110 CORRESPONDENCE . 



distances of lenses, pointed out by Messrs. McKenzie, Wharton Jones, 

 and Michael Levy, is, according to M. Desmarres, not perfect, but I 

 believe that it may be corrected by the microscope. 



The value of each division of the dial of the micrometric screw 

 already mentioned, must now be ascertained in the special case which 

 occupies us. There is a difference of 20 inches between normal vision 

 and mine. I assume that it may be represented on the dial by ten 

 divisions, each of which will be equal to 2 inches : 10x2 = 20. Let 

 us take an example. I wish to find out for a p-eshyofe the lens suit- 

 able for his sight. I begin by putting in focus an object, which for 

 this purpose will serve as a type. I next observe on what division the 

 index is, and then cause the dial to turn ten divisions from left to right 

 (from right to left when the patient is myopic) in order to bring it to 

 normal sight. This being done, I ask the person in question in his 

 turn to bring the object into focus. If we find that there is between 

 the two sights a difference of eleven divisions on the dial, each of which 

 is equal to 2 inches, we have 22 inches as the result. This we mul- 

 tiply by 10 (the number of inches for normal sight), and obtain the 

 number 220, which, when divided by the difference, that is by 12, gives 

 as quotient 18 inches (and a fraction which may be omitted) as the 

 focal distance of the lens, or the radius of curvature. 



The operation is evidently simjile and easy. Unfortunately, opti- 

 cians do not all mark their spectacle lenses according to a uniform 

 system of numbers. Some numbers indeed indicate the focal distance 

 or radius of curvature, but others indicate the diameter of the sphere on 

 which the lenses are formed. Other numbers, again, belong to series 

 peculiar to certain manufacturers, and do not afford the least informa- 

 tion. There are even a great many lenses not marked with any num- 

 ber. Hence arises an amount of confusion, very j)erplexing to those 

 who arc compelled to have recourse to spectacles to aid the eyes, in 

 their efforts at accommodation, at short distances. We know at least 

 through the foregoing operation, when the two surfaces have the same 

 curvature, the focal distance of the lenses suitable for our sight, and 

 we can select them ourselves at the optician's, or better still at home, 

 if he will intrust them to us, by means of the following very simple 

 process. One of the lenses is exposed to the solar rays in such a posi- 

 tion that its axis is directed towards the sun, that is, that the plane of 

 the circle bounding the lens is perpendicular to the rays. If the lens 

 is convex, and a screen is held parallel to this plane, the image of the 

 sun is seen to form a circle on the screen. This circle varies in size 

 according to the distance between the lens and screen, and the screen 

 must now be moved farther or nearer, without destroying its parallel- 

 ism, until the point is found at which the circle appears smallest and 

 brightest. The screen is then at the focus, occuj)ied by the little 

 luminous circle, and the distance from the lens to the focus can be 

 easily measured. 



It should be remarked that the numbers of spectacle lenses indicat- 

 ing the focus, either for myopian or i^resbyopian sight, are not conse- 

 cutive. Naturally that one must be selected which approaches nearest 

 to the figure given by the operation ; and although this operation may 



