THE MICROSCOPE. 69 



that L. at his death bequeathed the " greatest part of his 

 microscopes (Optics., p. 219) to the Roy. Soc. This is 

 evidently a mistake, for the number received by the society 

 was but twenty-six. A full description of them we find in 

 Philos. Trans., No. 380, by Martin Folkes, Esq. Another account 

 by Baker, in Phil. Trans., No. 458, in 1740, gives the magnifying 

 power, etc., but there is no drawing with either 



STUDIES WITH MICROMETERS. 



BY CHARLES K. WEAD, A.M. 

 Professor of Physics, University of Michigan. 



In view of the increased attention paid during the last few 

 years to the subject of micrometry, the following observations may 

 prove interesting to some readers. 



I. A comparison was made between a stage micrometer of 

 Fasoldt's, on which there were five spaces of yJ^^ in., ten of ^-^, and 

 some finer rulings, and a Rogers' "standard" on which were two 

 sets of rulings, one with t,ooo spaces in 0.4 inch, the other with 

 1,000 spaces in one centimeter. To use a longer space than could 

 be taken with a microscope in the ordinary way, a microscope 

 magnifying 200 diameters was clamped to the base of a dividing 

 engine, and the ruled plate was carried by the slide of the engine 

 just above the stage of the microscope. The engine has a screw 

 whose pitch is nearly one millimeter, and the divided head was 

 read to thousandths of a revolution, corresponding to microns. 

 Thus we combine the good illumination furnished by a microscope 

 stand, and the opportunity of measuring a considerable distance 

 by the long screw. The greatest distance available on Fasoldt's 

 plate was 0.1 inch. The value of this nominal distance on both plates 

 was then found in terms of the same part of the screw. A spot 

 on an eye-piece micrometer, cemented in the tube, and therefore pre- 

 vented from moving, served instead of cross-wires as the point to 

 bring the lines to. Of course the plate was leveled so that all 

 lines used were clearly in focus, and the lines were so accurately at 

 right angles to the line of motion that no measurable error could 

 arise here. To make sure that nothing had been displaced during 

 the work, the readings of the screw when the first line was in the 



