22 THE POPULAR SCIENCE MONTHLY. 



If a solution of silver nitrate is washed over a large sheet of paper, 

 which is then placed upon the wail or sci-een so as to receive tlie spec- 

 trum upon its surface, and is also made to cover the space consider- 

 ably above it, a transformation occurs where the radiations fall, pro- 

 ducing a blackening which defines the outline of the chemical spec- 

 trum. It is now found that the chemical rays are more refrangible 

 than the luminous, and that, wliile the darkening takes place in the 

 colored spectrum, it it strongest in the violet of all the colors, and 

 extends also through the dark space up to JB, as shown in the figure. 



It is now exactly 200 years since Newton published his " Optics," 

 in which was described the capital experiment of resolving white 

 light into its constituent colors by the prism. It was the first great 

 step toward showing that what was regarded us perfectly simple turns 

 out to be inexhaustibly complex ; and every succeeding step of re- 

 search, while clearing up some points, has led to others which are still 

 unresolved. One thing, however, seems to be quite clear : the mode 

 of action throughout the spectrum is fundamentally the same. There 

 are three sjjectra, one of which, the thermal, takes action upon all 

 kinds of matter ; another of which, the luminous, acts only upon a cer- 

 tain special foi-m of nerve-matter; while a third, the chemical, produces 

 changes in certain compounds. Although the luminous force acts 

 only upon the nerve of the eye to stir up a sensation, yet we know how 

 intinitely complex and varied is the world of color that results. Tliere 

 is evidence that the dark thermal and chemical radiations are of equal 

 variability and complexity, yet there can be no doubt that all these 

 multitudinous efiects are due to a sinole mode of action. The differ- 

 ence between the thermal and the chemical rays is simply the difi'er- 

 ence between the red and the green ; that is, a difference of wave- 

 length and degree of vibration. 



The unequal distribution of the forces of the spectrum is well illus- 

 trated by Fig. 2. The middle curve shows the varying intensity of 

 the luminous force. The maximum is at , in the yellow space ; and 

 from this point the intensity of the light rapidly declines each way, 

 its extent being shown by the space shaded with oblique lines. The 

 curve -4, with the vertical lines, represents the position and varying 

 force of the heat ; and the curve (7, horizontally shaded, exhibits the 

 distribution and unequal energy of the chemical force. The three 

 maxima are widely separated as if there were some antagonism among 

 them, and it is noticeable that where the light is strongest the chemi- 

 cal force quite disappears. Different prisms give somewhat diffei-ent 

 effects but do not change their order. 



It thus appears that, so far from light being the agent which pro- 

 duces sun-pictures, the intensest light is powerless upon the chemically 

 prepared plate. It looks as if the illumination neutralized or ex- 

 tinguished the chemical energy. Nevertheless, liglit and the chemi- 

 cal force are so intimately associated in reflection and refraction that 



