§2] 



CHEMICAL ACTION OF LIGHT 



161 



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N 



rays now pass through (4) a Nichol prism (capable of being rotated along- 

 side a graduated arc) set at 45°, in which position both rays pass through 

 without changed relative intensity. The rays emerging from 

 the collecting telescope are now dispersed by 

 passing through the vertically placed prism, 

 and the adjacent parallel spectra are observed 

 through a telescope. By a rotation of the 

 Nichol prism through an 

 observed number of degrees 

 the stronger light may be 

 brought to the intensity of 

 the weaker. The relative 

 intensity of two lights with 

 reference to a third (con- 

 stant) is as the squares of 

 the tangent of the angle 

 through which the Nichol 

 prism has been rotated. 

 Other modifications of 

 \o Glan's photometer are those of Lord Rayleigh ('81) and 

 of Lea ('85), upon which the spectrophotometer of the 

 Cambridge (Eng.) Scientific Instrument Co. is based. 



Fig. 43. — Diagrams showing con- 

 struction of VoGEL's spectropho- 

 tometer. 1. Horizontal section 

 through the optical axis. M, 

 mirror to reflect standard light, 

 by aid of a totally reflecting 

 prism p, into the optical axis. 

 S, shutter with slit divided into 

 an upper and a lower half by 

 means of a band q; C, coUi- 

 mating lens. D, doubly refract- 

 ing quartz prism; m, m, the 

 holder of the Nichol prism 2V, 

 which can be rotated through an 

 arc that can be read off from a 

 graduation on m, m; P, a flint 

 glass prism ; B, F, 0, observing 

 telescope, in which, at the focal 

 point, near F, is a diaphragm 

 cutting out the two outermost of 

 the four spectra coming through 

 B. 2. Front view of the shutter. 

 (From VOGEL, '77.) 

 M 



§ 2. The Chemical Action of 

 Light tjpon Non-living Sub- 

 stances 



The process of photography has 

 made us familiar with the fact 

 that daylight acts upon the halo- 

 gen salts of silver, gold, platinum, 

 and other metals, although the 

 nature of the chemical change 

 wrought by the light is uncer- 

 tain. It is not, perhaps, gener- 

 ally appreciated, but it is well 

 known to chemists, that light 

 can produce or further very 

 many chemical changes, particu- 

 larly among organic compounds. 

 The .effects are mainly due to the 

 blue and violet rays, hence are 



