148 PHENOMENA OF VISION. 



The question, therefore, arises, does magnetism act 

 directly upon the ray of light, or only indirectly, by pro- 

 ducing a molecular change in the body through which 

 the ray is passing ? This question, so important in its 

 bearings upon the connexion between the great physical 

 powers, will, no doubt, before long receive a satisfactory 

 reply. A medium is necessary to the production of the 

 result, and, as the density of the medium increases, the 

 effect is enlarged : it would therefore appear to be due 

 to a disturbance by magnetic force of the particles 

 which constitute the medium employed. 



Without any desire to generalize too hastily, we can- 

 not but express a feeling, amounting to a certainty in 

 our own mind, that those manifestations of luminous 

 power, connected with the phenomena of terrestrial 

 magnetism, which are so evident in all the circum- 

 stances attendant upon the exhibition of Aurora Borea- 

 lis, and those luminous clouds which are often seen, 

 independent of the Northern Lights, that a very inti- 

 mate, relation exists between the solar radiations and 

 that power which so strangely gives polarity to this 

 globe of ours. 



In connexion with the mysterious subject of solar 

 light, it is important that we should occupy a brief 

 space in these pages with the phenomena of vision, which 

 is so directly dependent upon luminous radiation. 



The human eye has been rightly called the ' ' master- 

 piece of divine mechanism;" its structure is complicated, 

 yet all the adjustments of its parts are as simple as 

 they are perfect. The eye-ball consists of four coats. 

 The cornea is the transparent coat in front of the 

 globe ; it is the first optical surface, and this is attached 

 to the sclerotic membrane, filling up the circular aper- 

 ture in the white of the eye ; the choroid coat is a very 

 delicate membrane, lining the sclerotic, and covered 

 with a perfectly black pigment on the inside ; and close 

 to this lies the most delicately reticulated membrane,. 



