science and philosophy of his period (488-385 B. C.), mentions
the "burning lens." Nearly every one is familiar with an achievement
attributed to Archimedes in which he destroyed the ships at Syracuse by
focusing the image of the sun upon them by means of a concave mirror.
The ancient Egyptians were proficient in the art of glass-making, so it
is likely that the "burning-glass" was employed by them. Even a crude
lens of glass will focus an image of the sun sufficiently well to cause
inflammable material to ignite.

The energy in sunlight varies enormously, even on clear days, because
the water-vapor in the atmosphere absorbs some of the radiant energy
emitted by the sun. This absorbed radiation is chiefly known as
infra-red energy, which does not arouse the sensation of light. When the
water-vapor content of the atmosphere is high, the sun, though it may
appear as bright to the eye, in reality is not as hot as it would be if
the water-vapor were not present. However, a fire may be kindled by
concentrating only the visible rays in sunlight because of the enormous
intensity of sunlight. A convex lens fashioned from ice by means of a
sharp-edged stone and finally shaped by melting the surfaces as they are
rubbed in the palms of the hands, will kindle a fire in highly
inflammable material if the sun is high and the atmosphere is fairly
clear. Burning-glasses are used to a considerable extent at the present
time in certain countries and it is reported that British soldiers were
supplied with them during the Boer War. Indicative of the predominant
use to which the glass lens was applied in the past is the employment of
the term "burning-glass" instead of lens in the scientific writings as
late as a century or two ago.

As civilization advanced, leading intellects began to inquire into the
mysteries of nature and the periods of pure philosophy gave way to an