786 SPECIAL SENSES. 



existence of an imponderable and a purely hypothetical ether. It is possible, indeed, that 

 scientific facts may, in the future, render the existence of such an ether improbable or its 

 supposition unnecessary ; but, at present, all we can say is that the theory of luminous 

 undulation is entirely in accord with the optical phenomena that have thus far been rec- 

 ognized. 



The different calculations of physicists with regard to the velocity of light have been 

 remarkably uniform in their results. The lowest calculations put it at about 185,000 

 miles in a second, and the highest, at about 195,000 miles. The rate of propagation is 

 usually assumed to be about 192,000 miles. 



The intensity of light is in proportion to the amplitude of the vibrations. The inten- 

 sity diminishes as the distance of the luminous body increases, and is in inverse ratio to 

 the square of the distance. 



In the theory of the colors into which pure white light may be decomposed by prisms, 

 it is assumed, as a matter of demonstration, that the waves of the different colors of the 

 solar spectrum are not of the same length. The decomposition of light is produced by 

 differences in the refrangibility of the different colored rays as they pass through a denser 

 medium than the air. The differences in the wave-lengths for different colors is very 

 simply set forth by Tyndall as follows : 



" The color of light is determined solely by its wave-length. The ether-waves grad- 

 ually diminish in length from the red to the violet. The length of a wave of red light is 

 about Tj^fjnr of an inch ; that of the wave of violet light is about -^-^ of an inch. The 

 waves which produce the other colors of the spectrum lie between these extremes. 



" The velocity of light being 192,000 miles in a second, if we multiply this number 

 by 39,000, we obtain the number of waves of red light in 192,000 miles ; the product is 

 474,439,680,000,000. All of these waves enter the eye in a second. In the same inter- 

 val 699,000,000,000,000 waves of violet light enter the eye. At this prodigious rate is 

 the retina hit by the waves of light. 



" Color, in fact, is to light, what pitch is to sound. The pitch of a note depends 

 solely on the number of aerial waves which strike the ear in a second. The color of 

 light depends on the number of ethereal waves which strike the eye in a second. 

 Thus the sensation of red is produced by imparting to the optic nerve four hundred and 

 seventy-four millions of millions of impulses per second, while the sensation of violet is 

 produced by imparting to the nerve six hundred and ninety-nine millions of millions per 

 second." In this way the scale of colors in the solar spectrum is compared to the scale 

 of musical notes and intervals. Indeed, Helmholtz has constructed a theoretical scale 

 of colors to correspond with musical tones and semitones. 



The analysis of white light into the different colors of the spectrum shows that it is 

 compound ; and, by synthesis, the colored rays may be brought together, producing 

 white light. Colors may be obtained by decomposition of light by transparent bodies, 

 the different colored rays being refracted, or bent by a prism at different angles. It is 

 not in this way, however, that the colors of different objects are produced. Certain 

 objects have the property of reflecting the rays of light. A perfectly smooth, polished 

 surface, like a mirror, may reflect all of the rays ; and the object then has no color, the 

 reflected light only being appreciated by the eye. Certain other objects do not reflect 

 all of the rays of light, some of them being lost to view or absorbed. When an object 

 absorbs all of the rays, it has no color and is called black. When an object absorbs the 

 rays equally and reflects a portion of these rays without decomposition, it is gray or 

 white. There are many objects, however, that decompose white light, absorbing certain 

 rays of the spectrum and reflecting others. The rays not absorbed, but returned' to 

 the eye by reflection, give color to the object. Thus, if an object absorb all of the rays 

 of the spectrum except the red, the red rays strike the eye, and the color of the object 

 is red. So it is with objects of different shades, the colors of which are given simply 

 by the unabsorbed rays. 



