432 SPECIAL PHYSIOLOGY. 



and is produced by a change in the condition of the transmitted light, 

 caused by the substance experimented on, and named internal disper- 

 sion (Stokes). The rays concerned in this phenomenon, exist also in 

 the colored prismatic spectrum, but they pass considerably beyond the 

 extreme violet end. They constitute, under ordinary circumstances, 

 invisible ultra-violet rays. When, however, a solution of quinine is 

 held beyond the violet end of the spectrum, it becomes bluish or fluo- 

 rescent, thus rendering these peculiar rays visible ; so also when a sheet 

 of paper, moistened with a solution of quinine, is held in the same 

 position, it becomes beatifully luminous. Clear water and ordinary 

 white paper, held in the same place, are not illuminated. The electric 

 light contains many of these invisible rays. 



Again, beyond the red rays of the solar and electric spectra, there 

 are invisible rays, some of which are found, in decreasing numbers, in 

 the rest of the spectrum. These rays give rise to the remarkable phe- 

 nomena occurring at this end of the spectrum, known under the name 

 of calorescence (Tyndall). By passing the rays of the electric light, 

 brought by means of a mirror to a focus, through a solution of iodine 

 in bisulphide of carbon, the luminous rays are completely stopped ; 

 but certain invisible rays, which, in the electric and solar spectrum, 

 are found chiefly near, and beyond, the red end of the spectrum, con- 

 tinue to pass, and produce at the focus, a heat sufficiently intense to 

 set fire to combustible substances. The phenomena of calorescence 

 occurring at, and beyond, the red end of the spectrum, have been 

 compared with those of fluorescence at, and beyond, the violet end. 

 The combustion of oxidizable substances by these dark rays, affords an 

 example of the conversion of obscure radiant heat into light. 



These heating rays have been called calorific rays ; whilst certain 

 of the rays at the violet end of the spectrum, are called chemical or 

 actinic rays, on account of their power of exciting chemical or photo- 

 graphic action. The colored rays are named colorific. 



Light is said to undergo a decomposition by absorption, as well as 

 by dispersion through a prism. Thus, the great variety of color pre- 

 sented by opaque bodies when viewed by solar light, is due to absorp- 

 tion by them, in most variable proportions, of the rays of one, or more, 

 of the three, or seven, primary colors, and the reflection of the re- 

 maining rays. In this manner, a blue body is said to absorb, more or 

 less completely, the red and yellow, and to reflect the blue rays ; a 

 red body absorbs the blue and yellow, and reflects the red ; whilst a 

 yellow body absorbs the red and blue, and reflects the yellow rays. 

 Secondary colors, or compounds of two primary colors, are produced, 

 when a body absorbs one primary color and reflects the other two ; 

 thus the absorption of the blue rays, and the reflection of the red and 

 yellow, give an orange color ; in the same manner, the absorption of 

 red alone gives a green color, and the absorption of yellow, a purple 

 color. Tertiary colors, as olives, grays, drabs, are produced when the 

 three primary colors all undergo more or less absorption and reflec- 

 tion. That color which is necessary, in regard to another, to complete 

 a white light, is called its complementary color ; thus orange is the 

 complementary of blue, and blue of orange ; again, yellow and purple, 



