114 



SELECTION" AHi> 



clips, which for this purpose are pushed through from below 

 upwards. In focussing, the objective is passed through the 

 stage if necessary. Great obliquity, and perfect safety against 

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

 microscopist is using valuable slides, costing from ten to one 

 hundred dollars, the latter feature is one of great importance. 



In many microscopes, however notably those of English 

 manufacture the under side of the stage is not flat and even, 

 so that a slide cannot be laid against it. The following simple 

 device obviates this difficulty: A rectangular plate of metal is 



pierced with a hole of the same 

 size as the interior diameter of 

 the sub-stage ring of the micro- 

 scope, and in this hole is fastened 

 a tube, which just fits this sub- 

 stage ring. The plate is provided 

 with two light spring clips, which 

 hold the object against its under 

 side, and it is easy to see that this 

 simple contrivance, which is shown 

 in Figure 26, serves three very im- 

 portant functions: 1. It affords 

 means for obtaining light of great 

 obliquity, since in reality it forms 

 a stage which has no thickness at 

 all. 2. It serves as a perfect safety 

 stage, thus enabling us to avoid all 



risks not only to valuable slides, but to still more valuabe ob- 

 jectives. 3. It enables us to rotate the object very nearly in 

 the optic axis of the instrument. 



The centricity of this rotation is not sufficiently accurate to 

 enable us to measure angles with any degree of precision, but 

 it gives us the opportunity of placing lined diatoms and other 

 objects in any position in regard to the illuminating ray. 



Sub-Stage. The sub-stage is used chiefly for holding and 

 adjusting illuminating apparatus beneath the stage, and may 

 justly be regarded as one of the most important parts of the 

 stand. 



Fig. 26. 



