150 THEORIES OF LIGHT. 



is impressed by light, and the communication made 

 with the brain possibly by a vibratory action. We may 

 trace up the phenomena of vision to this point ; we may 

 conceive undulations of light, differing in velocity and 

 length of wave, occasioning corresponding tremors in 

 the neuralgic system of the eye ; but how these vibra- 

 tions are to communicate correct impressions of length, 

 breadth, and thickness, no one has yet undertaken to 

 explain. 



It has, however, been justly said by Herschel : 



" It is the boast of science to have been able to trace 

 so far the refined contrivances of this most admirable 

 organ, not its shame to find something still concealed 

 from scrutiny ; for, however anatomists may differ on 

 points of structure, or physiologists dispute on modes of 

 action, there is that in what we do understand of the 

 formation of the eye, so similar, and yet so infinitely 

 superior to a product of human ingenuity ; such thought, 

 such care, such refinement, such advantage taken of the 

 properties of natural agents used as mere instruments 

 for accomplishing a given end, as force upon us a con- 

 viction of deliberate choice and premeditated design, 

 more strongly, perhaps, than any single contrivance to 

 be found whether in art or nature, and renders its study 

 an object of the greatest interest."* 



Has the reader ever asked himself why it is, having 

 two eyes, and consequently two pictures produced upon 

 the tablets of vision, that we see only one object ? Ac- 

 cording to the law of visible direction, all the rays pass- 

 ing through the crystalline lenses converge to one point 

 upon the retina, and as the two images are coincident 

 and nearly identical, they can only produce the sensation 

 of one upon the brain. 



When we look at any round object, as the ornamented 

 moderator lamp before us, first with one eye, and then 

 with the other, we discover that, with the right eye, we 



* Herschel, On Light, Encyclopaedia Metropolitan*. 



