370 SUMMAEY OF CURRENT RESEARCHES RELATING TO 



and, with the help of a Nelson or other collecting lens and a substage 

 condenser of a power suited to the objective in use, so arranged as to 

 just fill the back combination of the objective with a uniform solid cone 

 of light, at full aperture, and so as to give critical illumination and a 

 uniformly illuminated field on the ground glass of the camera. To 

 obtain these results it might be necessary, with objectives below h in. 

 (12 mm.) to employ a supplementary collecting lens, in the course of the 

 beam from the Nelson lens. The factors to be taken into consideration 

 in estimating the exposure, were : — 1. The magnification, which increases 

 the exposure in the direct ratio to the square. 2. The light : Exposure 

 tables were shown, based on a standard obtained with the edge of the 

 flame from a f in. wick kerosene lamp as a source of illumination. A 

 single 1 ampere filament Nernst lamp, working from 100 volt current, 

 required T \ of those exposures. 3. The plate : The photographic plate 

 on which the tables were based was the Ilford chromatic— a colour- 

 sensitive plate of medium rapidity. A standard was found by making 

 a series of strip exposures on such a plate, at a given magnification and 

 aperture (250 diameters, and 0*50 N.A.), from which a table of 

 exposures at that aperture could be calculated for all other magnifica- 

 tions. 4. Numerical aperture: It was a very common custom in 

 recordino- the conditions under which a photograph had been taken, to 

 say that a lens of 1-20 or 1-30 N.A., i.e. its full numerical aperture, 

 had been employed. What was, however, really required was the 

 aperture at which the lens had been worked — the working aperture — 

 which affects the exposure in the inverse ratio to its square. A second 

 table was therefore shown, giving the factors by which the exposures 

 found from the first table should be multiplied or divided, so as to give 

 the exposure required at varying working apertures. To use this second 

 table it was necessary to be able to estimate the working aperture which 

 had been found to give the result desired. If we take a finely divided 

 glass measure, giving say millimetres and tenths of a millimetre, and 

 hold it exactly in the position of the Ramsden disk of the eye-piece, and 

 focus both it and the Ramsden disk simultaneously with a focusing 

 glass, so adjusted that it is in focus when its mount rests on the surface 

 of the measure, it is possible to estimate the diameter of the Ramsden 

 disk of the ocular. Multiplying that diameter by the number of the 

 ocular (giving the number of times it magnifies the result obtainable by 

 the objective alone) we obtain the diameter of the cone of light emergent 

 from the back combination of the objective. If we now divide the 

 semi-diameter of that cone by the equivalent focal length of the objec- 

 tive, in millimetres (usually marked on objectives, now-a-days), we 

 obtain as a result the " working aperture " of the lens. 



For instance, if a 4 mm. apochromat be used, the full N.A. of which 

 is 0-95, and a No. 4 ocular, the diameter of the Ramsden disk of the 

 eye-piece may have been found to be 1 mm. This multiplied by the 

 power of the ocular (4), gives us 4 mm. as the diameter of the emergent 

 beam from the back of the objective ; the half of that (2 mm.) is the 

 semi-diameter of that beam, which divided by 4, the equivalent focal 

 length of the objective, gives us ■ 50 as the working aperture of the lens. 

 We have therefore all the information required to make use of the 

 following table : — 



