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into account as in ordinary photograph}-, and these were then 

 considered. First, Magnification. — This factor may he dismissed 

 with the remark that it increases the exposure in the direct ratio 

 to the square of the magnification. Second, Light ; and third, 

 the Plate. — With these two factors we have in photomicrography 

 the advantage that one usually keeps to the same source of light, 

 and also usually the same plate. A series of exposures is made 

 on a selected make of plate, with a certain light, and with the 

 condensing apparatus (consisting of a Xelson lens and achromatic 

 substage condenser) arranged so as to give critical illumination. 

 A lantern-slide of a diatom was then thrown on the screen. The 

 negative, which was taken at x 250 and with an aperture of 

 0*50 N.A., edge of kerosene-oil name, had received a series of 

 strip exposures ranging from two to twelve seconds. Of these 

 Dr. Reid considered that either six or eight seconds gave the 

 best result, and took eight seconds as standard. There is now 

 available material for making up a table of exposures for any 

 magnification at 0'50 N.A. But apertures vary just as do 

 magnifications, and as it is well known that exposures vary in 

 inverse ratio to the square of the aperture, it is quite easy 

 to prepare a second table of multiplying or dividing factors for 

 all possible apertures, by which one can multiply or divide the 

 exposure obtained by consulting the first table for a certain 

 magnification at 0'50 N.A. To make use of this second table 

 it is necessary to have some means of knowing what aperture 

 is being used, and the method must permit of the N.A. being 

 measured immediately before making the exposure. The method 

 employed and described by the lecturer is that due to Sir A. E. 

 Wright, who gives it in detail in his Principles of the Micro- 

 scope, and consists in the measurement of the Ramsden disc 

 of the ocular. To apply the method it is required to know the 

 equivalent focal length (say in millimetres) of the objective and 

 the semi-diameter of the beam of light as it emerges from the 

 back combination of the objective. The first factor is usually 

 known, and the second factor can be obtained as follows from 

 measurement of the Ramsden disc. A diagram of the passage 

 of light through the microscope from the condenser to the eye 

 was then shown. The rays, after emerging from the eyepiece, 

 converge to a point a little in front of the eye-lens, and there 

 form a circle of light — the " Ramsden disc " of the ocular. The 



