356 



PROCEEDINGS OF THE ACADEMY OF 



[1893. 



glass, e. g. a stage micrometer, be viewed under the microscope and 

 when seen in focus, that a spirit level be placed upon the upper 

 ends of the microscope and of the screw of the spharometer, 

 the spirit level of the latter having been temporarily removed. 

 The upper ends of the two instruments being now at the same 

 level, let a reading be taken oft' the spharometer and, for 

 example, suppose it to be 46*16. Now place upon the stage micro- 

 meter one of the glass plates of the ophthalmometer, the latter being 

 interposed between the lines and the objective of the microscope. 

 The tube of the microscope must now be elevated in order that the 

 lines may be brought again into focus. The upper end of the micro- 

 scope being now at a higher level than that of the spharometer, the 

 screw of the latter must now be raised until the upper ends of the 

 two instruments are again at the same level and a second reading be 

 taken which we will suppose, for example, to be 43*14. The differ- 

 ence between the two readings, 3"02=1 , 510 mm.,* will be the distance 

 through which the microscope was elevated in order to see the ruled 

 lines when the plates of the ophthalmometer were interposed between 

 the lines and the microscope. Now if this difference, 1*510 mm., be 



deducted from the thickness of 

 the glass, 4*4015, and the remain- 

 der, 2*8915 mm., be divided into 

 the thickness of the glass, the 

 quotient, 1*5222, according to the 

 principle above enumerated, will 

 be the index of refraction sought. 

 Thus, for example, let Fig. 11, 

 represent the micrometer resting 

 upon the stage of the microscope, 

 A G, the actual thickness of the 

 plate of the ophthalmometer 

 interposed between the lines and 

 the microscope, then according 

 to theory and experiment A G — 

 A B=BG or 4*4015 mm.— 1*510 



Fig. 11. 



8 Each division of the scale equals y 2 of a millimeter. 



Note. — The methods just described of determining the thickness of the glass 

 plates of the ophthalmometer and their index of refraction were suggested to the 

 authors by Professor A. W. Goodspeed of the University of Pennsylvania, 

 whose well known experimental skill is a sufficient guarantee of the accuracy of 

 the data submitted and the results obtained. 



