116 SKLECTION AND USE 



with pure silver, instead of with the amalgam of tin and 

 mercury, ordinarily used. The mirror should be so hung that 

 it may throw a beam of great obliquity through the object. In 

 many cases this is absolutely necessary, and even for ordinary 

 work it is of great advantage, since it not only enables us to 

 resolve lined objects, but to secure important changes in the 

 illumination of common objects. A very fair dark ground 

 illumination may be secured if the light is so oblique that none 

 of it can enter the object-glass directly. 



The mirror should not only be hung so as to swing to any 

 angle, but it should be movable on the mirror-bar so that the 

 rays which it reflects may be brought exactly to a focus on the 

 object. This is done by sliding the mirror out or in, according 

 as the rays are more or less divergent. 



The concave mirror should be large, so that it may collect 

 plenty of light. The plane mirror may be small without much 

 loss. The concave mirror is frequently used for the illumina- 

 tion of opaque objects, as when large it concentrates the light 

 very strongly. For this purpose it is either mounted on a 

 separate stand, or the mirror-bar is so hung that it may be 

 turned up over the stage, so as to reflect the light down upon 

 the object. 



The Body. The only points connected with the body of 

 the microscope which require consideration are its diameter and 

 its length, and these must of necessity vary so much according 

 to the purposes to which the microscope is to be applied, that 

 no rule can be laid down. Pocket microscopes are of necessity 

 small ; microscopes intended for use with objectives of low 

 power and large angles, must have a large diameter. And since 

 the distance of the eye-piece from the objective affects the cor- 

 rection of the latter, it has been found necessary to adopt a 

 standard length of body. This has been fixed at ten inches in 

 this country and in England. On the continent of Europe, 

 eight inches is the length that has been adopted, and most of 

 the continental objectives are corrected for this length of body. 

 Provided it is large enough to take the new broad-gauge screw, 

 the diameter is not of very great importance in bodies of 

 moderate length, but Beale tells us that in his long tubes, in- 



