880 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
so that the double wires in both Microscopes are parallel. The course 
of the measurement is similar to that in the comparator. 
The third instrument is the spherometer, shown in fig. 102. In prin- 
ciple it is the same as the ordinary spherometer, the radius of curvature 
R being given by the formula R = r 2 / 2 h -f h/ 2, where h is the height, 
as measured by the instrument, of a dome with base of known radius r ; 
but, as in Mr. E. M. Nelson’s instrument, described in the October number 
Fig. 102. 
of this Journal, p. 670, for the usual tripod arrangement is substituted 
a ring on which the lens to be examined rests. The steel ring R fits 
into the plate P, supported on two strong uprights, and can be replaced 
by other rings of less or greater diameter. The base-plate of each ring 
is provided with a cylindrical boring which fits on to a cylinder screwed 
to the plate P, so that the central position of the contact point K is in 
all cases ensured. 
