.pdf)

reported on their search-and-rescue exercise (p. 10, per protocol on bottom of p. 9), where one group was in a boat, and another in a search plane. The group on the boat were to signal to the plane via signal mirror. The plane spotted the boat by its markings, and made two passes. To quote the article:

"Only one mirror flash was observed during this approach, even though the sun angle was good for signaling"

Only one flash? With the people on the boat healthy, not in distress, expecting the plane in a narrow time frame, the plane expecting a flash from the boat, and two passes?

Not so good. No indication of the type of signal mirror used, but I'm thinking the boat team needed all the advantages they could get.

On the other hand....

We were helivacing an injured hiker from the Santa Catalina Mts., just north of Tucson, employing a Air Force bird from Davis-Monathan AFB (which lies within Tucson). As the sun rose, they asked for a mirror flash. I obliged, "painting" Tucson in the general area of D-M AFB, because I had no knowledge of their precise location. Sky conditions were moderately hazy, and they were at least ten miles distant.

They received flashes and had a fix on their objective before liftoff. It is always a good idea to make it as easy as possible for the helicopter pilot.

In most of our usage of signal mirrors, we did not know the specific location of our target. Painting was SOP....

I'm not worthy ... I'm not worthy *bowing and grovelling in Wayne and Garth fashion to the mirror master rafowell*

Seriously though (and this is a serious topic after all) I am seeing a lot of mention of 2" x 3" signalling mirrors. IIRC Doug Ritter stated that the reflectivity of the mirror is a function of its size. That said, I certainly hope everyone's dedicated signal mirror is larger than a 2x3. Granted it is extremely small, light and easy to carry but given a choice I would opt for a larger 3" x 5" or possibly a 4" x 6" for my main dedicated signalling mirror in a larger kit or pack. Or even in a pocket. I thought the smaller 2x3 was supposed to be a backup to your larger mirror but it appears that several people have turned it into their dedicated main signaling mirror. Not criticizing or belittling anyone here. Just want to make sure everyone is giving serious consideration to their signalling devices. I'm not an expert by any means just my opinion based on what I've read and learned thus far, mixed with a little common sense and stirred frequently over high heat ...

In terms of glass mirrors, a 2x3" mirror will be a lot less fragile than a 4x6" mirror. With plastic mirrors, a 4x6" will tend to warp a fair bit more. And, the more compact your mirror, the more likely you are to have it with you when you need it, i.e. a 4x6" isn't exactly pocketable.

We had been talking about aiming devices.

If you cant hit the target, it doesn't matter how big your mirror is. Even when painting an area, being able to accurately aim the device will increase the the probability of hitting a target in the area in a given amount of time (i.e. you will be more able to systematically scan the area). Same with scanning the horizon.

Not as crazy as you think ...the "Galton Sun Signal", invented by Sir Francis Galton in 1858, was very well thought of, and the fancier versions did indeed incorporate telescopic optics. Galton referred to his device as a "hand-heliostat".

Here's a nice example of a Galton sun signal in a London museum with several photos online. This example was purchased by the British government for official use in 1881.
http://www.nmm.ac.uk/collections/explore/object.cfm?ID=NAV0945

Whatever the British government thought, Galton's instrument was developed in the spirit of survival. Galton's popular description of it was in the (extensive) signal mirror section of his popular "Art of Travel" which has lots of other survival related sections, such as his 19(!) page section on methods of making fire.

Here's a link to the signal mirror section (pp. 277-281, with a lot of good information about signal mirrors, still relevant today) in the 5th edition of Galton's "Art of Travel" (1872, but still in print today). Galton's hand-heliostat (and its unique aiming principle) is described towards the end of that section.
http://books.google.com/books?id=73YBAAAAQAAJ&pg=PA277#v=onepage&q=&f=false

Galton's device was sold commercially by instrument makers Messrs Troughton and Simms as "Galton's sun signal":
"Fig. 3: a more elaborate form, which has a theodolite telescope A, and a plain tube B as a finder."
http://galton.org/cgi-bin/searchImages/search/pearson/vol2/pages/vol2_0034.htm

Galton's Sun Signal was used for a while - here's an 1891 mention:
http://www.archive.org/stream/preliminarysurv00unkngoog/preliminarysurv00unkngoog_djvu.txt

"The best heliographs are Galton's sun signal,, fitted with a telescope, and Mance's heliograph. Both are expensive
instruments, and need not enter into the outfit of the
preliminary surveyor."

This 1898 source, 40 years after Galton's invention, speaks highly of it and discusses it in some detail:
http://books.google.com/books?id=38NBAAAAIAAJ

Title Hydrographical surveying: A description of the means and methods employed in constructing marine charts
Author William James Lloyd Wharton
Edition 2
Publisher Murray, 1898

Page 35:

A better instrument is the excellent and convenient Galton's Sun Signal, now also supplied. This is fitted with a telescope, by looking through which and adjusting the mirror, a dim image of the sun is seen covering the object required to flash to. Nothing can be better adapted to the purposes of the nautical surveyor's work than this (when he is once accustomed to it, as at first it is a little awkward to manage), and when obtainable they should always be used. Care must be taken, however, that the instrument is in adjustment. This can be ascertained as follows: Place a board, with a sheet of white paper pinned on to it, about 50 yards off. Direct the sun signal flash on to it, and looking through the telescope, screen and unscreen rapidly with the hand the direct flash from the mirror. If the circular image formed by the direct flash on the sheet is not coincident with the image of the sun as seen through the telescope, take off the cap at the end of the tube and adjust with the screw that will be found underneath.


The 1911 Encyclopedia Britannica referred to "Galton's Sun Signal" as the most convenient form of heliotrope.

This 1996 book on surveying, refers to "Galton's Sun signal" as though it were still in use:

http://books.google.com/books?id=jAqM1cc...ope&f=false