292 Sir James Mackenzie Davidson [Feb. 2, 



made rapidly alternatino;, physiological effects will be produced. By 

 placing the head between the coils through which powerful alternat- 

 ing currents are passing, the observer will experience a flickering 

 sensation of light. The sensation of taste may also be stimulated in 

 the same manner. Professor Thompson suggests that these effects 

 are due to eddy currents set up in the nerves, and this seems highly 

 probable. 



Light Rays. — We may forego any further reference to heat rays, 

 and proceed at once to consider which of the rays of light have most 

 effect upon living matter. When a narrow beam of white light is 

 split up into its primary rays, a spectrum such as can be seen on the 

 screen is produced. In nature we are familiar with this phenomenon 

 in the shape of the rainbow. If, now, a strip of bromide paper is 

 stretched along the screen so as to receive the length of the spectrum, 

 the action of the light will take place upon its surface and can be re- 

 vealed by development in the ordinary way. On this exposed paper 

 being brushed over with developer, the affected parts will be found 

 gradually to darken, and it will be observed that the darkening takes 

 place more especially under the blue and violet rays, and even beyond 

 the violet end of the visible spectrum. 



[Strip of bromide paper across spectrum, develop 

 and fix, to show monochrome equivalent.] 



These have been called the chemical rays, and it is precisely these 

 rays which are most readily absorbed in theii- passage through matter. 

 The difference between the absorption of the various rays may be 

 observed in the case of the sun revealed through a fog. Yellowish at 

 first, the sun becomes red as the fog increases in density, the reason 

 being, that the red waves alone are able to get through the fog-bank, 

 while the violet and the shorter wave-lengths generally are scattered. 



[Grlass eeUs before lantern, fiUed with hypo solution, 

 with hydrochloric added, to show the absorption of the 

 blue and violet rays.] 



It is the shorter waves, at the violet end of the spectrum, which 

 have vital effects upon the tissues. They are the cause of sunburn — 

 a fact, which is well observed in the higher reaches of the Alps, where 

 the atmosphei'e, being comparatively free from suspended matter, the 

 blue, violet and ultra-violet rays get through so richly that, without 

 any special sensation of heat, severe effects are produced upon the 

 skin. If the ground be covered with snow, the rays are reflected, and 

 a more intense action results. It is interesting, further, to remember 

 that the blue and violet colour is the last to be seen at the close of 

 the day ; that when a person has his sight temporarily impaired by 



