THE DISSECTING MICROSCOPE. 17 



A polarization apparatus is occasionally employed 

 in investigating the optical properties of the sub- 

 stances which compose the tissues. (It is also of use 

 in helping to determine the nature of crystalline 

 deposits in urine and other fluids.) In connection 

 with the employment of this the stage-plate and 

 tuhe of the microscope are, in the best instruments, 

 made capable of rotating on a vertical axis. In 

 smaller instruments this movement is, as a rule, 

 not provided for, and indeed, although convenient, 

 is by no means essential. 



The mechanical stage movement, which is so often 

 fitted to microscopes of English construction, and 

 other appliances which tend to mar the perfect sim- 

 plicity of the instrument when in ordinary working 

 trim, serve rather to detract from its usefulness for 

 purposes of histology. 



Attention to the foregoing points, of which per- 

 haps perfect steadiness and rigidity of the stand and 

 stage is the most important to insist upon, will aid 

 the student in selecting a useful microscope so far as 

 the body of the instrument is concerned. The excel- 

 lence of the objectives can only be competently j udged 

 of by one who is already somewhat conversant with 

 the use of the microscope. Preparations of some of 

 the tissues or fluids of the body (connective tissue, 

 blood, salivary corpuscles) form the best test objects 

 for the high powers of an instrument that is to be 

 employed in histological studies. 



Jn addition to the ordinary instrument (which is 

 generally distinguished as the " compound" micro- 

 scope), it will be found very convenient to have a 

 smaller microscope of some sort ; so as to be better 

 able to follow the needles or other instruments when 

 the operator is engaged in the separation and manip- 

 ulation of minute objects. An instrument w r hich is 

 used for this purpose is termed a dissecting or pre- 

 paring microscope. Any simple lens which is mount- 

 ed on a stand will serve (Fig. 2), and even the hull's- 



eve condenser, which is generally furnished with the 



c o* 



