28o American Quarterly Microscopical Journal. 



ON PROFESSOR H. L. SMITH'S APERTOMETER. 



BY F. H. WENHAM. 



{Received June 4th, iS'/p.) 



I have read with much interest Professor Smith's paper on " An- 

 gular Aperture, and an Universal Apertometer," published in the 

 last number of the American Quarterly Microscopical Jour- 

 nal. The essay contains the most practical series of experiments 

 yet made, with results that must lead to a more correct knowledge 

 of the meaning of angular aperture, and to a sure method of 

 measuring it. After perusing Professor Smith's paper, any one 

 must be convinced that he is unbiassed and free from prejudice. 



I admire Professor Smith's courage in taking up a subject that 

 may give rise to acrimonious strictures, and which has been an- 

 nounced as " finally decided " by some who have complacently 

 agreed amongst themselves upon a verdict that mine is a " lost 

 cause," forgetting that up to the present time we have possessed 

 no instrument for measuring aperture satisfactorily, or with suf- 

 ficient accuracy to enable us to finally decide the question. 



Professor Abbe's apertometer is held by some as the only one 

 to be relied upon, but with it we take the angle of field instead 

 of the angle of aperture, for the outermost rays that extend to the 

 margin of the field render the index pointers visible through the 

 glass of this instrument, precisely as they would two diatoms 

 or other objects at the edges of the field of view, when mounted 

 in balsam. I have tested this apertometer ; using the inter- 

 mediate or examining lens supplied with it according to instructions. 

 I measured a very fine recent oil-immersion -|- inch by Zeiss, 

 professing to have a balsam angle of 113° : with a short micro- 

 scope body and the A eye-piece, the indices may be seen at 

 that angle in glass ; with the B eye-piece the indices will be out of 

 range, invisible at that angle; with the C eye-piece, 90° is indicated 

 on the inner scale, on which are degrees for air angles only ; with 

 the D eye-piece the air angle is 65° ; with the E it is 45", and 

 with the F it is the absurd angle of 35° only, in air. With such 

 enormous discrepancies, no reliance can be placed upon the instru- 

 ment, which I consider useless for measuring apertures, as it gives 

 a different result for every length of body and for each eye-piece, 

 and with the varying diameter of the stops by which the extent of 

 the field of view is determined. 



