MICROSCOPICAL DIAGNOSIS. 15 



the sky is clear, and when, best of all, the sunlight is reflected from 

 a white cloud. While artificial light is generally inferior to day- 

 light, and weakening to the eyes, direct sunlight is positively in- 

 jurious. Still, good lamplight is to be preferred to poor daylight. 



When the light passes directly through the specimen and micro- 

 scope, not reflected by the mirror, it is said to be direct" If the 

 mirror is placed so that the reflected rays are in the optical axis of 

 the microscope, the light is said to be axial or central. If the mir- 

 ror be turned to one side so that the rays pass through the object 

 at an acute angle, oblique light is obtained. In the illumination of 

 opaque objects, when ordinary daylight or lamplight is allowed to 

 fall on the object without any means of concentration, we have dif- 

 fused light. When it is desirable to have stronger illumination, 

 some means of concentration must be employed. For this purpose 

 a bull's-eye condenser is used. This consists of a plano-convex 

 lens, either mounted on a stand or attached to the stage of the 

 microscope. It should be placed at right angles to the direction of 

 the illuminating rays, with its plane side toward the object. 



In the care of the microscope the following practical hints may 

 not be out of place: 



When removing from or placing on the stage a specimen, if the 

 higher powers are in use, always raise the body of the instrument. 



It is rarely necessary to clean a good microscope. Always use 

 soft chamois, well beaten, or silk to clean the instrument with, and 

 camel's hair brushes to remove dust. 



To remove balsam, etc., from objectives, moisten the chamois 

 slightly in alcohol or benzole and carefully wipe it off, remembering 

 that any excess of the solvent may work around the lower glass and 

 dissolve the balsam that unites the glasses of the lower system. 

 Handle the instrument as little as possible, always carrying it by the 

 arm. 



Clean immersion objectives thoroughly and immediately after 

 using. 



When the instrument is not in use it should be placed in its 

 case or under a bell-jar. 



A microscope will suffer more injury at the hands of a careless 

 and dirty person in twenty minutes than it need sustain by proper 

 care in twenty years. 



