Any sort of magnifying lens may be used to start a fire on a
sunny day. This would include a regular magnifying lens, some sort of
special magnifier (as shown in the photo below), binoculars,
glass bottle bottoms, eyeglasses (far-sighted prescriptions), and so on. A
drop of water on the glass can intensify the sun light even further. You can
even buy specialized "burning glasses", which are specially made
for concentrating the sun's light for the purpose of starting fires.
A pocket magnifier that is about one and a half inches in
diameter, with the plastic case that swing away for use, is very handy. Any
avid tracker/naturalist will usually want to carry one around anyway to examine minute plant details for identification, examine micro-details
of tracks, start a fire, read fine print, or even hold it in front of your
flashlight to disburse the beam in a wider area.
Simply hold the lens at such an angle as to focus the
sun's light into as small an area as possible. Place some tinder under this
spot and it will soon start to smoke and hopefully catch fire.
** This is one reason why glass thrown out into the
woods can be a forest fire hazard - if it lands at just the right angle,
perhaps with a drop of morning dew in the right place **
Given the right conditions, this method could even be used
to make fire from ice!