THE GROWTH OF PHYSICAL IDEAS 97 



experimentally that a prism splits a ray of light refracted 

 through it into a band of colors. White light could there- 

 fore be considered to contain rays of various degrees of re- 

 frangibility, the least refrangible rays being red and the most 

 refrangible, violet. Thus Newton discovered the spectrum 

 and with it much relating to the nature of color. Newton 

 made another observation which later became of the utmost 

 importance, namely, that when a thin film of air occurs be- 

 tween two plates of glass, the film shows colors, and these 

 colors depend upon the thickness of the film. 



The distinction between the physical and the psychological 

 properties of color was first made clear at the beginning of 

 the nineteenth century by Thomas Young, who advanced a 

 theory of color vision according to which the eye perceives 

 three fundamental sensations— red, green, and violet— and all 

 other color sensations arise from combinations of these three. 

 Yellow, for instance, arises from simultaneous sensations of 

 red and green. The distinction between the psychological 

 basis of color and its physical basis in the differing refrangi- 

 bility of the rays of light has been a difficulty for scientific 

 workers and artists ever since the days of Newton. The pig- 

 ments of the artists have as their fundamental colors the com- 

 piementaries to Young's sensation primaries, and only with 

 the advance of color photography in recent years have the 

 relations between the sensation primaries and the pigment 

 colors become familiar to the general public. 



As a result of his work on the refraction of light through 

 prisms, Newton inferred that the dispersion of a prism is 

 always proportional to the deviation it produces; that is, he 

 didn't realize that by the use of glass of different kinds prisms 

 could be made that for a given refraction would give different 

 deviations between the rays of varying colors. Newton con- 

 cluded that it was not possible to correct the variation in the 

 focal length of a lens for different colors, an effect which is 

 generally known as the chromatic aberration of the lens. He 

 abandoned the idea of making telescopes of great power by 



