114 KANSAS ACADEMY OF SCIENCE. 



tomary in our own times to ascribe the existence of the aznre to atmospheric 

 • absorption. 



In order to show the distinction between these blues, permit me to repeat a few 

 well-known experiments. We have, in the rays of light issuing from the lantern, all 

 those components which go to make up sunlight, only in slightly different propor- 

 tions. If we place in the path of the ray a piece of cobalt glass, or this solution of 

 sulphate of copper, nearly all the colors are absorbed, and their energy goes to heat 

 the interposed medium. A few rays, however, are transmitted and fall upon the 

 screen, of which the blue predominate to so great an extent as to determine the 

 character of the resulting color, and to give us good examples of what are known as 

 absorption blues. 



There is, however, another way in which these other colors may be disposed of. 

 Whenever white light is reflected from the surfaces of very minute transparent 

 bodies, or of extremely thin films, the light-waves thus reflected interfere with each 

 other, by which process some colors are destroyed, and others increased in intensity. 

 One of the most convenient ways of showing this phenomenon, to which we owe 

 many of the most striking and beautiful of nature's tints, the magical coloring of 

 the peacock and the humming-bird, the varied hues of tropical beetles and butter- 

 flies, is by means of the familiar soap-bubble film. Here we have a substance which, 

 without any intricate or difficult manipulation on our part, grows rapidly thinner 

 and thinner, until it begins to show a play of interference colors of surpassing 

 brilliancy. When we reflect the ray of the lantern from such a film and focus its 

 image upon the screen, we have before us a shifting field of these interference col- 

 ors. As the increasing tenuity of the film destroys each component of the ray in 

 turn, each color of the spectrum in turn predominates, and we obtain at certain 

 stages of the experiment interference blues of such intensity and delicacy that they 

 may, with much more propriety, be compared with the blue of the sky than any 

 absorption blue which can be produced by artificial means. What wonder that New- 

 ton, who made the first thorough investigation of this class of colors, and that many 

 observers after him, should have ascribed the sky-blue to this source ! 



Experimental treatment of the subjective blues before a large audience is a much 

 more difficult matter, and I shall content myself with a single very simple experi- 

 ment, in illustration of one of the methods by which this class of colors may be 

 produced. The distinguishing feature of the subjective blues is, that they do not 

 depend upon the destruction of any portion of the spectrum by absorption or inter- 

 ference, but arise from temporary peculiarities of the retina of the eye, by which it 

 becomes incapable, in one way or another, of receiving all the impressions which, in 

 the normal retina, would be sent to the brain, there to be united in the complex sen- 

 sation which we call white. The study of the color-sense shows that color-sensation, 

 which appears at first sight exceedingly complicated, may be very simply explained 

 by supposing the existence of three classes of nerve-fibers in the eye, each capable, 

 whatever may be the character of the stimulus acting upon it, of transmitting a 

 single message. One set conveys the impression which we call red, another green, 

 and the third violet — the three primary color sensations by the union of which, in 

 varying proportions, we are enabled to perceive the many thousands of hues which 

 the outside world presents to vision. Now, the production of a subjective color 

 depends upon this, that when white light enters the eye one or more of these nerves 

 is unable to properly perform its function, so that the brain, instead of receiving 

 the sensations due to white light, gets an incomplete impression, in which some 

 necessary component is enfeebled or missing. Whenever this happens the observer 

 perceives color, the nature of which depends upon which nerve it is that fails to act. 

 The sole distinction, then, between objective and subjective colors lies in the man- 



