1912] on Vital EfcrfK of Radivm mid other Rdijs. 29?. 



over-indulgence in tobacco he loses in the centre of his field of vision 

 the perception of red and green but rarely that of violet ; and that, 

 in cases of blindness coming on gradually from wasting of the optic 

 nerve, blue or violet is the last colour to go. Many years ago Sir 

 James Dewar, in conjunction with Professor McKendrick, demon- 

 strated that when light fell upon the retina of the eye an electrical 

 current was generated in the optic nerve. 



Finsen Light Treatment. — For a long period advantage was taken 

 of the physiological effect of sunlight in the treatment of certain 

 diseases, but Finsen, of Copenhagen, was the first to make a sys- 

 tematic study of the curative properties of light. The obvious draw- 

 back to this method of treatment is that sunlight is not always 

 available. It is evident that if we were to depend upon sunlight in 

 London for the treatment of certain diseases, cures would indeed be 

 tardy. Finsen, therefore, adopted the method of using powerful 

 artificial lights, and in this manner he soon discovered that the most 

 effective rays were those of the violet and ultra-violet. He used the 

 electric arc'for the purpose, and, as ultra-violet light is intercepted by 

 glass, he employed quartz lenses. The readiness with which these 

 active rays were absorbed rendered them only useful in superficial 

 diseases, and it was found that even blood intercepted them to so 

 great an extent that treatment could be made more effective by intro- 

 ducting compression, whereby the blood was forced out of the area 

 upon which the light fell. Finsen's results were very brilliant. More 

 recently, a quartz mercury- vapour lamp has superseded the arc for the 

 production of active violet light in large quantities. Its action upon 

 living cells is very powerful, and it is most destructive, even after 

 short exposures, to all forms of bacteria. But its ready absorption 

 confines its influence to the production of surface effects, and it fails 

 to reach deeply. The effects of light upon plant life are well-known, 

 but the limited time does not permit us to deal with the subject. A 

 familiar instance is that of a plant grown in the dark, which is de- 

 prived of its colouring matter. 



X-rays. — We pass now to the consideration of the Eontgen or ar- 

 rays. It was in 1895 that Professor Rontgen's discovery that a 

 Crookes tube emitted rays, which would pass through opaque sub- 

 stances as readily as light passes through glass, created immediate 

 and world-wide interest. The fact that these rays could pass through 

 our bodies and reveal the shadow of our bones so absorbed the 

 interest of the early workers in this field as to mask for a time the 

 terrible effects upon our living cells which followed over-exposure. 

 The precise nature of the «-rays has not yet been ascertained. It is 

 highly probable, as the late Sir Gabriel Stokes suggested, that they 

 are impulsive in ether of extremely short wave-length. As they travel 

 in straight Hues from their point of origin into space, and are not 

 affected in their course by a magnet, and can be neither diffracted 

 nor refracted, their wave-length cannot be determined. 



X 2 



