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prism for polariscope purposes. In some the prism is fixed in the 

 nose-piece, whereas it ought to be capable of rotation. Lastly we 

 have a revolving nose-piece for the purpose of testing objectives. 

 Mr. Nelson, in a paper read before the Quekett Microscopical Club, 

 February 1885, stated that he had observed that certain objectives 

 performed better when the object was placed in a definite azimuth. 

 With a view to eliminate any possible alteration which might arise 

 from the revolution of the object with regard to the light, he had 

 designed a revolving nose-piece which enabled the objective itself to 

 be revolved true to the optic axis when any imperfection in its 

 performance in a particular azimuth could be immediately noted. 

 This plan had, however, been previously in use by Professor Abbe 

 for a similar purpose, but not, as w r e believe, made public. 



Finders. A finder is a very important and valuable addition to 

 a microscope. By its means the, position of any particular object or 

 part of an object in a mount can be noted, so that it may be found 

 again on any subsequent occasion. In working on a microscope 

 without a finder it frequently happens that in the prosecution 

 of special research, or in the examination of unknown objects, 

 something is seen w r hich it would be of the utmost value to recur to 

 again ; but the amount of time lost in transferring the object to a 

 stand with a finder is so great that most experienced microscopists 

 do all their search and general w r ork on their best instruments with 

 finders. 



The usefulness of the finder has caused a large number to be de- 

 vised ; but, as in all cases, we consider only those which we believe 

 embody the best practical principles. 



The first, and by far the best, is the graduation of the stage 

 plates of a mechanical stage by dividing an inch into 100 parts, both 

 on the vertical and horizontal plates. The vertical stage-plate will 

 then indicate the latitude, and the horizontal plate the longitude of 

 the object, the slip being always pressed close home against a prepared 

 stop. For many years Messrs. Powell and Lealand have supplied 

 their No. 1 stand with this kind of finder ; and its permanent position 

 and ease in use not only give greater facility in special researches, 

 but in reality attach a new value to every slide in the cabinet. Such 

 a worker at critical images as Mr. Nelson has weeks of close work 

 ' logged ' on the labels of his slides. A still better plan is to ' log ' 

 in books in which the slides are numbered. The result is that the 

 labour of days and weeks can be in a moment recalled for demon- 

 stration ; and so accurate is this method that an object so small as 

 a Bacterium termo or a specified minute diatom in a thickly scattered 

 mounting may be at once, and as often as we please, replaced in the 

 field with even high powers. 



These finders of course are only suitable for the microscope on 

 which the 'log' was taken. It is beneficial, and even needful at 

 times, to interchange specimens or refer an object to an expert at a 

 distance. In that case a minute dot may be placed on the cover, or 

 a single selected diatom or other object may be fixed upon and its 

 latitude and longitude as read on the microscope of the sender marked 

 on the slide. If the receiver then places this on his microscope and 



