CH. xxxiv. PHOTOGRAPHY. 333 



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. Th<i 

 printed sensitized paper is now bathed in hyposulphite of 

 soda, which dissolves off the unaltered chloride of silver, 

 leaving the photograph permanently printed in reduced or 

 metallic silver. All this work is done by those 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 photograph, because these colours con- 

 tain so few of those particular chemical rays, while the 

 darkest blue and violet come out nearly white, because they 

 act strongly upon the nitrate of silver. 



It was formerly thought that the chemical rays of the 

 spectrum acted almost entirely at the violet end, but Captain 

 Abney has discovered that by using a peculiar film of silver 

 bromide, he can produce photographic impressions by rays 

 far into the red of the spectrum, showing that the chemical 

 action does not so much depend upon peculiar rays as upon 

 the sensitiveness of the molecules of certain substances to 

 the different rates of vibration in the various parts of the 

 spectrum. 



Wollaston first observes the Dark Lines in the 

 Spectrum, 1802. In the same year that Ritter discovered 

 the chemical 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 (born 1766, died 1828), 

 first saw the da:k 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 priom, and instead of letting in the light by a round hole 



