﻿62 



A. M. Mayer — Well-Spheromete?\ 



The instrument which I now describe is, I think, free from 

 these objections and has, in my practice with it, given accurate 

 results. 



The points of the tripod A, B, C, fig. 1, rest on a plane sur- 

 face. A disc of steel H, H, is screwed into the plate of the 

 instrument, this disc has a cylindrical aperture, or well, accu- 

 rately formed. The surface of the mouth ?, Z, of this well is 

 turned to a sharp edge by a hard tool with a sharp and properly 

 formed cutting edge. • 



To measure the radius of a lens, a steel disc is selected whose 

 aperture is smaller than the diameter of the lens, and it is 

 screwed firmly into the plate. A piece of flat glass is now 

 pressed by a spring against the lower opening of the steel 

 cylinder, and the reading of the head of the screw is taken 

 when the pressure of the point of the screw on the glass plate 

 makes the contact-lever, Gr, read zero. The flat plate is now 

 replaced by the lens, D, and the screw is approximately centered 

 in the steel well. The screw head is turned till the contact- 

 lever reads zero, then the spherometer is slid over the top of the 

 lens till the contact-lever gives the highest reading, the axis of 

 the screw then coincides with the axis of the well, and is in line 

 with a radius of that portion of the lens contained above the 

 plane passing through the mouth of the well. The instrument, 

 now stationary, is adjusted till the contact-lever reads zero. 

 The difference of this reading and the one obtained when the 

 screw was in contact with the plane surface gives the height of 

 the section of the lens above the plane cf the bottom of the well. 



