1. A companion paper in the present issue of this journal describes this instrument.
2. A valuable improvement might indeed be made by using a well-fitted dovetail slide instead of the roller and straight edge. The sliding carriage would then be pivoted to the bar 5 at the point 7.
3. We wish to mention with praise the careful work done in preparing the quartz plates by the Penn Optical Company of Pasadena to whom we are also indebted for suggesting to us the procedure (to be explained later) using a pair of auxiliary cast-iron laps to correct the stainless-steel blocks.
4. We are much indebted to Professor W. Hansen of Stanford University for recommending the use of the lapping compounds of this firm and to the U.S. Products Corporation, 518 Melwood Street, Pittsburgh, who very kindly and ably advised us regarding the proper compounds for this job and generously presented us with samples more than adequate for our use. These compounds have proved extraordinarily satisfactory for the purpose.
5. The times of lapping stated here are in every case the net periods during which the surfaces were actually being rubbed together, excluding time out for cleaning, testing, etc.