3i8 NINETEENTH CENTURY. pt. hi. 



poured upon the plate to melt away any nitrate of silver 

 which remains, so that when the sun next falls upon it it 

 may not blacken the rest of the plate and destroy the 

 picture. Then the glass plate is again placed in the sun 

 with properly prepared paper under it ; and now the shades 

 are reversed. Under the dark parts of the plate the sun 

 will act feebly on the paper, and produce light patches, 

 while through the light parts it will act strongly and produce 

 shadows. And in this way the lights and shades of your 

 image will appear in their right places on the paper. All 

 this work is done by the chemical rays which are chiefly at 

 and beyond the violet end of the spectrum, and this explains 

 why bright red and yellow objects come out dark in a photo- 

 graph, because these colours contain so few chemical rays, 

 while the darkest blue and violet come out nearly white, 

 because they act strongly upon the nitrate of silver. 



Wollaston first observes the Dark Lines in the Spectrum, 

 1802. — In the same year that Ritter discovered the chemi- 

 cal rays at the dark end of the spectrum which have given 

 us the whole art of photography. Dr. Wollaston, one of 

 our most celebrated chemists (bom 1766, died 1828), first 

 saw the dark lines in the spectrum which have enabled us to 

 discover the actual materials which exist in the sun and 

 stars. Dr. Wollaston, who made many good experiments 

 on light, was one day examining ordinary daylight through 

 a prism, and instead of letting in the light by a round hole 

 in the shutter as Newton had done, he made only a very thin 

 slit, so that the colours of the spectrum were prevented from 

 overlapping each other, as they had done in Newton's expe- 

 riment. The result was that seven dark upright lines or 

 spaces appeared in the band of colour, which seemed to show 

 that no light fell on those parts. Wollaston did nothing more 



