1893.] NATURAL SCIENCES OF PHILADELPHIA. 357 



mm.— 2*8915, A G=^ index of refraction or 4*4015 mm.= 



BG 2*8915 mm. 



l*5222=index of refraction. 



The average index of refraction as obtained from ten experiments 

 with each plate of the ophthalmometer by the method just described, 

 was 1*5249, as shown by the following resume: — 



Resume of experiments with spharometer and microscope to determine 



the value of the index of refraction of the glass plates of the 



ophthalmometer. 



R. L. 



1*5222 1*5276 



1-5222 1-5276 



1*5249 1-5249 



1-5276 1*5249 



1-5276 1-5222 



1*5222 1-5222 



1*5249 1*5276 



1-5276 1-5249 



1*5249 1-5222 



1*5276 1-5222 



1*5252 1-5246 



1-5252 

 1*5246 



2)3-0498 



l-5249=index of refraction. 

 For the sake of simplicity, we have hitherto assumed that the 

 luminous object was single, two images being then seen by means of 

 the ophthalmometer when the glass plates of the latter were made 

 to diverge. In the practical working of the instrument it will be 

 found, however, that if three luminous objects be used, three gas jets 

 for example, placed at right angles to the long axis of the telescope, 

 six images being then perceived, the plates diverging, that it will be 

 easier to determine the displacement of the images than if one be 

 used. The principle involved, however, is the same whether one or 

 three luminous objects be used. This will be made apparent by 

 Fig. 12, in which the distance between the first image and a point 

 midway between the second and third images corresponds to a single 

 image, and 1 C and C 3' to the displacement of the images to the right 



