GENEKAL PKELIMINAEY PKEPAKATIONS 187 



be so adjusted that when placed on the plain focussing-screen 

 the diamond lines on the far side of the screen are exactly in 

 focus. This is an adjustment that must be made for each 

 individual user ; but, having once been carried out, it remains 

 always in the correct position ready for use. The adjustment 

 is effected by placing the eye-piece on the focussing-screen until 

 the image of the diamond line or of the ground -glass surface is 

 sharply focussed; this is done by screwing the cell carrying 

 the optical portion in or out by means of the milled head shown. 

 When this operation has been carried out, any alteration of the 

 position of the lens system is prevented by firmly screwing 

 down a clamping-ring, which prevents any alteration during use. 



In general, these focussing eye-pieces are made of too high 

 power for photo-mi crographic work, as the image at least 

 in high-power work is not one that will stand any great 

 amplification, especially when the focussing-glass is not well 

 achromatised. The writer has had made by Messrs. J. Swift & 

 Son a focusser which he has found answer better than any of 

 those on the market. It is a well-achromatised system, but of 

 very low power, so that the image is magnified less than two 

 diameters. In effect it only takes up the image at the focal 

 plane, and transmits it to the eye with but slight alteration. 

 The only objection to it is that the field is small, but in the 

 large majority of cases this is of no moment. 



In actual practice such a focussing system is fitted into a 

 long mount, and owing to this it can only be used at or near 

 the centre of the focussing-screen. If it is away from the 

 centre to any considerable extent, the image is not transmitted, 

 but is cut off by the sides of the mount. The great advan- 

 tage of having such a low-power eye-piece for focussing 

 is that the image is transmitted practically unaltered, and 

 it is much easier therefore to focus images formed by high 

 powers. 



Some workers have advised the use of ordinary spectacle 

 lenses for focussing, and this would appear to be quite practi- 

 cable where a ground-glass screen only is used. Better still is it 

 to use a pair of spectacle lenses mounted to form a binocular 

 focussing-glass. This type is made in a convenient form by 

 Messrs. Newman & Guardia, and is supplied by them for use 

 in their reflex cameras. The binocular, as supplied, should be 



