1893.] NATURAL SCIENCES OF PHILADELPHIA. 355 



As the average thickness of the two glass plates is the same, viz. : 

 4*4015 mm., the result is what might have been anticipated since 

 the two glass plates were the halves of one plate. 

 Resume of 25 determinations of the thickness of the right and left glass 

 plates of the Ophthalmometer by the Sphdrometer. 



Readings from the Instrument. 



Corrected and reduced. 



4-4015 mm. 4*4015 mm. 

 In determining the index of refraction of the two plates of the 

 ophthalmometer the spharometer was also made use of, its applica- 

 tion for this purpose being based upon the principle of the index of 

 refraction being equal to the ratio of the actual thickness to the 

 apparent thickness when viewed normally to the surface. Let us 

 suppose that a number of equidistant, parallel lines ruled upon a 



