I did (note to myself: quit trying everthing you read on ETS).
The material:
- clear ziplock bag 3" x 6"
- lid of CD jewelcase
- 4 clothespins (I´m aware this is close to cheating)
- tap water

I fill the bag with some water and poured out some until the focal length was manageable. The bag was pinned to the lid with the clothespins. When the bag was filled to about one third I was satisfied. I found that air bubbles are no problem as they rise to the top where the geometry isn´t good anyway. The geometry of the lens deteriorates when it is tilted to the vertical. The focussing is not too good and the intensity is even worse than with the fresnel lens. It gets slightly better when there´s some water between the bag and the plate (that seems to take care of itself). You have to be really careful to avoid soaking the tinder.

I tinkered with the plastic bag/water lens for about an hour once. Never even bothered with tinder, since it was pretty clear I wasn't getting a very good focus. I didn't think of pinning it to a sheet of clear plastic, that may have helped. I can also see this might be a little easier in southern latitudes around noon; my so-called "lens" sagged down because it had to be at an angle at my northern latitude, throwing the shape way off.

In Jules Verne 1874 novel, "The Magnificent Island", balloon-wrecked survivors start a fire by taking the glass faces off two watches and filling the space between them with water.

The whole novel is online here and other places, with the relevent bit in chapter 10, and chapter 9 is also interesting for their failed attempts. Indeed, the rest of the book is good too; I loved it as a child.

I agree... as small, light and thin as they are it's pretty hard to come up with an excuse not to carry one although I admit that I myself do not currently carry one though I have in the past and may again in the future.

Looked at the website posted cheap prices but the shipping costs more than the lenses.
Lee

Try bookshops.

Sue

I went to a Grand & Toy (big chain of stationery supply stores) here in Calgary and asked about the credit card sized fresnel lens. They didn't have any in stock but they ordered a couple in for me - $0.99 each (Canadian). (I ordered two because I felt guilty about making a special order for less than a dollar.) <img src="/images/graemlins/blush.gif" alt="" />

Many bookstores will sell a fresnel lens bookmark which will work at least as well, but may cost a bit more. (Otoh, you may be able to cut it in two and get two lenses out of it.)