1893.] 



NATURAL SCIENCES OF PHILADELPHIA. 



359 



D 



It has already been incidentally mentioned that the size of 

 the corneal image having been determined, we are enabled 



thereby to deduce 

 the radius of the cur- 

 vature of the cornea. 

 This is accomplished 

 by means of the for- 

 mula d — I : I : : D : 

 R or R =2 D I in 

 •2 d— I 



Fig. 13. which Fig. 13 R is 



the radius sought, D the variable distance of the luminous 

 object, the three gas jets, from the cornea C, I the size of the corneal 

 images as determined by the ophthalmometer, and d the constant 

 length of the luminous object 480 mm., that is the distance between 

 the first gas jet and a point midway between the second and third 

 gas jets, the latter being separated by a distance of 80 millimeters. 

 The following example will illustrate the manner in which the 



formula is used : — 



R=2DI 



d— I 

 D=900 mm. d=480 mm. 

 d=480-000 

 1= 1-957 



1=1-957 mm. 



d— 1=478-043 

 2 D=1800- 

 Log 2 D= 3-255273 

 Log I = 0-291506 



3-546779 

 Log (d— 1)= 2-679467 



Log R = 0-867312 



R = 7'367 mm.=radius of curvature of the cornea. 

 In connection with the formula? just made use of in determining the 

 radius of the curvature of the cornea, it may not be superfluous to call 

 attention to the extent to w r hich the result so obtained will be affec- 

 ted by small errors made in determining the values of T, n, I, D, d, 

 incidental to experimentation. Thus, for example, if an error of 

 - 02 mm. be made in T, the error in n will amount to 0'0002 mm. 

 It is not necessary, therefore, to use the value of the thickness of the 



