86 



A NEW OPHTHALMOSCOPE. 



nated by a thin brass diaphragm having a central circular aperture 

 of f of an inch in diameter. 



OONSTEUCTION : THE TUBES. 



Fig. 1. 



At the juncture of the tube A witb B there is a circular aperture 

 of one inch diameter, and between and B an aperture of ^ incb 

 diameter — affording a communication between A and G through B. 



THE PLATE GLASS. 



At the juncture of the tubes there is placed an elliptical piece of 

 highly polished thin plate glass with parallel surfaces, which is in- 

 clined at such an angle to the tubes that a portion of a ray of light 

 falling upon it through the centre of the tube A from the direction 

 M Q is reflected at right angles to its original direction, and in 

 the same plane with the centre of the tube B which will be through 

 the centre of the aperture in the diaphragm. A portion of the ray 

 will be refracted by the plate glass and pass through the tube C par- 

 allel to its original direction. 



THE LENSES. 



At the inner extremity of the tube A and as close as possible to 

 its juncture with the tube- B, a double convex lens G- is placed li 



