﻿A. M. Mayer — Well-Spherometer. 



67 



Simple forms of Spherometers with which the well is used. — 

 Other and simpler forms of the well-spherometer suggest them- 

 selves. They may be worth the recording, as they may be 

 considered useful by those desiring only approximate measures 

 with simple and inexpensive apparatus. 



Fig. 4 shows a form in which the tripod is replaced by a 

 hollow cylinder carrying in its axis a micrometer-screw. The 

 figure explains itself. The micrometer-screw may be replaced 

 by a graduated rod sliding along a fixed vernier, or, a rod may 

 be used without graduation, and two marks cut on it by a 

 sharp edge drawn along the upper surface of the top of the 

 cylinder. One of these marks is made when the point of the 

 sliding-rod touches a plane closing the bottom of the cylinder ; 

 the other when the rod rests on the spherical, or other surface, 

 to be measured. The distance separating the marks thus made 

 is then measured with a micrometer-microscope. 



Fig. 5 shows how the common micrometer-screw calipers can 

 be used to determine the ra- 5 



dius of curvature. With it 

 we measure the united thick- 

 ness of the well w and the 

 plate £>, cemented to its top. 

 Call this measure i. Then we 

 measure the thickness of the 

 lens ; call this I. Then the 

 lens covers the bottom of the 

 well, and the thickness of well 

 and plate and lens is measured 

 in this position. Call this t' . 

 Then (t+l) — i'= height of sec- 

 tion of lens above plane of 

 mouth of well. If the axes of 

 screw and well coincide, the 

 pressure of the micrometer on 

 the lens tends to center it in 

 the well. If the plate cover- 

 ing the well be rigid, and the 

 pressures in all the measures 

 be made as equably and as 

 gently as possible, one can thus 

 get the height of the section of 

 the lens to T ^„ 7 th of an inch. 

 The same measures can be made with the vernier-calipers. 



Fig. 6 shows a simple optical method in which the well is 

 used. The well W, is placed with the plane of its mouth 

 vertical. The well is attached to a stand whose base is firmly 

 screwed to the table. A rod, r, slides neatly through a hole in 



