54 
Dr. Young’s Lecture 
of the eye; and has described, under the name of an optometer, 
a very excellent instrument, founded on the principle of the 
phenomenon. * But the apparatus is capable of considerable 
improvement ; and I shall beg leave to describe an optometer, 
simple in its construction, and equally convenient and accurate 
in its application. 
Let an obstacle be interposed between a radiant point (R, 
Plate II. Fig. 4,) and any refracting surface, or lens (CD), 
and let this obstacle be perforated at two points (A and B) only. 
Let the refracted rays be intercepted 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 fall 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. 5,) 
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 coming from the first point, the images 
of the second and third points must both be double (s s, tt;) 
since the plane (EF) is without the focal distance of rays 
eoming from the furthest 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 somewhat inclined to the axis of the lens, 
* Edinb. Med. Ess. Vol. IV. p. 185.. 
