THE MICROSCOPE AND ITS PARTS. 23 



by a fierce glare. Such a blaze is objectionable, not 

 only because it tends to obscure the finer parts of the 

 object, but it may lead the student or his friends to 

 condemn the microscope as injurious to the sight an 

 unjust accusation more than once made. If too much 

 light is undesirable, do not go to the opposite extreme 

 and strain the eye by forcing it to work in semi-dark- 

 ness. Keep the field sufficiently lighted to be pleasant 

 to the sight. Turn the diaphragm until the opening 

 giving the most agreeable effect and illuminating the 

 object enough to show the parts clearly is under the 

 centre of the stage opening. If the object is very thick 

 or opaque, more light will be needed than if it were per- 

 fectly transparent ; in such cases use a larger diaphragm 

 opening. 



The mirror is one of the most important parts of the 

 stand. It should have both a concave and a plane sur- 

 face, and it ought not to be less than two inches in di- 

 ameter, so that it may reflect enough light and be easily 

 handled. In the newest styles of stands the mirror is 

 arranged to swing from side to side, so as to throw an 

 oblique beam of light on the object, as well as to rise 

 above the stage, so that light may be reflected down 

 upon an opaque specimen, since it is used below the 

 stage for the illumination of transparent substances only. 

 This swinging arrangement is very convenient, and 

 should be had if possible. It is, however, not absolute- 

 ly necessary, as similar illumination of opaque bodies 

 can be obtained by the "bull's eye condensing lens," a 



