692 SPECIAL SENSES. 



tinct vision, especially for near objects, may readily be appreciated in watch- 

 ing a person who is attentively reading a book, when the eyes will be seen to 

 follow the lines from one side of the page to the other with perfect regular- 

 ity. When it is considered that in addition to these qualities, which are not 

 possible in artificial optical instruments, the eye may be accommodated at 

 will to vision at different distances, and that there is correct appreciation of 

 form, etc., by the use of the two eyes, it is evident that the visual organ gains 

 rather than loses in comparison with the most perfect instruments that have 

 been constructed. 



Certain Laws of Refraction, Dispersion etc., bearing upon the Physiology 

 of Vision. Physiologists have little to do with the theory of light, except as 

 regards the modifications of luminous rays in passing through the refracting 

 media of the eye. It will be sufficient to state that nearly all physicists of 

 the present day agree in accepting what is known as the theory of undula- 

 tion, rejecting the emission-theory proposed by Newton. It is necessary to 

 the theory of undulation to assume that all space and all transparent bodies 

 are permeated with what has been called a luminiferous ether ; and that light 

 is propagated by a vibration or an undulation of this hypothetical substance. 

 This theory assimilates light to sound, in the mechanism of its propagation ;, 

 but in sound the waves are supposed to be longitudinal, or to follow the line 

 of propagation, while in light the particles are supposed to vibrate trans- 

 versely, or at right angles to the line of propagation. It must be remem- 

 bered, however, that the undulatory theory of sound is capable of positive 

 demonstration, and that the propagation of sound by waves can take place 

 only through ponderable matter, the vibrations of which can always be ob- 

 served ; but the theory of luminous vibrations involves the existence of an 

 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 the theory of luminous undulation seems to 

 be 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 (297,725 kilometres) in a second, and the highest, 

 at about 195,000 miles (313,818 kilometres). The rate of propagation is 

 usually assumed to be about 192,000 miles (309,000 kilometres). 



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

 The intensity 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 decom- 

 posed by prisms, it is assumed to be 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 medium denser than the 

 air. 



The analysis of white light into the different colors of the spectrum shows 



