The Magnifying Power of the Microscope. By A. E. Wright. 285 



ledge of the refractive index of the medium which bathes the front 

 surface of the objective. 



The former measurement (a) may be obtained, where the back 

 lens of the objective is fully filled, and where it is directly accessible 

 to measurement by the application of a millimetre scale to the 

 objective. In the case where the back lens is sunk, or where it is 

 only partially filled by the transmitted beam, we obtain the measure- 

 ment required by measuring, by the procedure explained in the last 

 section, the Eamsden disc of the eye-lens, and multiplying by the 

 magnifying power of the ocular. 



The focal length of the objective is arrived at most simply by 

 measuring its magnifying power by means of the eikonometer, and 

 dividing this magnifying power into 250 mm. or 10 in. 



From these measurements and a knowledge of the refractive 

 index of the medium which bathes the front face of the objective, 

 we obtain the numerical aperture of the beam which enters the 

 objective in accordance with the formula 



» » _ semi-diameter of beam x refractive index of the immersion medium 



focal length of the objective 



Determination of the Numerical Aperture of the Beam, which 

 is furnished to the Eye by the Eye-lens of the Microscope. — The 

 numerical aperture of the beam, which is furnished to the eye by 

 the eye-lens of the Microscope, is obtained by dividing the semi- 

 diameter of the Eamsden disc of the eye-lens, measured as 

 explained in the last section, into 250 mm. or 10 in. 



The magnifying power of the Microscope is obtained from the 

 numerical apertures of the opening and closing beams in accordance 

 with the formula 



Mao-nifvins 1 Dower — numer * ca l aperture of beam which enters the objective 



numerical aperture of the beam furnished to the 

 eye by the eye-lens. 



Measurement of the Magnifying Power of the Microscope 

 by the Exploitation of a Fiduciary Phenomenon obtained 

 by the Aid of a Diffraction Grating. 



The method of measuring the magnifying power of the Micro- 

 scope, which I am about to suggest to you, is a direct outcome of 

 Mr. Gordon's critical study of the Abbe theory of microscopic 

 vision, which was laid before this Society some time ago. You 

 will remember that Mr. Gordon's paper dealt with the phenomena 

 of diffraction which come into view when lines and rulings are 

 viewed with the Microscope, or as the case may be, with the 

 unaided eye, through restricted apertures, and in particular through 

 slit apertures and diffraction gratings. Let me recall to your 

 memory the following: — 



Jane 15th, 1904 x 



