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Simple Methods of Focometry and Apertometry. 

 By Frederic J. Cheshire. 



{Read May 16th, 1902.) 



In presenting the following simple methods of microscopical 

 focometry and apertometry to your notice, I would, in the first 

 place, observe that they are the outcome of considerable experience 

 and numerous experiments extending over the last two or three 

 years. I may claim for them then, I hope, that they are some- 

 thing more than mere paper methods ; in fact, I am sanguine 

 enough to think that, so far as the subject of apertometry is 

 concerned, I shall be able to present to you a very simple and 

 inexpensive method by which any one can, with care, determine 

 and compare the N.A's of dry objectives to a degree of accuracy 

 quite sufficient for all practical purposes. 



Focometry. 



Some of the methods to be subsequently described for deter- 

 mining N.A. necessitate a careful preliminary determination of 

 the focal lengths of objectives and condensers. These I find, in 

 practice, are best found by using the microscope as a simplified 

 but less accurate modification of the fccometer invented by Abbe, 

 and described in the Jour^ial Roy. Mic. Soc, 1892. This modifi- 

 cation, which does not appear to be known to microscopists to the 

 extent that its merits deserve, I first saw described in a publica- 

 tion some three or four years ago ; but unfortunately I have lost 

 the reference, and am therefoie unable to do the author the 

 justice of mentioning his name. 



Abbe's focometer is based upon the fact that if an image of an 

 object is produced by a lens system, first in one plane normal to 

 the axis, and then in another, the focal length of the system is 

 equal to the distance between these two image-planes divided by 



