﻿66 



A. M. Mayer — W ell-Spherometer , 



be obtained by this rattling-test to IF J R th of an inch. It 

 must not be inferred, however, that other contacts, like those 

 on the top of a small lens, or on a small and thin plate, can be 

 determined with the same accuracy. The very test requires 

 vibration and unsteadiness in the instrument; and these condi- 

 tions are not' those for precise work. This ordinary spherome- 

 ter cannot, in my opinion, be regarded as an instrument of 

 precision, like the lever-contact spherometer. 



8. 



D 



In measuring radii of lenses with the ordinary spherometer 

 we use a plate A, B, fig. 3, with a hole in its center. In this 

 bole is cemented the steel well W, whose lower opening rests 

 on another plate. This rests on the resonant box. Paral- 

 lelism of the surface at the bottom of the well and that of A, 

 B, on which stands the spherometer, is determined by the 

 rattling test. If this parallelism of the plates does not exist, it 

 must be obtained by re-cementing the well with thin foil 

 placed under the flange of the well. This cement is not placed 

 under this flange, but only on the outside of the cylinder of 

 the well. In attaining the height of the section of the lens 

 (shown in dotted lines in fig. 3), it is best to rest this in a hole 



i 



mmmwmm t 





llll II 1 1 1 



1 1 in = 







■mm- 



= 





J VI 



j 



I 



nn 1 



j 



in a plate C, D, placed on the resonant box, and then place the 

 plate with the well on top of the lens. Small corks may be 

 placed between the plates to prevent too much motion of the 

 well around the spherical surface of the lens. 



