270 THEORY Of LIOHT. 



the course of the ray, it would not, as we see in the ordinary diagrams, 

 pass like a straight line of the pen, but in a curved line, showing the 

 gradual manner in which the ray is refracted through successive trans- 

 parent layers. As it eaters in the anterior half of its passage, it 

 encounters media of increasing density : but as it passes out behind, it is 

 transmitted through media diminishing in density. The ray is nowhere 

 opposed by that sudden increase of density which gives a disposition to 

 reflection ; and it passes through the vitreous humour still refracted, the 

 density of that humour having a just correspondence with the posterior 

 surface of the lens. In the atmosphere there is a similar arrangement for 

 receiving the light proceeding from the sun or stars : for as the density 

 of the air diminishes as the height above the earth increases, the surface 

 of our atmosphere, from its rarity, must almost resemble free space; 

 consequently the light falling into it will penetrate more abundantly than if 

 the air were compressed as it is near the earth, and were of uniform 

 density. We thus see the obvious superiority in the structure of the eye 

 to any thing that can be composed of glass, which is of uniform density 

 throughout, and must therefore present a succession of surfaces where 

 rare and dense media are abruptly opposed to the rays transmitted. 

 We may observe another happy result from the peculiar structure of the 

 lens. A magnifying glass is never true : an aberation of the rays takes 

 place in the pencil of light, as the rays are drawn to a focus. The rays 

 which penetrate near the centre are projected, so as to be drawn to their 

 focus beyond those rays which pierce through nearer the edge. The rays 

 penetrating this double centre of the convex glass willproject the image 

 to a greater distance then those penetrating nearer the circumference, and 

 consequently falling more obliquely will form a focus nearer the lens. 

 But in the crystalline humour of the eye, which corresponds with the 

 optician's lens, the exterior layer having less density, and therefore a 

 diminished property of refracting the ray, the image is carried farther off; 

 and by this means it is ordered that wherever the ray penetrates, it shall 

 be drawn to an accurate focus. Some modern philosophers have 

 asserted that the eye is not perfectly achromatic in every adjustment. 

 The term implies the property of the instrument to represent an image 

 divested of the prismatic colours; those false colours which attends the 

 refraction of the rays of light. If the statement be correct, it is nothing 

 against our argument ; nor have those inquiries advanced in any such 

 view. We know that in all the ordinary exercises of the eye the image is 

 perfect, having neither penumbra nor prismatic colours. This property 

 of the eye results from the different media through which the rays are 



