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VIII. — On certain New Methods of Measuring the Magnifying 

 Power of the Microscope and of its Separate Elements. 



By A. E. Weight, M.D. 



(late processor of pathology, army medical school, netley ; 

 pathologist to st. mary's hospital, paddington, w.) 



(Bead March 16th, 1904.) 



The methods of measuring the magnification of the image which 

 are ordinarily in use in connection with the Microscope are the 

 following two. 



1. A micrometric scale placed upon the stage of the Micro- 

 scope is viewed with one eye through the Microscope ; an ordinary 

 scale placed at the standard distance of 10 in. is viewed with the 

 other eye ; and by an intellectual effort the images in the two eyes 

 are combined into a composite image. 



2. By a suitable arrangement of reflectors the system of rays 

 proceeding from a scale placed at 10 in. from the observer is 

 brought across in such a manner as to form an unmagnified image 

 in the eye, which is viewing the magnified image of a micrometric 

 scale furnished by the Microscope. 



Both these methods involve the simultaneous observation of 

 two scales, and as a pre-condition of such simultaneous observation 

 a careful balancing of the light which falls into the eye from the 

 two sources. 



I do not propose to concern myself with either of these methods 

 to-night. I propose with your permission to consider four other 

 methods of measuring the magnification of the microscopic image. 

 The first and last of these are, so far as I know, novel in principle ; 

 and all four have, I think, certain advantages over the methods 

 commonly in use in connection with, the Microscope. Before pro- 

 ceeding to the discussion of these methods I may enumerate them. 



Method 1. — A lens functioning as the counterpart of the re- 

 tracting system of the eye is placed above the eye-lens of the 

 Microscope, in such a manner as to bring the image of a micro- 

 metrical ruling to focus upon a measuring scale, which occupies 

 a position corresponding to that normally occupied by the retina 

 of the observing e} T e. The dimensions of the image which is 

 furnished by the Microscope (working in- conjunction with the 

 focussing lens) are read off upon this measuring scale by means of 

 an eye-piece. 



Method 2. — The magnifying power of the objective and ocular 

 are separately measured, and the magnifying power of the Micro- 



