II. OPHTHALMOLOGICAL. 929 



The mirror apparatus is fixed into the opening of the index disc, which 

 is placed at the further end of the ophthalinometer. The instrument must, 

 when put into use, be so placed upon a stand that the long rods of the mirror 

 apparatus clear the disc of the stand, when it is rotated. A weight, passing 

 over the telescope, balances the instrument. 



3686. Coccius' Double-refracting Ophthalmometer. 



E. Stohrer, Leipzig. 



Design of Apparatus. To measure the principal curved surfaces by means 

 of two movable sources of lights and Iceland spar with doubly refracting 

 prism of Iceland spar whose angle of dispersion is about 3 mm. 



Practical Application. The cornea, on which three points of light (the 

 distance of the images of the respective lights being three millimeters) or 

 four points of light (the distance of the images from the respective sources of 

 light thrown on the cornea being 1^ mm.) can be shown. In the last case 

 the distance of the lights from each other is the half of the distance which 

 three or four points of light show. When the position of the lamps is vertical, 

 the Iceland spar must be turned round. 



b. OPTOMETERS. 



3710a. New Optometer, with double refracting lens of calc- 

 spar, give double readings, and greater precision in determining 

 the distance of sight. Prof. Carl Wenzel Zenger, Prague* 



3714a. Ferrin and Mas cart's Optometer. 



M. Roulot, Paris. 

 3713a. Ferrin and Mascart's Optometer (small pattern). 



M. Roulot, Paris. 

 3713. Dr. Badal's Optometer. M. Roulot, Paris. 



3697. Graefe's Binocular Optometer. 



Dr. Weber, Darmstadt. 



3710a. Optometer, for determining the condition, degree of 

 refraction, and acuteness of vision. 



William Laidlaw Purves, M.D. 



The optometer consists of a disc of convex spherical, a disc of concave 

 spherical, a disc of convex cylindrical, and a disc of concave cylindrical 

 lenses. These move upon a central axis, so that any of the four discs can be 

 superimposed upon another, and thus any combinations made which the par- 

 ticular lenses used can arrive at. The cylindrical lenses, having their axes 

 all placed in the same direction, and at any one meridian of the instrument, 

 the different powers are speedily brought before the eye, so as to act on the 

 desired meridian of the eye, and on it only. (Vide "British Medical 

 Journal" for January 1875.) 



371Ob. Boxwood Scale, on which the combinations are 

 calculated : belongs to the optometer last described. 



William Laidlaw Purves, M.D. 



40075. 3 N 



