120 MECHANICAL ARRANGEMENT OF THE MICROSCOPE. 



purposes at the present day ; it should not, however, be placed at 

 too great a distance from the object-stage. One of the most 

 convenient arrangements for the disc of diaphragms is to hollow 

 out the stage from below, and construct the diaphragm-plate with 

 a corresponding curvature. The larger stands of Zeiss are pro- 

 vided with calotte diaphragms on this plan. 



Besides these revolving discs the cylinder-diaphragms introduced 

 by Oberhseuser have recently come more and more into use. 

 They are cylindrical caps with apertures of varying size, which 

 drop into a larger cylinder sliding in a socket beneath the stage. 

 They are convenient for adjustment at different distances beneath 

 the object, and thus regulate the amount of light very perfectly. 



We have already referred to various contrivances for shutting 

 off the central rays. 



The Base. The chief purpose of the base is to give stability to 

 the whole instrument ; it must, therefore, be neither too small nor 

 too light. As a model of firmness and convenience the horse-shoe 

 stand of Oberhneuser may, perhaps, be named ; it combines great 

 solidity with the advantage that the illuminating mirror can be 

 lowered to the level of the table, by which the necessary space is 

 gained for other contrivances to be applied below the stage. Less 

 convenient, because arranged only for axial illumination, and, 

 therefore, applicable only for the smaller instruments, is the so- 

 called drum stand of Oberhseuser; it rests upon a circular foot 

 weighted with lead, which is connected with the stage by a 

 cylindrical tube with an opening in front. Some opticians unite 

 a similar foot by one or two pillars to the object-stage and to the 

 whole upper part of the stand. Others, again, adopt a tripod base, 

 of which the feet are sometimes fixed and sometimes arranged to 

 fold up, &c. Small instruments, such as travelling and pocket 

 Microscopes, are so constructed that the stand may be screwed 

 on the lid of the case containing them. For daily use, how- 

 ever, we should not recommend such an arrangement. 



Length and position of the body-tubes. Most modern opticians on 

 the Continent give the body-tube a length of 200 220 mm. from 

 the anterior surface of the objective to the terminal surface of the 

 eye-piece, so that the total height of the instrument amounts to 

 300 360 mm. Only the older large Microscopes of Ploessl and 

 his imitators (Pistor, Schieck, &c.) differ essentially from this, 

 reaching a height of about 450 mm., with a tube-length of about 



