MICROSCOPES. 



337 



ACCESSORY APPARATUS. 



In addition to those parts of the instrument which are more or 

 less essential for examining nearly all classes of objects, various 

 forms of accessory apparatus are often very useful, 'or even neces- 

 sary, for particular purposes. Amongst these, attention may be 

 drawn to micrometers for the stage, or to be inserted into one of 

 the eye-pieces, consisting of plates of glass on which lines are 

 ruled with a diamond at known small intervals. These micro- 

 meters are used to determine the true size of objects seen under 

 the microscope, which in some cases it is very important to 

 know. Live boxes are used for examining animalcules in water, 

 and compressoria for applying more or less pressure to soft 

 tissues. For holding insects and many other objects, various 

 kinds of forceps are used, made so that they can be turned round 

 and moved into different positions. Maltwood's photographed 

 and other finders are very useful in enabling us to refer at pleasure 

 to any part of an object previously examined. A camera lucida 

 placed over the eye-piece is employed in making drawings. 



POLARISING APPARATUS, &c. 



Minerals and rocks, and even some organic bodies, cannot be 

 studied in a satisfactory manner without the means of examining 

 their action on polarised light. For this purpose a polariser must 

 be fixed below them, and an analyzer above them, over the eye- 

 piece ; or, if binocular effects are required, just over the object- 

 glass. Plates of selenite are used to raise or lower the tints of 

 the light caused by the action of the object under examination, 

 and to ascertain whether it has positive or negative double refrac- 

 tion. A double image prism over the eye-piece, and a plate with 

 a round hole inserted into the eye-piece, are very useful in 

 studying the dichroism of minerals. A double image prism 

 goniometer may be employed with advantage to measure the 

 angles of minute crystals. 



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