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Description of an Instrument proposed as a Standard 



Dynamometer, for determining the Magnifying Powers of 



Microscope Objectives. 



By John E. Ingpen, F.R.M.S. 



{ReadSept.21th,ld>12,) 



This instrument has been contrived in accordance with the views 

 expressed by Dr. Ward, in a paper " On Uniformity of Nomen- 

 clature in regard to Microscopical Objectives and Oculars," in "The 

 American Natural," and reprinted in " The Monthly Microscopical 

 Journal" of July 1, 1872, p. 15, to which reference should be made. 

 It consists essentially of two scales, ruled on glass, one of which is 

 magnified by the objective, the other not. The ratio of equal 

 spaces on the two scales gives tbe magnifying power of the objec- 

 tive, at the distance between the scales. 



Description. — A tube (plate v. fig. i.) A carries at one end a glass 

 scale micrometer seen edgeways at B, and also a positive eyepiece, C ; 

 and at the other end a tube D, carrying the glass scale micrometer E, 

 which is capable of being focussed on the objective by the milled 

 head F. Inside the tube A, slides another tube G, into which H, 

 the objective to be tested, is screwed. By means of the milled 

 head K, the front surface of the objective H can be set at exactly 

 ten inches from the scale B. To measure this distance, D is tem- 

 porarily removed, and a cap L, substituted, which cap is of such a 

 length as to place the front surface of the objective at the required 

 distance when flush with the aperture M, or touching a rule laid 

 across it. The cap L is then removed, and D slid on to A, when 

 the scale E can be adjusted to the focus of the objective, and the 

 comparison between the two scales made. The distance of ten 

 inches from the front of the objective to the scale B has been 

 selected instead of a fixed distance between the two micrometers, 

 for the reason that the objectives are thereby put more nearly under 

 their usual working conditions ; if the distance between the scales 



JouRN. Q. M. C. No. 21. H 



