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Transactions of the Society. 



field of vision, meaning thereby each constituent point of it, is 

 equally opposed to the simplest law of optics. 



With reference to the various lens combinations interposed in 

 the course of the incident light, it is easy to show that their effect 

 is exerted only under special circumstances, as has been shown of 

 the form and position of the mirror. Thus, if we take Sir D. 

 Brewster's condenser (Fig. 8), where F is a point in the plane of the 



Fig. 7. 



Fig. 8. 



Fig. 8 (from pp. 95 N. and 1 S. ' Das Mikroskop'), to illustrate theory of action of 

 Cata-dioptric illuminator. 

 p, Plane of object and focal point of illuminator. 

 a-h, Diaphrafj;m aperture regulating the angle of convergence. 

 L, Lens emitting converging pencil. 

 M, Mirror. 

 E, Bull's-eye condenser. 



Fig. 7, Condenser designed by Sir D. Brewster. 

 A B C D, Lens combination sending converging pencil on F focal point 



in plane of object m n. 

 A' B'C D', Lens combination for collecting light from light-source and 



transmitting to mirror M. 

 G H, Diapliragm aperture. 

 In both diagrams the luminous source is practically the surface of the lens 

 immediately under the plane of object, and a b Is manifestly better placed than 

 G H for regulating the size of illuminating pencil. 



object, Gr H the diaphragm aperture which determines the size of 

 the incident cone,* then A B, the last refracting surface of the combi- 

 nation, simply represents an area of light-source, and is in effect self- 

 luminous, since the light received by the lens system A B C D is 



* It should be noted that H G is a screen supplied with different circular 

 apertures and " a variable rectilinear aperture," the image of the slit being thrown 

 upon a lined object parallel with its lines. And a final recommendation is made 

 by Sir David Brewster, which I here quote. " It would be desirable to have 

 circular and rectilinear apertures of different sizes to be placed immediately 

 beneath F, so as to allow no part of the field to be seen excejiting that which is 

 occupied by the object or part of it under examination." The action of such 

 supplementary diaphragms would modify (beneficially) the incidence of light if 

 the aperture were not so fine as to occasion diffractive effects. (!) 



