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March 14th, 1884. — Conversational Meettng. 



The fourth demonstration of the second series was given by Mr. E. M. 

 Nelson, the subject being " How to work with the microscope." 



He began his subject by a short description of the instrument. He 

 advocated the simple tripod as being the lightest and at the same time 

 the most steady form of stand. He noticed the tendency there was to 

 shorten the spread given to the legs, which made the instrument more 

 portable, but at the expense of steadiness. He said that microscopes might 

 be divided broadly into two classes, viz., the bar-movement and the Jackson- 

 Lister. In point of steadiness he did not think there was much to choose 

 between them in first-class stands. As the bar-movement was the more diffi- 

 cult and expensive to make properly, in many instances the work would be 

 found to have been scamped, and shaky instruments would be the result. 

 When carefully made the bar-movement was as steady as the Jackson- 

 Lister, and had the decided advantage of giving plenty of room on the stage. 

 Unless a Jackson-Lister were made enormously large and heavy the com- 

 plete rotation of an ordinary mechanical stage was impossible. The thin 

 stages with pinions on the top, which have been made, both here and in 

 America, to obviate this difficulty, have introduced fresh elements of un- 

 steadiness, errors far worse than the one they were intended to correct. 



Until quite lately it may be said that there was no Jackson-Lister which 

 had a satisfactory fine adjustment. Every one will know the common 

 model, with the abominable little lever at the side of the nose piece. When 

 delicate focussing was required one was always obliged to give up the fine 

 in preference to the coarse adjustment. That this objection was really felt 

 is evident from the number of appliances that have been invented to get 

 over the difficulty. He only alluded to one, which was probably the most 

 common, viz., the plan of cutting the solid Jackson-Lister arm, and putting 

 the body, coarse adjustment, rackwork, pinion, etc., on the fine adjust- 

 ment lever. It is only necessary to say that when the solid arm is cut the 

 special point of the Jackson-Lister model is lost, and the instrument at 

 once placed far below the bar-movement in efficiency. Messrs. Swift and 

 Son were the first to rescue the Jackson-Lister model from its unfortu- 

 nate position, by the invention of the vertical lever movement. This was 

 a great stride in the right direction, not so much on account of the 

 vertical lever itself, but because it did away with the necessity for an 

 accurate fitting to the movable nsse piece. This fitting is left loose, 

 the movement being steadied by a bar in guides, which can be adjusted 

 at pleasure by two screws, without taking anything to pieces. This fine- 

 adjustment has the slowest motion, is the steadiest, and at the same time 

 is the strongest that has been yet made. With regard to Continental 

 models they may be considered as Jackson-Listers, with cut arms placed on 

 direct acting screw fine-adjustments. They are not equal in efficiency to 

 those which have the cut arms, and whose fine-adjustments have been 

 geared down by the interposition of a lever. It may be accepted as an 

 axiom that no direct acting screw fine -adjustment is slow enough for the 



