278 Transactions of the Society. 



(2) It is impossible for the movements to become shaky from wear, 

 since every movement is held up by a spring. (3) The alignment 

 of the several slides is obtained optically. 



These are the essential advantages. There are also some in- 

 cidental points, namely, easy removal of the fine-adjustment tube 

 for cleaning ; possibility of replacement of any part without the 

 need of special fitting — for example, it would be possible to replace 

 a fine-adjustment tube, whiclrcarried a sliding coarse adjustment, 

 by one carrying a mechanical movement, without skilled work ; 

 convenient position of the substage focussing screw ; accessibility 

 of the condenser ; and the shape of the limb, which enables it to 

 be finished entirely by machinery. 



Of all these points, those which make for cheapness appear to 

 me to be of the greatest importance. This was the primary object 

 with which the instrument was designed. 



The particular instrument from which the photographs re- 

 produced with this paper were taken, was made throughout with 

 the roughest of workmanship. In spite of this, i the movements all 

 worked smoothly, and without shake, a result which could certainly 

 not have been obtained with similar workmanship in a Microscope 

 of the usual pattern. This fact affords the strongest proof of the 

 £ uperiority of the geometric slide. 



