﻿A. M. Mayer — Well- Spherometer. 61 



Art. YII. — On the Well- Spherometer ; an instrument that meas- 

 ures the radius of curvature of a lens of any linear aperture ; 

 by Alfred M. Mayer, Professor in the Stevens Institute 

 of Technology, Hoboken, New Jersey. 



During the past ten years I have used in my laboratory a 

 spherometer that gives measures from which we can compute 

 the radii of curvature of lenses of very small linear aperture. 

 This instrument I exhibited, and its mode of use I explained 

 before the National Academy of Sciences on Nov. 15th, 1882. 

 The spherometer, as generally made, cannot measure the radii 

 of lenses of small diameter, by reason of the large spread of the 

 feet of the tripod that support the micrometer screw. On some 

 spherometers these feet are made to slide toward the screw till 

 they are equidistant from it, and so near to it, that they can 

 stand on the surface of the small lens whose radius is to be 

 measured. The objections to this arrangement are: (1) that 

 when the feet are thus brought near the screw the instrument 

 is top-heavy, and it becomes difficult to determine nicely the 

 contact of the point of the micrometer screw ; (2) that it re- 

 quires expensive workmanship to make these slides carry the 

 feet so that the plane passing through their points shall remain 

 at right angles to the axis of the screw ; (3) the difficulty of ad- 

 justing these feet so that they are equidistant from the axis of 

 the screw ; (4) the difficulty of measuring what is known as 

 "the radius of the instrument;" i. e. the distance of the points 

 of contact of the feet from the point of contact of the screw. 

 This objection (4) applies to all spherometers except the one 

 described in this paper. The necessary curvature of the ter- 

 minal points of the tripod renders the radius of the circle, 

 including these points of contact, variable with the radius of 

 lens ; and these points of contact cannot be determined with 

 precision except by difficult measurements on the form of the 

 points and subsequent troublesome computations; and then 

 both the zero reading of the screw and the radius of the instru- 

 ment are changed from what they were when referred to a plane 

 surface. It has been attempted to overcome this objection by 

 making the points exceedingly fine; but in this condition they 

 are soon blunted by use and deformed by the least carelessness. 

 (5) To be able to bring the feet of the tripod sufficiently near 

 the axis of the screw to measure lenses of very small diameter, 

 these feet have to be made very delicate and the screw has to 

 be terminated by a long pointed rod ; thus making the feet 

 liable to spread from the weight of the instrument pressing them 

 upon a curved surface of small radius. 



