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useful for the following kinds of illumination : — 1. Oblique with double slot. 

 2. Dark ground with central stop. 3. Transmitted light with ground glass 

 for low powers. 4. Lieberkuhns. 5. Polarised light with low powers. He 

 did not recommend the bull's eye for critical work with transmitted light, 

 snch as testing object glasses, nor with oblique light with single slot ; the 

 simple edge of the flame without any bull's eye being far better. In dark 

 ground illumination, the lower the power of the objective the larger would 

 be the field that would have to be illuminated, consequently the larger the 

 disc ought to be. The size of the disc can be regulated by approaching or 

 removing the lamp, with the bull's eye attached, to or from the condenser. 

 The bull's eye ought always to be fixed to the lamp itself, and not be 

 on a separate stand, or even move independently on the same stand. 

 Diaphragms, he said, were elements of the greatest importance to the 

 critical microscopist. The ordinary wheel of diaphragms, if carefully made 

 so that the apertures were all centred, was very convenient. The apertures 

 could be rapidly changed, and the effects could be exactly reproduced by 

 using the same aperture. Another plan was to have a disc holder fastened 

 below the condenser, and made so that it could be turned on one side to 

 allow the discs to be changed. This plan permits of easy centring, also the 

 exact reproduction of the effects; its disadvantage is that it is slow. An 

 iris diaphragm is the most rapid, as well as true in centring, but one can 

 never be sure of reproducing the same effects. Therefore, while it is the 

 best for ordinary work, it can hardly be considered suitable for testing 

 object glasses, where the exact reproduction of the illuminating conditions 

 is absolutely essential. He then explained, by means of diagrams, the 

 various methods of using stops, slots, etc., and the consequent results in 

 the images at the backs of the objectives. The lieberkuhn, he thought, was 

 the best illumination for opaque objects, with low powers- Excellent 

 results could be got by it with a i of 100°. It was unfortunate that many 

 opaque objects were mounted on slides either covered up with paper, or on 

 opalescent glass slips, so that the lieberkuhn could not be used. Side and 

 parabolic reflectors ought always to be fitted to a part of the stand, and 

 not, as is usually the case, either to the objective, or nose piece, or movable 

 part of the stage. The reason for this is that when the light has been once 

 arranged it is not disturbed by moving the object, or changing the object 

 glass. He next showed how very delicate measurements could be made by 

 having the screw micrometer mounted on a separate stand. He recom- 

 mended the fixed wire to be placed five revolutions of the screw to one side, 

 instead of in the centre, of the field, where it is always found. The wire, 

 he said, could not be too fine. The separate stand could be of the ordinary 

 bull's-eye condenser form, but the circular foot ought to rest on three points 

 only. With such an instrument he had passed the movable wire over, and 

 counted 100 striae on an A. Pellucida, which had 9G striae in the T o^u of an 

 inch. As to drawing, he fully endorsed what Mr. Hardy had said at a pre- 

 vious demonstration, with regard to Beale's neutral tint. With this simple 

 appliance he had outlined delicate objects, under a magnification of 2,500 

 diameters, without difficulty. He regretted that time did not permit him 



