676 



ON THE JIKCHANISM OF THE EYE. 



Let an obstacle be interposed between a 

 radiant point (11, Plate 15. Fig. 109j) and any 

 refracting surface, or lens (CD), and let this 

 obstacle be perforated at two points (A and 

 B) onl3'. Let the refracted rays be inter- 

 cepted by a plane, so as to form an image on 

 it. Then it is evident, that when this plane 

 (EF) passes through the focus of refracted 

 rays, the image formed on it will be a single 

 point. But, if the plane be advanced for- 

 wards (to GH), or removed backwards (to 

 IK), the small pencils, passing through the 

 perforations, will no longer meet in a single 

 point, but will fail on two distinct spots of 

 the plane (G, H ; I, K :) and, in either case, 

 form a double image of the object. 



Let us now add two more radiating points, 

 (S and T, Fig. 110,) the one nearer to the lens 

 than the first point, the other more remote ; 

 and, when the plane, which receives the 

 images, passes through the focus of rays com- 

 ing from the first point, the images of the se- 

 cond and third points must both be double 

 (« s, t t ;) since the plane (EF) is without the 

 focal distance of rays coming from the fur- 

 thest point, and within that of rays coming 

 from the nearest. Upon this principle. Dr. 

 Porterfield's optometer was founded. 



But, if the three points be supposed to be 

 joined by a line, and this line to be some- 

 what inclined to the axis of the lens, each 

 point of the line, except the first jKiint (K, 

 Fig. 1 1 1,) will have a double image; and each 

 pair of images, being contiguous to those of 

 the neijiibouring radiant points, will form 

 with them two continued lines ; and the 

 images being ntore widely separated as the 

 point which they represent is further fron^ 

 the first radiant point, the lines (s t, s t,) will 

 convergeon each side towards (r) the image of 

 this point, and there will intersect each other. 



The same happens when we look at any 

 object through two pin holes, within the li- 

 mits of the pupil. If the object be at the 

 point of perfect vision, the image on the re- 

 tina will be single ; but, in every other case, 

 the image being double, we shall appear to 

 see a double object : and, if we look at a line 

 pointed nearly to the e3'e, it will appear as 

 two lines, crossing each other in the point 

 of perfect vision. For this purpose, the 

 boles may be converted into slits, which ren- 

 der the, images nearly as distinct, at the same 

 time that they admit more light. The num- 

 ber may be increased from two to four, or 

 more, whenever particular investigations 

 render it necessary. 



This instrument has the advantage of show- 

 ing the focal distance correctly, by inspec- 

 tion only, without sliding the object back- 

 wards and forwards, which is an operation 

 liable to considerable uncertainty, especially 

 as the focus of the eye may in the mean 

 time be changed. 



The optometer may be made of a slip of 

 card paper, or of ivory, about eight inches 

 in length, and one in breadth, divided lon- 

 gitudinally by a black line, which must not 

 be too strong. The end of the card must be 

 cut as is shown in Plate 9. Fig. 71, in order 

 that it may be turned up, and fixed in an in- 

 clined position by means of the shoulders : or 

 a detached piece, nearly of this form, maybe 

 applied to the optometer, as it is here engrav- 

 ed (Fig. 72.). A hole about half an inch square 

 must be made in this part ; and the sides so 

 cut as to receive a slider of thick paper, with 

 slits of different sizes, from a fortieth to a 

 tenth of an inch in breadth, divided by 

 spaces somewhat broader ; so that each ob- 

 server may choose that which best suits the 

 aperture of his pupil. In order to adapt the 



