ACCESSORY INSTRUMENTS. 



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are subservient to the iUumination of objects, and, secondly, 

 into those for the purpose of keeping objects in, whilst they 

 are^ being examined, or for preparing them for exami- 

 nation. Amongst the former may be mentioned all the 

 various kinds of diaphragms, condensers, illuminators, po- 

 larizing apparatus, dark wells, &c. ; and amongst the latter, 

 the live boxes, animalcule cages, fishing tubes, &c., all of 

 which require special notice. 



The Diaphraffm. — A very useful piece of apparatus applied 

 to the under surface of the stage in most microscopes is the 

 diaphragm, represented by fig. 56 ; it consists of two or more 

 plates of brass, one of which is perfo- 

 rated with four or five holes of different 

 sizes, this plate is of a circular figure, 

 and is made to revolve upon another 

 plate by a central pin or axis ; this last 

 plate is also provided with a hole as 

 large as the largest in the diaphragm- 

 plate, and corresponds in situation to 

 -. ; ^^ the axis of the compound body. To 



f I «Ly wK ascertain when either of the holes in 



i I ^ py the diaphragm-plate is in the centre, a 



' bent spring is fitted into the second 



plate, and rubs against the edge of the 

 diaphragm-plate, which is provided 

 with notches, so that when either of 

 the holes is brought into its proper 

 position, the end of the spring drops 

 into the notch. The space between 

 the largest and smallest hole is greater 

 than that between any other two ; this answers the purpose of 

 stopping off all the light if necessary. The diaphragm is 

 attached to the under surface of the stage, either by a sliding- 

 plate, as seen in the figure, or by a short piece of tube 

 fitting into the hole of the stage, and securely fixed in the 

 proper position by a bayonet-joint. The former method is 

 adopted by Mr. Eoss and Mr. Smith, and the latter by Mr. 

 Powell ; every part of this instrument through which the 



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Fig. 56. 



