Hence it follows that the blackness which the luna cornua acquires from the sun’s light, and likewise the solution of silver poured on chalk, is silver by reduction.  I mixed 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.  Artificial light is playing a great part in this branch of photochemistry, and the development of artificial light for the various photographic needs is best emphasized by reminding the reader that the sources must be generally comparable with the sun in actinic or chemical power.  The intensity of illumination due to sunlight on a clear day when the sun is near the zenith is commonly 10,000 foot-candles