FLUORESCENCE. 431 



water, or glass, it is almost all transmitted ; but absorption of some 

 rajs takes place. 



Light is either colorless or colored. White or colorless light, when 

 reduced in intensity, forms a bluish-gray tint, gradually passing into 

 blackness, which is usually regarded as dependent on the relative or 

 nearly total absence of light. Black, however, is by some considered 

 to be a positive sensation. 



Luminous bodies generally give off rays of light composed of several 

 colors. Thus, solar light, though apparently white, may be decomposed, 

 by aid of a prism, into several colored lights. When a small beam of 

 solar light, admitted through a circular opening in a shutter or other 

 septum, falls on one side of a prism, or three sided piece of glass, its 

 component rays are so dispersed or spread out, that if an opaque screen 

 be placed behind the prism, an elongated luminous image is produced. 

 This, which is named the prismatic solar spectrum, is not white, but 

 colored, like the rainbow, presenting bands of violet, indigo, blue, 

 green, yellow, orange, and red. These colors appear to consist of 

 various combinations of three different colored lights, viz., red, yellow, 

 and blue, or, according to Sir J. Herschel, red, green, and blue, which 

 therefore are named the three primary colors. Others maintain that 

 the seven colors of the spectrum, as they cannot be further analyzed, 

 are the primary, simple, optical, or homogeneous colors. 



The different colored lights are said to differ as regards the number 

 of undulations of the hypothetical luminiferous ether which excites 

 them. The extreme red rays of the spectrum, for example, are calcu- 

 lated to undergo undulations numbering 399 billions in a second ; whilst 

 the undulations of the other colors of the spectrum are said to increase 

 progressively in number, the extreme violet rays performing 831 bil- 

 lions of undulations in the second. The more numerous the undula- 

 tions, the shorter are their component waves. Color in the eye is due 

 to specific sensations in the retina, excited, according to the theory 

 just mentioned, by undulations of different velocity and length ; yet 

 why such relations of color to differences in the number and measure- 

 ment of the undulations, should exist, is not obvious. 



Besides the visible rays, or rays capable of exciting luminous sen- 

 sations in us, solar light contains certain invisible rays, or rays inca- 

 pable of exciting such sensations, excepting under certain conditions. 

 These rays are also dispersed in the prismatic spectrum, and project, 

 some beyond the violet end, and some beyond the red end, where they 

 form the so-called ultra-violet and ultra-red rays. It may be con- 

 ceived that these rays undulate, in the former case too rapidly, and in 

 the latter too slowly, to act upon the retina. 



There are certain bodies, such as fluor-spar, and many decoctions of 

 organic substances, such as the bark of the horse-che'stnut, and the 

 seeds of stramonium, also an alcoholic solution of chlorophyll, and, 

 more especially, a solution of sulphate of quinine in water, which give 

 rise to the formation of internal color from the passage through them 

 of solar light. The color, in the case of a solution of quinine, has a 

 beautiful pale blue tint ; in other solutions, it may be yellow, yellow- 

 ish-orange, or red. This appearance of color is known as fluorescence, 



