THEORY OP COLORS. 



54? 



called a prism, at D. The parts of that ray in passing through tho prism will 

 diverge from each other, and falling upon the second surface of the prism at 

 ?, will issue from it still more divergent. If the prism had not been murno*- 

 ed, a circle of light would be formed upon a white screen at E N, which would 

 correspond with the magnitude of the opening in the window-shutter. But 

 when the light is made to pass through the prism an oblong spectrum will be 

 formed on the screen, the breadth of which will correspond with E N, but 

 which will have considerable length. This spectrum will exhibit a series of 

 colors, the lowest of which will be red, and the highest violet. They will 

 succeed each other in the following order, proceeding upward : red, orange, 

 yellow, green, blue, indigo, and violet. These colors will not, however, have 

 distinct boundaries, but will pass gradually, by insensible tints, one into another, 

 so that it will be impossible to say exactly where the red ends and the orange 

 begins, and so of the others. 



Fig. 3. 



White. 



This remarkable phenomenon was explained by Newton by showing that the 

 solar light was composed of a number of different kinds of light, which were 

 capable of being refracted in different degrees by the prism, those lights which 

 were least refrangible passing to the lower extremity, and those that were most 

 refrangible to the upper extremity of the spectrum. By inspecting the figure 

 it will be evident that the red light is less deflected from its straight course 

 than the orange ; the orange less than the yellow ; the yellow less than the 

 green, and so on. Newton, therefore, inferred that there were lights of seven 

 distinct kinds, having seven different degrees of refrangibility, and seven dif- 

 ferent colors. 



This conclusion, however, has been subject to much modification by subse- 

 quent optical investigators. 



It is found that rays of light of the same color differ slightly in refrangibility, 

 and the investigations of Brewster, and others, appear to justify the conclusion, 

 that the solar light, instead of consisting of seven elementary colors, ia only 

 composed of three. 



At so early a period as the year 1775, it was suspected that the conclusion 

 of Newton, that the spectrum was divisible into seven different simple con- 

 stituent lights, was fallacious. Mayer maintained that there were but three 

 elementary colors, red, yellow, and blue, and at a later epoch, Dr. Young sug- 

 gested that all colors were compounded of red, green, and violet. 



Let us, however, for a moment contemplate the actual result of the prismatic 

 experiment of Newton, and let us separate, carefully, that which is matter of 

 observation in it, from that which is, properly speaking, matter of hypothesis 

 or theory. 



In passing through the prism, and being, thereby, submitted to a considerable 

 refracting action, a single beam of light is spread out into a fan of rays as rep- 



