THE SENSES 773 



patch, but scattered over the tongue and palate ; and both tongue 

 and palate are at least as much concerned in mastication and 

 deglutition as in taste. The olfactory portion of the nasal mucous 

 membrane, although a continuous area with fairly distinct boun- 

 daries, is still a part of the general lining of the nostril. The 

 epithelial surfaces which minister to the supreme sensations of sight 

 and hearing the retina and the sensitive structures of the cochlea 

 are the most sequestered of all the sensory areas, as the organs of 

 which they form a part are, of all the organs of sense, the most highly 

 specialized in function, and anatomically the most limited. But 

 although hidden in protected hollows, they are endowed, either in 

 virtue of their own movements or of those of the head, with the 

 power of receiving impressions from every side, and their actual size 

 is thus indefinitely multiplied. 



VISION. 



Physical Introduction. Physically a ray of light is a series of 

 disturbances or vibrations in the luminiferous ether, which radiates 

 out from a luminous body in what is practically a straight line. The 

 ether is supposed to fill all space, including the interstices between 

 the molecules of matter and the atoms of which those molecules are 

 composed. Suppose a bar of iron to be gradually heated in a dark 

 room. In the cold iron the molecules are moving, on the average 

 at a relatively slow rate, and the waves set up in the ether by their 

 vibrations are comparatively long. Now, the long ethereal vibrations 

 do not excite the retina, because it is only fitted to respond to the 

 impact of the shorter waves ; and, indeed, the long waves are largely 

 absorbed by the watery media of the eye. As the temperature of the 

 iron bar is increased, the molecules begin to move more quickly, and 

 waves of smaller and smaller length, of greater and greater frequency, 

 are set up, until at last some of them are just able to stimulate the 

 retina, and the iron begins to glow a dull red. As the heating goes 

 on the molecules move more quickly still, and, in addition to waves 

 which cause the sensation of red, snorter waves that give the sensa- 

 tion of yellow appear. Finally, when a high temperature has been 

 reached, the very shortest vibrations which can affect the eye at all 

 mingle with the medium and long waves, and the sensation is one 

 of intense white light. 



We have said that a ray of light travels in a straight line, and the 

 direction of the straight line does not change so long as the medium 

 is homogeneous. But when a ray reaches the boundary of the 

 medium through which it is passing, a part of it is in general turned 

 back or reflected. If the second medium is transparent (water or 

 glass, e.g.), the greater part of the ray passes on through it, a smaller 

 portion is reflected. If the second medium is opaque, the ray does 

 not penetrate it for any great distance ; if it is a piece of polished 

 metal, e.g., nearly the whole of the light is reflected ; if it is a layer of 

 lampblack, very little of the light is reflected, most of it is absorbed. 



Reflection. The first law of reflection is that the reflected ray, the 



