4S8 



diameter of the circle of light varies with the aperture of the 

 objective. The greater the magnifying power of the ocular, the 

 smaller the disc. If, therefore, we hold a finely graduated 

 measure — divided, say, into millimetres and tenths — exactly in 

 the plane of the Ramsden disc, and focus both it and the disc 

 simultaneously with a pocket-lens, we can read off the diameter 

 of the disc in, say, tenths of a millimetre. Multiplying half 

 this diameter by the magnifying power of the ocular — which is 

 usually marked on the mount — and dividing by the focal length 

 of the objective, gives us the required N.A. of the objective in 

 use. Dr. Reid then gave an example. Assume that a 4-mm. 

 objective, with an ocular magnifying four diameters, is being 

 employed, and that the diameter of the Ramsden disc of the 

 ocular, obtained by measurement, is 1 mm. This multiplied by 

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

 the objective beam ; half of this (2 mm.) is the semi-diameter 

 of the beam, which, divided by 4 (the focal length of the 

 objective), gives 0'50 as the N.A. in use at the time. 



Tables for exposure, calculated for various magnifications from 

 xlOO to x 4,000, and with N.A. 0'10 to 1*30, were then pro- 

 jected on the screen. The lecturer had found that a single-fila- 

 ment Nernst lamp working at 100 v. was sixteen times more 

 powerful than a |-in.-wick oil-lamp. With variously coloured, or 

 stained, or thick objects, the exposures indicated would, of course,, 

 have to be suitably modified. If detail is required in a coloured 

 object, it is necessary to use a screen of about the same colour as 

 the object. All screens, whether green, yellow, or other colour, 

 must be standardised by test exposures with and without the 

 screen, and with the illuminant to be used, in order to know by 

 what factor the originally obtained exposure should be multiplied. 

 It was found that with the oil-lamp a rather dark yellow screen 

 (Wratten & Wainwright's K3) required three and a half times 

 the normal, and a rather dark green screen (Wratten & Wain- 

 wright's B) required six times the normal. In concluding, Dr. 

 Reid said that those who wished to studv the tables he had 

 projected on to the screen would find them reproduced at length 

 in The Photographic Journal ("Transactions" of the Royal 

 Photographic Society) for January, 1909. 



Mr. W. B. Stokes (Hon. Secretary) deprecated the use without 

 verification of the power marked on eyepieces, and suggested that 



