1887.] 



On a Perspective Microscope. 



49 



Shelford (W.) and A. H. Shield. On some points for the considera- 

 tion of English Engineers with reference to the Design of Girder 

 Bridges. 8vo. London 1886. The Authors. 



Trois (Enrico E.) Annotazione sopra un esemplare di Trygon 

 Violacea. 8vo. Venezia [1886] ; Considerazioni sul Dentex 

 Gibbosus. 8vo. Venezia 1886. Prof. T. R. Jones, E.R.S. 



January 27, 1887. 



Professor G. G. STOKES, D.C.L., President, in the Chair. 



The Presents received were laid on the table, and thanks ordered 

 for them. 



The following Papers were read : — 



I. "On a Perspective Microscope." By George J. Burch. 

 Communicated by J. Russell Reynolds, M.D., F.R.S. 

 Received January 7, 1887. 



(Abstract.) 



In 1874 the author discovered a form of microscope giving constant 

 magnification along the optic axis, so that objects were shown by it 

 in microscopic perspective. 



By writing (f l -\-f 2 + 'H) for the distance between two thin lenses, 

 he obtained for the formula of the system 



/ 3 (/s + H)«-/ 1 /,(/ 1 +/ 3 + H) _ 

 H«-/i(/i+H) 



u being the distance from the object to the first lens, and v that from 

 the second lens to the image. 



Putting H = in this equation, three things result. 



1. dv/du, which represents the longitudinal magnification, becomes 

 constant, namely —(fjfi) 2 ; 



2. The lateral or angular magnification, f^/fi, is also constant ; 



3. A picture of an object so magnified, drawn with the camera 

 lucida, when viewed from a distance fo/fi times less than that at 

 which it was drawn has the perspective belonging to an object 

 magnified (/ 2 //i) 2 times. 



The distance at which the eye must be placed is great, but may be 

 reduced by employing three lenses, the distance between the first and 



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