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ia the most efficient, viz., of hanging the microscope in a horseshoe, supported 

 by three legs ; but for this class of instrument that mode of mounting was 

 quite out of the question, for cost immediately put it outside the category of 

 students' microscopes. There is a great difference between the steadiness of 

 a microscope perched up on the top of its trunnions and one that is hung in 

 a tripod. This microscope is placed in a kind of stirrup hanging from the 

 trunnions, a most ingenious device of Mr. Curties. The body is large 

 enough to take Zeiss' full-sized eyepiece, viz., If in., and is lOin. long when 

 the draw-tube is pulled out to a mark. When the draw-tube is pushed home 

 the length is 6*3in., or Continental gauge. It, therefore, will suit both kinds 

 of apochromatic object glasses. The optic axis of the instrument, when in 

 a horizontal position, is 8|in. from the table. It has rackwork coarse adjust- 

 ment, and Campbell's fine adjustment. It is to this fine adjustment that 

 the instrument owes its origin. The moment Mr. Campbell explained to me 

 the principle of his fine adjustment, I foresaw the construction of an 

 efficient students' microscope. The direct-acting screw is only suitable for 

 low powers and small apertures. I will put it even stronger : delicate 

 work with high powers and wide apertui-es is not possible with any micro- 

 scope having a direct-acting screw fine adjustment. The stage is of the 

 cut horseshoe form, which I had the honour of bringing before you some 

 little time back. The principal object of this is to enable you to feel your 

 working distance. A great improvement has been made in the sliding 

 bar, its guiding lugs being stowed away underneath the stage ; I have no 

 hesitation in saying that next to a perfect mechanical stage this is the best. 

 Most of the mechanical stages are so defective in design, and so scamped 

 in their workmanship, as to be w orse than useless. The substage is fitted 

 with a tube, having a spiral slot for focussing, which I will pass over, 

 as I have described it here on a former occasion. There is a novel feature 

 about the stops for dark ground illumination, viz., there is a three-legged 

 carrier which Lolds them all. This cariier has a pin in the centre of it on 

 which the various sized discs fit. The stops, diaphragms, &c, have a 

 separate 'tube-fitting for them, so that it is unnecessary to move your con- 

 denser when changing either a stop or a diaphragm. This substage will 

 carry either of Prof. AbbtS's condensers, or a cheap condenser made 

 especially for this microscope. The weight of the instrument complete is 

 7lbs. 



Mr. John Mayall, jun., said that knowing well Mr. Nelson's thorough 

 competence to give a reliable opinion upon any matter connected with the 

 mechanical construction of the microscope, there could not be the slightest 

 doubt that the one he had just described had some very excellent points. 

 Indeed, he thought they might take it for granted that if the instrument 

 before them had successfully passed Mr. Nelson's criticism, it must be 

 pretty nearly all right. It occurred to him, however, that the spiral 

 arrangement employed for focussing the condenser was, perhaps, not likely 

 to prove quite so true as could be desired, and he should like to ask if Mr. 

 Nelson had found that it introduced any very great error in centring. 



