22 AQUATIC MICROSCOPY FOR BEGINNERS. 



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-darkness. 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 center of the stage open- 

 ing. If the object is very thick or opaque, more light 

 will be needed than if it were perfectly 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 surface, and ought not to be less than two 

 inches in diameter, 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 effect oblique illumination of 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 sub- 

 stances only. This swinging arrangement is con- 

 venient, and should be had if possible. It is, how- 

 ever, not absolutely necessary, as similar illumination 

 of opaque bodies can be obtained by the "bull's-eye 

 condensing lens," a rather expensive piece of appara- 

 tus, and somewhat difficult to manipulate successfully. 

 But as the newest and best stands have the swinging 

 mirror, the condensing lens need not be described, 

 especially since the beginner will not care to examine 

 many opaque objects that will demand stronger 

 illumination than that of ordinary diffused daylight or 

 common lamplight. 



