﻿A. M. Mayer — Well-Spherometer. 65 



It is better to use the spherometer of the form given in fig. 1, 

 and to bring the planes to true parallelism ; which can be ac- 

 complished by placing thin foil under the flange of H, H (not 

 shown in the figure). This tilts the axis of the well so much 

 from its previous position that the instrument retains its zero 

 reading when the screw is moved over the plane at the bottom 

 of the well. 



The contact-lever on the Briinner spherometer differs from 

 that shown in the figure. The lower end of a delicate contact- 

 lever is bent at right angles and at the angle of this short arm 

 of the lever is the fulcrum. At the other end of this short arm 

 is a point at right angles to it. A delicate spring constantly 

 presses this short arm of the lever downward. The point on 

 the short horizontal arm of the lever is the contact-point used 

 in making measures. This delicate lever is carried on a piece 

 of metal which slides in a guide attached to a horizontal arm 

 of the tripod. The ends of this piece of metal are bent at right 

 angles, and between these are embraced the screw. The lower 

 end of the screw rests in a conical hole in the bent arm, and a 

 set-screw enters a conical hole in the upper end of the screw. 

 On the longer and vertical arm of the contact-lever is a lug on 

 which is engraved a fine line. When this line is made to 

 coincide with a line, drawn on a standard, the screw is at zero 

 of contact. The coincidence of the lines is obtained by means 

 of a microscope attached to the instrument. 



Mode of measurement with the xuell and the ordinary sjoher- 

 ometer. — The ordinary spherometer is simply a tripod, sup- 

 porting a nut N in which runs a screw P. The head of 

 the screw is divided into- fractions of a revolution, and the 

 pitches of the screw are read on a vertical scale attached 

 to one of the legs of the tripod. The zero reading of this 

 apparatus is obtained when the four points, those of the tripod 

 and that of the screw, are in the same plane. The most accu- 

 rate way of getting this adjustment is to place the plane on the 

 top of a resonant box and to run the screw down till it 

 lifts slightly two of the feet of the tripod above the plane. 

 Then, on quickly and gently tapping the plate a rattle will be 

 caused by the instrument vibrating about the screw-point, 

 which is its highest point of support. If the screw be run 

 slightly upward this rattle will become just audible and then, 

 on a very small amount of rotation of the screw, it disappears. 

 The reading of the screw may now be taken for the zero-read- 

 ing. If this reading is obtained by an upward motion of the 

 screw then all subsequent measures referred to this one should 

 be made by contacts obtained by the same direction of motion; 

 otherwise, the " back-lash " of the screw will enter to vitiate 

 the measures. I have found that contacts on a large plate can 

 Aji. Jour. Sci.— Third Series, Yol. XXXII, No. 187.— July, 1886. 



