82 



SELECTION AND USE 



stage if necessary. Great obliquity, and perfect safety against 

 breakage of the object by pressure of the objective, are secured. 

 When the microscopist is using valuable slides, costing from 

 fifty to two hundred dollars, the latter feature is one of great 

 importance. 



Sub-Stage. The sub-stage is a device for holding and 

 adjusting illuminating apparatus beneath the stage. It should 

 be moved by rack work, so that paraboloids, achromatic con- 

 densers, etc., may be brought accurately into focus, and it 

 should also have adjustments for accurately centering these 

 various pieces of apparatus. We regard the sub-stage as one of 

 the most important parts of the stand. 



The Mirror. In microscopes of modern construction, the 

 mirror is made of glass, coated with pure silver, (ordinary mir- 

 rors are coated with an amalgam of mercury and tin) and the 

 best instruments are provided with two mirrors, one plane and 

 the other concave. In both mirrors the surfaces of the glass 

 should be accurately ground and polished. Blown glass will 

 not answer. The plane mirror reflects the light just as it falls 

 on it, while the concave one causes parallel rays (such as those 

 from the sun) to converge and meet at a point, and renders 

 divergent rays (such as those from a lamp) either less 

 divergent, parallel, or convergent, as the case may be. 

 The concave mirror should be large, but the plane mirror is as 

 efficient when small as when large. Where but one mirror is 

 provided, the concave is the one usually selected. 



In English and American microscopes of even the cheapest 

 construction, the mirror is so arranged that it may be made to 

 send a beam of light through the object very obliquely. This 

 is absolutely necessary for some purposes, but not for the ex- 

 amination of ordinary objects. The ability to use oblique light, 

 as it is called, is, however, a great advantage. 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 also be secured by placing the 

 mirror in such an oblique position that none of the light can 

 enter the object-glass directly. 



