1893.] 



On a Portable Ophthalmometer. 



3 



source, a virtual image of the disc will be formed at the virtual focus 

 of the convex reflecting surface. This image will only be seen 

 distinctly by the emmetropic eye through the neutralised portion of 

 the prism, when the focus of the lens in front coincides with the 

 virtual focus of the convex surface. The ratio of the object to the 

 image will be as shown above. If now a double-image prism be 

 inserted behind the neutralising prism, which exactly doubles this 

 image, its power with the combination is easily determined, and 

 therefore the exact size of the image can be measured. The size of 

 the object being known, we have the three elements necessary for 

 determining the fourth proportional, the curvature of the convex 

 reflecting surface. 



The instrument in this simple form presented a number of prac- 

 tical difficulties in its manipulation, which were overcome by the 

 introduction of a short telescope behind, with double-image prism 

 fixed in front of its object-glass. 



Fig. 2. 



In its present form the instrument consists of the following parts 

 [vide fig. 2*) : — An aplanatic lens Ob, a rectangular prism P neu 

 tralised in the visual axis by a smaller prism P 2 and a telescope, 

 with the double-image prism BP fixed in front of the object glass of 

 the telescope Ob 2 . The focal length of the object-glass Ob z is pre- 

 cisely the same as that of the aplanatic lens 06, and cross wires CW 

 at its principal focus are viewed by a Ramsden eye-piece. 



Before using the instrument it is necessary and sufficient that the 

 cross wires should be distinctly seen at the punctum remotum of the 

 observer. The adjusted instrument is held in the observer's left 

 hand, which rests on the forehead of the patient, the disc being 

 directed to a luminous source to the right of the observer. The point 

 of coincidence of the principal foci is found by moving the instrument 

 to and fro. When the observed eye is directed to the central or 



* The object-glass lias been specially constructed according to the formula of 

 Professor Abbe, and contains barium glass as one of its constituents. 



B 2 



