THE ANALYSIS OF LIGHT. 327 



formed a circular disc of white light ; but by the interposition of the prism 

 the ray spreads out in a fan-shape, and forms an obloug colored image on the 

 opposite waiL This image, called the solar spectrum, is divided horizontally 

 into seven colored spaces, or bands, of unequal extent, which succeed each 

 other in an invariable order, viz., red, orange, yellow, green, blue, indigo, violet. 



uponwhatdoes The Separation of the seven different rays 

 of''wMfe"iigh't composing white light from one another, de- 

 depend? pends entirely upon a difference in their re- 



frangibility in passing through the prism ; those which 

 are refracted the least falling upon the lowest part of the 

 screen, and those which are refracted the most upon the 

 upper part. 



Thus the red rays, which are the least refracted, or the least turned from 

 their course by the prism, always occur at the bottom of the spectrum, while 

 the violet, which is the most refracted, occurs at the top ; the remaining colors 

 being arranged in the intermediate space in the order of their refrangibility. 



What additional Thc sevcu diifcrcnt rays of light, when once 

 ofThe compoli! Separated and refracted by a prism, are not 

 ii-ht?°^ ^^^^° capableof being further analyzed by refraction ; 

 . but if by means of a convex lens they are col- 

 lected together and converged to a focus, they will form 

 ■white light. 



If the spectrum formed by a prism of glass bo divided into three hundred 

 and sixty parts, it is found that the red ray, or color, occupies forty-five of 

 those par' , the orange twenty-seven, the yellow forty-eight, the green sixty, 

 the blu sixty, the indigo forty, and the violet eighty. 



If we take a circle of paper and paint upon it in divisions of proportionate 

 size the seven colors of the spectrum, and then cause it to rotate rapidly about 

 a center, the colors by combination wiU impart to it a white appearance.* 

 From this and other experiments, therefore, it is inferred that light which wo 

 call colorless, or white (as that coming immediately from the sun), really con- 

 tains light of all possible colors so mixed as to neutrahzo each other. 



687. The separation of the difterent rays of light which 

 lakes place in their passage through a prism, is designated 

 by the term Dispersion. i 



E lain what '^^^° order of refrangibility of the seven different rays of 

 is meant by the light, or the arrangement of the seven colors in tho spcc- 

 ero^dSi'r'ent t^um, is always the same and invariable, whatever way the 

 Bubstanc«s. prism may be turned ; the lower end of the spectrum being 



• It is very common to find it stated in hooks of science that by niixinjr powders of tho 

 seven different colors together a white, or grayish-whito compound may be produced. 

 The exporimont, is not, howcTcr, satisfactory 



