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wide angled oil immersion glasses of the present day. In his experience he 

 thought the bar-movement as made by Powell and Lealand the best, and 

 found it capable of doing everything that any other instrument would do. 

 Passing on he mentioned that an instrument could only be called a 

 microscope which had a mechanical centring and focussing snbstage con- 

 denser ; those without this apparatus might be termed magnifying glasses. 

 With regard to object-glasses he considered that four only were necessary to 

 form a complete battery, viz., 1^ inch N.A. '17, §, N.A. '39, ±, N.A. '82, and 

 a T ^-N.A. 1*43. A beginner might get a 1| and a |, or a medical student a 

 f and i. In either case the series can be made complete by the addition of 

 other glasses. Much money is wasted in buying glasses which, as the stu- 

 dent advances in knowledge and experience, have to be sold at a loss to make 

 room for others to perfect the battery. Anyone going in for fancy work might 

 add to the above series a 3-inch N.A, '08a }, N.A. *77, and a ^ N.A. 1*38. 

 In respect of the proper aperture that ought to be given to objectives, he 

 referred to a paper he had already communicated to the Club, in which he 

 had stated, that a theoretically perfect lens ought to have sufficient aperture 

 given it to enable it to resolve anything that was capable of being ap- 

 preciated by the eye, with a magnification of ten times the initial magnify- 

 ing power of the lens, the visual angle for definition being taken at l'*23" 

 (250 lines to the inch at 10 inches). Example : — The eye, assisted by a mag- 

 nification of 400 diamaters (= the initial magnification of a \ X 10), could 

 perceive with the above visual angle 100,000 lines to the inch at 10 inches 

 (i.e., 250 X 400), provided the ^ had a numerical aperture of 1*04, so as to 

 be capable of resolving them. Therefore a^, to be a theoretically perfect 

 lens, ought to have a numerical aperture of 1'04, otherwise all that is capable 

 of being seen will not be resolved ; in other words, all the details of any object 

 will not have the greatest amount of perceptible sharpness. As to eye-pieces, 

 he feared that the Huygenian formula was not strictly adhered to in many 

 instances. The curve of the field glass was often too shallow. He did not 

 approve of the plan of stopping off a large portion of the field by a 

 diaphragm. A 2 inch, 1 inch, and ± inch would form a complete battery 

 of eye-pieces, the first two being for work, and the last for adjusting 

 the object glass and testing purposes. He would not recommend the \\ 

 inch except where only one eye-piece was allowed, and he deprecated the 

 use of Kellner's altogether. 



With respect to condensers he said that it was not sufficiently realised 

 that the power and aperture of the condenser ought to bear a certain rela- 

 tion to the power and aperture of the objective. He considered the 

 best condenser for use with a ^ and upwards was the achromatic by Powell 

 and Lealand. It had a numerical aperture of 1*0, was perfectly achromatic, 

 and had a very long focus, in fact, no critical work with high powers could 

 be done without it. For lower poAvers that of Swift and Son was excellent. 

 The top lens was removable, so that its range could be expanded from a 3 

 inch to a ^ or dry J-. This condenser gave with low powers the best dark 

 field known, and when used with the ground glass gave splendid effects 

 with transmitted light. The equipment of a microscope was not complete 



