'a<So On the Mechamfm of tht Eye. 



But the apparatus is capable of confiderable improvement ; and I fliall beg leave to defcribe 

 an optometer, fimple in its conflru€tion, and equally convenient and accurate in its 

 application. • , , , 



Let an obftacle be Interpofed between a radiant point (R, Plate Xlll. Fig. 4,) and any re^ 

 frafting finrface, or lens (CD), and Jet this obftacle be perforated at two points (A and B) 

 only. Let the refra£led rays be intercepted by a plane, fo as to form an image on it. Then 

 it is evident, that when this plane (EF) pafles through the focus of refraded rays, the 

 image formed on it will be a fingle point. But, if the plane be advanced forwards (to 

 GH)j or removed backwards (to IK), the fmall pencils pafling through the perforations, 

 will no longer meet in a fingle point, but will fall on two diftinft fpots of the plane (G, H.; 

 I, K ;) and, in either cafe, form a double image of the objeft. 



Let us now add two more radiating points, (S and T, Fig. 5,) the one nearer to the 

 lens than the firft point, the other more rem«te ; and, when the plane which receives the 

 images pafles through the focus of rays coming from the firft point, the images of the 

 fecond and third points muft both be double (s s, 1 1 ;) fince the plane (EF) is without the 

 focal diftance of rays coming from the furtheft point, and within that of rays coming from 

 the neareft. Upon this principle, Dr. Portcrfield's optometer was founded. 



But, if the three points be fuppofed to be joined by a line, and this line to be fomewhat 

 inclined to the axis of the lens, each point of the line, e«cept the firft point (R, Fig. 6,) will 

 have a double image ; and each pair of images, being contiguous to thofe of the neighbour- 

 ing radiant points, will form with them two continued lines, and the images being more 

 widely feparated as the point which they reprefent is further from the firft radiant point, 

 the Vines- (s f, sty) will converge on each fide towards (r). the image of this point,, and 

 there will interfcft each other. 



The fame happens when we look at any objefl through two pin holes, within the limits 

 of the pupil. If the objedl be at the point of perfedl vifion, the image on the retina will ba 

 fingle i but, in every other cafe, the image being double-, we (hall appear to fee a double 

 obje£t: and, if we look at a line pointed nearly to the eye, it will appear as two lines, 

 croflifig each other in the point of perfe£l vifion. For this purpofe, the holes may be con- 

 verted into flits, which render the images nearly as diftindt, at the fame time that they ad- 

 mit more light. The number may be increafed from two to four, or more, whenever 

 particular inyeftigations render it neceflary. 



The optometer may be made of a flip of card- paper, or of ivory, about eight inches in 

 length, ^nd one in breadth, divided longitudinally by a black line, which muft not be too 

 flrone. The end of the card niuft be cut as is (hown in Plate XIV. Fig. 7, in order that it 

 may be turned up, and fixed in an inclined pofition by means of the (houlders : or a de* 

 tached piece, nearly of this form, may be applied to the optometer, as it is here engraved; 

 A hole about half an inch fquare muft be made in this part j and the fides fo cut as to re- 

 ceive a Aider of thick paper, with flits of different fizes, from a fortieth to a tenth of an inch 

 3 »a 



