90 A NEW OPHTHALMOSCOPE. 



same horizoiatal line. A screen, to shade the ground glass and the 

 observer's eyes, is placed between the light and the back of the cam- 

 era ; or what I have found to be much better, a metallic tube or shade 

 is placed around the lamp, from an aperture in which, projects a 

 collar somewhat resembling that of a magic lantern, of the right size 

 to allow the illuminating tube of the instrument to fit closely. In- 

 deed with this apparatus the camera can be dispensed with, that is, in. 

 making examinations of the eye simply, but when the object is to 

 demonstrate the fundus of the eye to a number of persons the camera 

 is used both with and without the ground glass. 



Since this paper was read before the Institute, Mr. Potter has constructed for me a modi- 

 fication of tlie above instrument which I find to be very convenient. 



It shews the fundus of the eye of the cat or dog beautifully, but it remains to be seen whe- 

 ther the illumination is sufficient for examining the fundus of the human eye. 



The light is supplied by an ordinary coal oil lamp which is placed in a box about six inches 

 Bquare and fifteen inches high. Opposite the flame of the lamp there is an aperture in one 

 side of the box from which projects a brass tube or collar to which is adjusted the illumina- 

 ting tube of the instrument. 



In the outer or camera tube is a double convex lens of 2 inch focus instead of the tw o 

 lenses of 5 inch focus each. At the outer extremity of this tube a moveable eye piece is at- 

 tached three inches in length, and containing a convex lens of tliree inch focus. 



OPTICS. 



1st. Illumination : — Let MQ (fig. 1) represent pai'allel rays of solar 

 light incident upon the double convex lens G: at the points NR they 

 are refracted and emerge from the lens convergingly towards a focus 

 V in the tube C, but at O and S they are intercepted by the plate 

 glass D, a portion of the rays are reflected by its polished surface in 

 the direction E, and rays not reflected or absorbed are transmitted 

 and pass to form a focus at V — the principal focal distance of the 

 lens G, and again diverge in the direction WX. The rays reflected 

 from the surface of the plate glass form a focus at U (which is also 

 the focal centre of the eye E), at the same distance in front of the 

 plate glass D as V is behind it ; these rays again diverge and illumi- 

 nate a portion of the fundus at TP. 



2nd. Reflection : — Let E (fig. 2) represent the same eye illuminated 

 as just described ; D the plate glass ; and HI the lenses in the 

 camera tube. Rays from any portion of the illuminated fundus 

 as a, are reflected from the fundus and emerge from the cornea at 

 bo, the width of the dilated pupil, and proceed to the plate glass 

 D, parallel, where some of its rays are reflected from the plate 

 glass through the lens G in the direction of the source of illumi- 



