452 LECTURE xxxvrir. 



limits of perfect vision, unless we allowed the irregularity of the form as- 

 sumed by the marginal parts of the crystalline lens. The iris is also pecu- 

 liarly useful in excluding such parts of lateral pencils of light as fall very 

 obliquely on the cornea, and are too much refracted, while a smaller pencil 

 only, which enters the eye more directly, is admitted into the pupil. 



The refractive powers and properties of the eye may be very conveniently 

 ascertained by means of an instrument to which I have given the name 

 optometer, a term first employed in a sense nearly similar by Dr. Porterfield. 

 If two or more separate parcels of the rays of the same pencil be admitted 

 at distant parts of the pupil, they will only be reunited on the retina when 

 the focus is perfect, so that if we look through two small perforations, or slits, 

 at a minute object, to the distance of which the eye is not accommodated, it 

 will appear as if double; and when the object is aline directed nearly towards 

 the eye, each point of it will appear double, except that which is at the 

 distance of perfect vision, and an image of two lines will be seen, 

 crossing each other in this point; so that the measurement of the focal 

 length of the eye is immediately performed by inspection of the optometer 

 only. The scale may be extended by the addition of a lens, which enables 

 us to produce the effect of a longer line, while the instrument still remains 

 portable. 



When the eye is possessed of too great a refractive power for the distinct 

 perception of distant objects, the pupil is generally large, so that the con- 

 fusion of the image is somewhat lessened by partially closing the eyelids; 

 and from this habit an eye so formed is called myopic. In such cases, by 

 the help of a concave lens, the divergence of the rays of light may be 

 increased, and a virtual image may be formed, at a distance so much smaller 

 than that of the object as to afford perfect vision. For a long sighted or 

 presbyopic eye, on the contrary, a convex lens is required, in order to 

 obtain a virtual image at a greater distance than the object; and it often 

 happens that the rays must be made not only to diverge less than before, but 

 even to converge towards a focus behind such an eye, in order to make 

 its vision distinct. Presbyopic persons have in general a small pupil, and, 

 therefore, seldom acquire the habit of covering any part of it with their 

 eyelids. 



