THE RECEPTORS AND EFFECTORS 71 



series of ethereal vibrations may extend farther indefinitely both 

 downward and upward, but of its ultimate limits we have no 

 knowledge. 



There is no human sense organ which can respond directly to 

 the electric waves, the ultra-violet rays, and the x-rays. 

 These have, accordingly, remained wholly unknown to us until 

 revealed indirectly by the researches of the physical laboratories. 

 Some ten octaves of this series are contained in the solar spec- 

 trum, from an infra-red wave length of about .1 mm. to an ultra- 

 violet wave length of .00035 mm. The light from metallic arcs 

 and from incandescent gases has, however, been found to contain 

 wave lengths as short as .00006 mm. The human eye is sensi- 

 tive to something over one octave of this series (waves from 

 .0008 to .0004 mm. in length, whose rates lie between 400,000 

 and 800,000 billions of vibrations per second), with six octaves 

 in the infra-red and three in the ultra-violet. The lower mem- 

 bers of this 'series of vibrations of the solar spectrum, and to a 

 less extent the higher also, are capable of stimulating the tem- 

 perature organs of the skin. 



Thus it appears that of the complete series of ethereal vibra- 

 tions, we can sense directly only about one octave by the eye and 

 a number of others through the sense organs for temperature in 

 the skin, while to the lowest and highest members of the series 

 our sense organs are entirely insensitive. The sensitivity of the 

 skin to these vibrations is limited subjectively to a small range 

 of temperature sensations, while the retinal excitations give us 

 subjectively an extensive series of sensations of color and bright- 

 ness. The human eye can discriminate from 150 to 230 pure 

 spectral tints, besides various degrees of intensity and purity of 

 tone, making a total of between 500,000 and 600,000 possible 

 discriminations by the visual organs (von Kries). Some of the 

 preceding data are summarized in the table 1 on page 73. 



1 In the preparation of this table I have been assisted by Professor R. A. 

 Millikan, of the University of Chicago, whose kindness I gratefully ac- 

 knowledge. The figures given are based upon the formula 



velocity 



i znr = rate 

 wave length 



and the velocity of transmission is taken as 3 x 10 10 cm. per second. The 

 actual velocity of light waves as worked out experimentally by Michelson 

 is 299,853 kilometers per second. 



