1893.] 



NATURAL SCIENCES OF PHILADELPHIA. 



351 



mometer. It should be mentioned, however, that in the ophthal- 

 mometer as devised by Helmholtz 6 the two images of the object 

 are produced by means of two glass plates such as were suggested 

 by Clausen to obtain two images of a star. The ophthalmometer 



as constructed by Meyerstein of Gottingen 

 consists of a small telescope (Fig. 4, T) the 

 objective of which, b, is enclosed within a 

 brass box C, the latter supported upon a 

 tripod and capable of being leveled in all 

 azimuths. The brass box contains two plane 

 parallel glass plates P 1 P 2 , which are situated in 

 front of the objective of the telescope. By 

 means of a screw mechanism the glass plates 

 can be so turned around a common axis that 

 they can be brought either into the same plane 

 (Fig 5) or into two different planes (Fig. 6). 

 The angular deviation produced by the move- 

 ment of the glass plates just referred to can be determined by two 

 graduated wheels which rotate as the glass plates diverge from each 

 other to the one-tenth of a degree, the instrument being provided 

 with a vernier. 



By means of the ophthalmometer we are enabled to determine 

 the size of the virtual image produced when rays of light emanating 



Fig. 3. 



cC 



a 



a 



Fig. 4. 

 from an external object are allowed to fall upon the cornea and, 

 further, as we will show presently, by determining from the size of 

 such corneal image the radius of the curvature of the cornea. The 

 principle by which the size of the corneal image is determined by 

 the ophthalmometer is the same as already mentioned asthat of obtain- 

 ing the apparent diameter of a heavenly body by means of a helio- 

 meter. The only essential difference in the theory of 

 the two instruments is that while in the heliometer 

 one semi-lens alone moves, and moves in the same 

 plane with the other fixed semi-lens, (the displace- 

 6 Archiv fur Ophthalmologic Band ], Abth. II S 1. 



Fig. 5. 



