26 THE AMERICAN MONTHLY [Jan., 



at one side for the admission of light to the mirror, 

 which, swung between pivots of which the ends project 

 through the sides of tlie drum, is focused vertically by a 

 rack and pinion of which the milled head is seen behind. 

 At the top of the drum is a slit which gives passage to 

 the edge of a circular diaphragm, and the stage, like that 

 of the simple microscope, is round and fitted in the same 

 way for numerous stage forceps ; but, inasmuch, as its 

 rotation with these in place would bring their project- 

 ing ends against the pillar of the microscope, the direct 

 connection of the latter with the foot is severed, and it 

 is inserted instead on a projecting lip of the stage upon 

 which it is centered by being fitted loosely in a hole, 

 where it is clamped in concentricity with the diaphragm 

 by the clamping screw seen underneath. It is expressly 

 admitted by Stranss-Durckheim that the working out of 

 this feature was effected by M. Trecourt, Oberhaeuser's 

 senior partner ; and here we may ask for the original 

 source of such a model. The inventor having stated that 

 he originally worked " with a microscope which, by kind 

 of chance, was of small dimensions," there was only one 

 pattern extant at the time, which corresponds to this 

 description and could have served as a basis for the de- 

 sign. This is the so-called drum microscope, shown in 

 fig. 3, which is now only used for toys, but was adopted 

 by the optician Fraunhofer, of Munich, as a regular pat- 

 tern about 1815, and was certainly manufactured in 

 Paris by Chevalier, Lerebours, Trecourt, and perhaps 

 others from 1830 onwards ; and it is not diflicult to see 

 how the combination of the stand with a rotating stage 

 would result in the design of fig. 2. Returning to this 

 instrument it will be noticed that the pillar which car- 

 ries the body is enveloped in two outer tubes or sleeves, 

 the inner of which, by means of a micrometer screw and 

 spring, slides up and down to aff'ord the now^ familiar 

 form of fine-adjustment, whose milled head is seen below 



