33^ Trajisactions of the [Sess. 



as Parkes's slipping-tube or the "turntable" adapter, being too 

 clumsy, and hardly worth a passing consideraton. 



Changing the power in the usual way, by unscrewing the objec- 

 tive and screwing in another, has always seemed to me both a 

 tedious and needless way of working ; and I have often revolved 

 in my mind other means of arriving at the same result. The out- 

 come of these cogitations is the small piece of apparatus now ex- 

 hibited. It consists essentially of a small face-plate or " chuck," 

 which screws into the ordinary " nose " of the microscope. On its 

 face this has a slide which has fitted into it another sliding-piece, 

 and into which the objective is screwed. As many of the other ob- 

 jectives as belong to the instrument are fitted with similar sliding- 

 pieces, which also fit into the first. Once, therefore, an objective 

 is fitted and centred with one of these sliding-pieces, having a suf- 

 ficient length of tube to bring it very nearly into focus, it can be 

 substituted in a moment for one of lower or higher power, as the 

 case may be ; and if an object has been previously centred on the 

 stage with a low power, it will be found accurately centred in the 

 field of that of the higher. I also wish to draw your attention to 

 the fact that all the face parts of this appliance are finished on the 

 lathe, which enables the optical axis of the eyepiece, instrument, 

 and objective to be truly maintained, and does away with the fail- 

 ings of the ordinary double nosepiece in this respect. Another 

 form of this adapter is to have two, three, or more objectives 

 mounted together on one of the sliding-pieces, having on each 

 objective a sufficient length of tube to bring it accurately into focus, 

 and sliding one objective on another, as may be wished, central 

 with the tube of the instrument — a small spring-point retaining it 

 in that position. It is a matter of choice, however, as to which is 

 the better form — whether it will be more convenient to have two 

 or three objectives mounted together, or to have them separate. 



Having thus described the appliance, I think I may fairly claim 

 for it that it will change the objective of a microscope with great 

 rapidity, with very accurate centering, and very close approximate 

 focusing. Having made these claims for it, I commend the ap- 

 paratus to the attention of all workers with the microscope, 

 whose time is generally too valuable to waste on matters such 

 as this.-^ 



Throughout the session various meetings were held in the Club- 

 room, No. 35, at 20 George Street, for practical work. One series 

 embraced the study of the Optical Principles of the Microscope ; 



1 [Since the above was exhibited to the Microscopic Section of the Field 

 Club, the Royal Scottish Society of Arts has awarded a Silver Medal to Mr 

 J. M. Turnbull for his invention.] 



