The Magnifying Power of the Microscope. By A. E.Wright. 283 



Measurement of the Magnifying Power of the Ocular. 



The total magnifying power of the Microscope corresponds to 

 the total angle through which the most obliquely incident ray is 

 refracted in its passage from the object plane on the stage of the 

 Microscope to the retina of the observer's eye. The magnifying 

 power of the ocular which we are here concerned to measure cor- 

 responds, as reflection will show, to the angle through which the 

 ray in question is refracted while in passage between the aperture 

 of the objective and the aperture of the eye-lens. 



In order to obtain our measurement we must be clear, on the 

 one hand, as to the situation of the apertures which are in question, 

 and, further, we must have some means of locating in the respec- 

 tive apertures the points of origin and arrival of the rays from 

 which we have to take off in making our measurements. 



Position of the Apertures which come into consideration: — The 

 true aperture of a lens is positioned where the beams which are 

 transmitted through it mutually interfuse and overlap. This 

 interfusion disc (variously denoted the " Lagrange disc," the 

 Eamsden disc, and the " pupil of entrance," or, as the case may be, 

 " pupil of exit ") is positioned, in the case of the objective, in 

 close proximity to the posterior surface of the back lens of the 

 combination, and in the case of the eye-lens, at a little distance 

 superficial to the upper surface of the eye-lens. 



Method by which the Points of Origin and Arrived of the Rays 

 which arc transmitted from Aperture to Aperture can be located. — 

 It is manifestly impracticable to identify an isolated ray, or to 

 recognise the point of origin or arrival of such an isolated ray. 

 What is impracticable in the case of the isolated ray is, however, 

 eminently practicable in the case of any beam. We can readily, 

 in case of a beam, identify its point of origin and its point of focal 

 impact. By this means we can, in the case where a particular ray 

 passes through the radiant point (pole of origin) of a beam and 

 again passes through the focal point (terminal pole), identify its 

 position at two points of its course. 



Taking this principle as our guide, and bearing in mind that the 

 aperture of the objective is everywhere traversed by rays which 

 intersect with each other to form radiant points ; and bearing in 

 mind, further, that the rays diverging from these radiant points will 

 in each case re-intersect in the aperture of the eye-lens, consti- 

 tuting as they do so focal points ; we can manifestly re-identify in 

 the image of the objective-aperture, which is formed in the Eamsden 

 (Use of the eye-lens, the position of any ray which has emerged 

 from a radiant point in the aperture of the objective. 



The position of the rays which pass through the extreme margin 

 of the aperture of the objective can manifestly most readily be 



