ed so
much of distilled water with the well-washed horn silver as
would just cover this powder. The half of this mixture I poured
into a white crystal phial, exposed it to the beams of the sun,
and shook it several times each day; the other half I set in a
dark place. After having exposed the one mixture during the
space of two weeks, I filtrated the water standing over the
horn silver, grown already black; I let some of this water fall
by drops in a solution of silver, which was immediately
precipitated into horn silver.

This extract shows that Scheele dealt with the reducing action of light.
He found that silver chloride was decomposed by light and that there was
a liberation of chlorine. However, it was learned later that dried
silver chloride sealed in a tube from which the air was exhausted is not
discolored by light and that substances must be present to absorb the
chlorine. Scheele's work aroused much interest in photochemical effects
and many investigations followed. In many of these the superiority of
blue, violet, and ultra-violet rays was demonstrated. In 1802 the first
photograph was made by Wedgwood, who copied paintings upon glass and
made profiles by casting shadows upon a sensitive chemical compound.
However, he was not able to fix the image. Much study and
experimentation were expended upon photochemical effects, especially
with silver compounds, before Niepce developed a method of producing
pictures which were subsequently unaffected by light. Later Daguerre
became associated with Niepce and the famous daguerreotype was the
result. Apparently the latter was chiefly responsible for the
development of this first commercial process, the products of which are
still to be found in the family album. A century has elapsed since this
earliest period of commercial photography, and during each year progress
has been made, until at the present time photography is thoroughly woven
into the activities of civilized mankind.

In those earliest years a person was obliged to sit motionless in the
sun for minutes in order to have his picture taken. The development of a
century is exemplified in the "snapshot" of the present time.
Photographic exposures outdoors at present are commonly one thousandth
of a second, and indoors under modern artificial light miles of
"moving-picture" film are made daily in which the individual exposures
are very small fractions of a second. Art