ORGANS OF SENSE. 173 



upon the surface of the cornea, which being convex and much more dense 

 than air, refracts them, and causes them to converge towards a distant 

 focus. In passing through the anterior chamber, this convergence is slightly 

 counteracted, and the degree of refraction is about equal to that which 

 would have occurred had they impinged at once upon the convex surface 

 of the aqueous humor, supposing the cornea not to have been interposed. 

 JMuch of the light that traverses this fluid falls upon the iris, which, like the 

 stop or diaphragm in any optical instrument, excludes it, and this light is 

 reflected back again through the cornea. Hence we see the fibrous texture 

 and variegated color of the iris when we look upon the eye of another. 

 Those pencils of rays only which pass through the pupil are subservient to 

 vision. As these pass through the lens they landergo two refractions, 

 because this medium is convex on both surfaces, and is more dense than 

 either the aqueous or vitreous humor. These refractions increase the con- 

 vergence of the rays, and bring them to their proper focus upon the sentient 

 surface of the retina, on which is formed an accurate image of the various 

 external objects from which the rays of light have been received. This 

 image is inverted. What is above in the object is below in the image. 

 The right side of the object is to the left, and the left to the right, while 

 the relative position of its different parts remains the same. But as all sur- 

 rounding objects, if seen, must be equally reversed, the relative position of 

 all objects, therefore, remains unchanged, and there can be no comparison 

 between what is erect and what is inverted, and, therefore, the sense ac- 

 quires a correct idea of the position of objects. The fact, however, of our 

 seeing objects erect, notwithstanding their images are inverted on the retina, 

 as also that of single vision, from the simultaneous action of both eyes, 

 and many other phenomena, has given rise to some interesting inquiries 

 and to much speculation respecting the functions of the retina and optic 

 nerves. 



As the accurate convergence of all these pencils of rays to their respec- 

 tive foci upon the retina is necessary for the formation of a perfect image, 

 and for clear and distinct vision, certain nice adjustments in the appa- 

 ratus are required, and have accordingly been supplied, and which in 

 artificial optical instruments are attained with difiiculty by mechanical 

 skill. An image of an object, formed by a simple refracting medium, is 

 liable to certain imperfections or aberrations of light. These are principally 

 three, and are technically termed, first, aberration from sphericity ; second, 

 from parallax ; and, third, chromatic aberration. Each of these we shall 

 briefly explain, with a view of considering how they are obviated in the 

 living eye. 



Aberration from sphericity arises from this cause: a refracting spherical 

 surface does not unite the parallel or diverging rays of a pencil exactly 

 into one focus, because the lateral rays converge sooner than the central ; 

 hence arises a certain degree of confusion, which requires much attention to 

 correct in the construction of the microscope or telescope. Tliis correc- 

 tion is effected in the eye partly by the iris shutting off all circumferential 

 rays, and partly by the form and texture of the lens. Its surfaces are not 



879 



