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1-32 „ Of Optics.

eye. This humour is transparent like chrystal, !s much of the con-listence of hard jelly, and exceeds the specific gravity of water inthe proportion of 11 to 10. It is inclosed in a fine transparentmembrane, from which proceed radial fibres 00, called the ligamentumciliare , ali around its edge ; and join to the circumference of the iris.These fibres have a power of contracting and dilating occasionally, bywhich means they alter the shape or convexity of the chrystallinehumour, and also fhift it a little back ward or forward in the eye, soas to adapt its focal distance at the bottom of the eye to the differentdistances of objects; without which provision, we could only fee thofeobjects distinctly, that were ali at one distance from the eye.

At the back of the chrystalline, lies the vitreous humour KK,■which is transparent like glafs, and is largest of all in quantity, fillingthe whole orb of the eye, and giving it a globular shape. It is much.of a consistence with the white of an egg, and very little exceeds the‘specific gravity and refractive power of water.

As every point of an object ABC sends out rays in all directions,i o forne rays, from every point on the side next the eye, will fall uponthe cornea between E and F ; and by pasting on through the humoursand pupil of the eye, they will be converged to as many points onthe retina or bottom of the eye, and will thereon form a distinctinverted picture cba of the object. Thus, the pencil of rays qrs thatflows fröm the point ^ ofi the object, will be converged to thepoint a on the retina; thofe from the point B will be converged tothe point b 1 thofe from 1 the point C will be converged to thepoint c\ and so of all the intermediäre points : by which meansthe whole image abe is formed, and the object made vifible; althoughit must be owned, that the method by which this fensation is carriedfrom the eye by the optic nerve to the common sensory in the brain,and there discerned, is above the reach of our comprehension.

But that vision is effected in this manner, may be demonstratedexperimentally. Take a bullock’s eye whilst it is fresh, and having cutoff the three coats from the back part, quite to the vitreous humour,put a piece of white paper over that part, and hold the eye towardsany bright object, and you will fee an inverted picture of the objectupon the paper;

Linee the image is inverted, many have wondered why the objectappears.upright. But we are to consider, i. that inverted is only arelative term ; and 2. that there is a very great difference betweenthe real object and the means, or image by which we perceive it.When all the parts of a distant profpect are painted upon the retina,

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