The Evolution of Optics. 271 



pression, and it is this movement, this constant attention 

 to the different parts of the picture, which we miss in blind 

 or very defective eyes. Again, as the terminal filaments of 

 the optic nerve are alone sensitive to light, it follows that 

 at the point where the nerve enters the eye and spreads out 

 upon the retina there is a portion of the background of the 

 eye which is entirely blind, and you have no doubt all dem- 

 onstrated the existence of this blind spot for yourselves by 

 that simple and familiar experiment made with a dot and a 

 cross upon a piece of white paper. 



Since light of varying wave-lengths is unequally bent in 

 passing through a refracting substance, it follows that light 

 having passed through a lens will not be united in a single 

 focus, but, we may say approximately, in different foci, cor- 

 responding to the different colors; this gives us what is 

 known as chromatic aberration. It was once supposed that 

 the eye was free from this defect. This, however, is not 

 the case, as may be easily demonstrated ; but it is an inter- 

 esting fact that the supposed freedom of the eye from this 

 defect led to its imitation in the manufacture of lenses. 

 Lenses of different materials were thus combined, and with 

 the most happy effect, as is witnessed by our present highly 

 perfected achromatic objectives. 



From any point of view save that of evolutionists these 

 defects are inexplicable. What useful purpose can be served 

 by the facts that but one little spot on the retina possesses 

 accurate vision, that one portion of the field is entirely 

 blind, that the retinal vessels cast shadows upon the retina, 

 which, if we were conscious of them, would give us the im- 

 pressions of viewing the world through the branches of a 

 forest ? Why should not the optical apparatus be corrected 

 for color and for form ? It signifies but little that these 

 defects are compensated in one way or another ; there is no 

 excuse for their existence unless we look at the eye as a 

 nerve of common sensation which has been developed and 

 modified through various steps of evolution, and which, 

 from mere light-perception, has come by gradual stages to 

 its present visual perfection. When we understand this, our 

 criticisms are turned to admiration, and we look upon the 

 eye as one of the most wonderful, one of the most instruct- 

 ive, of all Nature's works. Darwin has well said that " it 

 seemed quite inconceivable that such an organ could be de- 

 veloped ; but," he continues, " reason tells me that if nu- 

 merous gradations from an imperfect and simple eye to one 



