COLOR VISION 879 



tained with the aid of the so-called Zollner's lines, represented in 

 Figure 478. This illusion may be explained upon the basis that we 

 tend to overvalue the size of acute angles. Figure 479 shows two 

 horizontal lines of the same length which, however, are made to appear 

 distinctly unequal by oblique lines affixed to their end-points. Illu- 

 sions of movement are now extensively employed in cinematographic 

 pictures. A series of instantaneous photographs having been taken 

 of a moving object while assuming its successive positions, these pic- 

 tures may in turn be thrown upon the retina in rapid sucession repro- 

 ducing the original movement. 



CHAPTER LXXV 

 COLOR VISION 



Qualities of Light. The ethereal vibrations which are capable 

 of affecting our retinae, have different vibratory qualities. White light, 

 such as is emitted by the sun, is made up of rays of different wave 

 lengths or rapidity of vibration. Consequently, if a beam of this light 

 is made to impinge upon a plane medium of greater density, it is split 

 into its component rays. Those possessing the more rapid vibratory 

 rate, are retarded or refracted more sharply than those characterized 

 by a slow vibration. If the aforesaid medium is arranged in the form 

 of a prism, this "dispersion" or spreading of the different rays will 

 be even more apparent. We then obtain the so-called prismatic or 

 solar spectrum (Newton 1657), consisting of seven primary colors, 

 namely red, orange, yellow, green, blue, indigo and violet. These 

 colors, however, form a continuous series and gradually shade into 

 one another. Those which stimulate our retinae vary in their vibra- 

 tory rate between 392,000,000,000,000 and 757,000,000,000,000 in a 

 second. In round numbers, therefore, it may be said that we are 

 subject to rays, the wave lengths of which vary between 400/iM to 

 800 A^i. 1 Under ordinary conditions, however, we do not recognize 

 the existence of these rays, because our eyes do not possess the means 

 of resolving white light into its constituents. Consequently, this 

 analysis can only be made outside of this receptor, and only when the 

 retina is subjected to these rays separately, are we in a position to 

 recognize colors. In this regard, our eyes differ very materially from 

 the ear, because the latter is equipped with a mechanism for analyzing 

 sound, i. e., for resolving the compound waves into their simple com- 

 ponents. It should also be remembered that the spectrum contains 

 other rays beyond its red and violet ends, and while the latter do not 

 activate the retinae, they may be made to do so by accessory means. 



