284 Transactions of the Society. 



identified for the purposes of measuring the interval which separates 

 them. 



Procedure. — Take a low-power lens, preferably one whose back 

 lens is flush with the back of the mount,* measure the diameter of 

 its back lens, and fit it to the Microscope tube. Focussing the 

 objective upon the focal plane of the condenser, and opening up to 

 its fullest extent the iris diaphragm in the substage, project a beam 

 of light upwards through the Microscope. Take in hand now a 

 pocket lens and a millimetre rule. Bringing the former up quite 

 close to the eye, and disposing the latter in the neighbourhood of 

 the bright Ramsden disc, seen on looking down from a distance 

 upon the upper surface of the ocular, bend down over the Micro- 

 scope, until an image of the illuminated back surface of the objective 

 comes clearly into view. Now readjust the position of the milli- 

 metre scale in such a manner as to bring it accurately into the focal 

 plane occupied by the image of the objective aperture. Eead off 

 the diameter of the image, and divide this measurement into the 

 measurement previously obtained by the direct application of the 

 rule to the back lens of the objective. The quotient represents the 

 magnifying power of the ocular. 



Measurement of the Magnifying Power of the Microscope 

 by the Determination of the Restriction undergone by 

 the Beam in passing through the Microscope. 



The magnifying power of an optical system can, as is well 

 known, be determined by measuring the total angle through which 

 the most obliquely incident ray is refracted in its passage through 

 the system. Put otherwise, the magnifying power corresponds to 

 the diminution in the numerical aperture of the beam which is 

 effected in its transmission through the system. A word or two 

 may be in place in connection with the application of this system 

 of procedure to the Microscope. 



Determination of the Numerical Aperture of the Beam which 

 enters the Objective. — The determination of the numerical aperture 

 of the beam which enters the objective involves (a) the measure- 

 ment of the linear diameter of the aperture of the objective, or, in 

 the case where the objective is not fully filled, of the linear diameter 

 of the illuminated area of the back surface of the objective, (b) the 

 measurement of the focal length of the objective, and (c) a know- 



* Where a lens of this kind is not available, we may, in conformity with a sug- 

 gestion made to me by Mr. Gordon, drop upon the back lens of the objective a square or 

 a triangle of paper, whose sides measure, say, exactly 1 mm. By the procedure de- 

 scribed above, we now apply our measurements to the image of this triangle or 

 square. An even simpler method of procedure is to dispense with the objective, and 

 to take as our object the vacant lower aperture of the barrel of the Microscope tube, 

 and to apply our measurements to the image of this aperture. 



