778 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



are often spoken of as heat rays, light rays, and actinic or chemical rays, but 

 it must be remembered that the same wave may produce all three classes of 

 phenomena, the effect depending upon the nature of the substance upon which 

 it strikes. It will be observed that the range of vibrations capable of affecting 

 the retina is rather less than one octave, a limitation which obviously tends to 

 reduce the amount of chromatic aberration. 



In this connection it is interesting to notice that the highest audible note is 

 produced by about 40,000 sonorous impulses in a second. Between the high- 

 est audible note and the lowest visible color there is a gap of nearly thirty-four 

 octaves in which neither the vibrations of the air nor those of the luminifer- 

 ous ether affect our senses. Even if the slowly vibrating heat-rays which 

 affect our cutaneous nerves are taken into account, there still remain over 

 thirty-one octaves of vibrations, either of the air or of the luminiferous ether, 

 which may be, and very likely are, filling the universe around us without in 

 any way impressing themselves upon our consciousness. 



Qualitative Modifications of Light. All the ethereal vibrations which 

 are capable of affecting the retina are transmitted with very nearly the same 

 rapidity through air, but when they enter a denser medium the waves having 

 a rapid vibration are retarded more than those vibrating more slowly. Hence 

 when a ray of sunlight composed of all the visible ether waves strikes upon a 



plane surface of glass, the greater 

 retardation of the waves of rapid 

 vibration causes them to be more 

 refracted than those of slower vibra- 

 tion, and if the glass has the form 

 of a prism, as shown in Figure 237, 

 this so-called " dispersion " of the 

 rays is still further increased when 

 the rays leave the glass, so that the 

 emerging beam, if received upon a 



f , a .2SV.-Diagra m illustrat ingthe dispersion of light wh ; te ^^ instead of forming a 



spot of white light, produces a band 



of color known as the solar spectrum. The colors of the spectrum, though 

 commonly spoken of as seven in number, really form a continuous series from 

 the extreme red to the extreme violet, these colors corresponding to ether vibra- 

 tions have rates of 392,000,000,000,000 and 757,000,000,000,000 in 1 second, 

 and wave lengths of 0.7667 and 0.3970 micromilli meters l respectively. 



Colors, therefore, are sensations caused by the impact upon the retina of 

 certain ether waves having definite frequencies and wave-lengths, but these 

 are not the only peculiarities of the ether vibration which influence the retinal 

 sensation. The energy of the vibration, or the vis viva of the vibrating mole- 

 cule, determines the " intensity " of the sensation or the brilliancy of the light. 2 



1 One micromillimeter = 0.001 millimeter = one p. 



2 The energy of vibration capable of producing a given subjective sensation of intensity 

 varies with the color of the light, as will be later explained (see p. 786). 



