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ON THE USE AND CHOICE OF SPECTACLES. 
By EGBERT BRUDENELL CARTER, F.R.C.S. 
A COMPLETE explanation of all the conditions that may 
render the use of spectacles desirable, and of all the points 
that may require to be considered in their selection, would fill a 
work of very considerable magnitude, and would demand a long 
period of patient application from the reader. It is the special pe- 
culiarity of the eye that its functions fall under the laws of two 
distinct sciences — optics and physiology — and a considerable 
degree of proficiency in both is necessary to a full understanding 
of the way in which each may disturb calculations founded only 
upon the other. It would, at present, be hopeless to attempt to 
popularise some of the questions that come continually under the 
consideration of the ophthalmic surgeon ; and I propose to 
attempt an account only of those defects of sight which are most 
frequent, and concerning which intelligent people may easily be 
instructed to choose spectacles for themselves. 
The eye, as an optical instrument, bears a very close resem- 
blance to the camera obscurain daily use for taking photographic 
pictures. In both the essential parts are a dark chamber, a 
lens to refract the rays of light and bring them to foci, and a 
screen on which these foci fall and form a picture. This screen, 
in the eye, is called the retina, and consists of a delicate layer of 
expanded nerve tissue. How the picture on the retina is made 
an object of mental perception we do not knowq but we do know 
that there cannot be distinct vision unless this picture is clear 
and well defined. If we return to the camera, we may soon 
satisfy ourselves that the clearness of the picture depends upon 
the maintenance of a certain proportion between the distance of 
the object, the power of the lens, and the interval between the 
lens and the screen. If the object be quite distant, the lens 
must either be less powerful or else a little nearer the screen 
than when the object is itself brought nearer. The reason of 
this can be readily made apparent by a simple diagram. If 
rays of light proceed from the point A (fig. 1), pass tlirough the 
lens c, and are brought by it to a focus at b, it is manifest that 
