OF THE MICROSCOPE. 81 



with perfect accuracy have been applied both to the mechanical 

 stage and the glass stage, though the latter is very frequently 

 so constructed that rotation is an impossibility. Mr. Brown- 

 ing's device for securing rotation in the optic axis has been 

 already described. Another ingenious device for securing 

 coincidence of the optic axis and the centre of rotation is the 

 centering nose-piece, by means of which the objective is moved 

 so as to bring it exactly over the centre of the stage. 



Stages for Special Purposes. It may be safely asserted 

 that there has never yet been constructed a stage which would 

 suit the requirements of every worker with the microscope. 

 Indeed, each investigator seems to require special modifications 

 of his own. Thus, it will be found that the ordinary stage, 

 with all its appurtenances, is too thick to admit the use of that 

 very oblique illumination which, is required by the worker on 

 diatoms,while if the stage be made thin enough it* loses the 

 necessary rigidity. Some makers have sought to obviate this 

 by supplying two stages a stout one for common work, and a 

 thin one for diatoms. A microscope now in our possession is 

 furnished with an extra thin stage, which, by a very simple 

 and ingenious device, can be instantly substituted for the heavy 

 one. The microscope is said to have been made by Spencer or 

 Tolles, and must be over twenty years old. A thin stage on the 

 same principle, and called by' the maker a Diatom Stage, has 

 been recently brought out by Mr. Zentmayer. 



The same object is also attained by means of the secondary 

 stage, invented by Mr. Lewis Rutherfurd. This is simply a 

 skeleton stage, which is placed on the ordinary stage, and is 

 raised so far above it that the illumination may be applied 

 between them. Rays of great obliquity may thus be passed 

 through the object. Rutherfurd's skeleton stage forms also an 

 admirable safety stage, since the object, being held against the 

 under side of the skeleton stage, yields to the slightest pressure 

 of the objective. Mr. Spencer has also taken advantage of this 

 principle, and so formed the under side of the stage in some of 

 his stands, that the object may be pressed against it by the 

 clips, which for this purpose are pushed through from below 

 upwards. In focussing, the objective is passed through the 



