APPENDIX. 495 



One of these cells, mounted as just described, is shown at /, 

 the ceU being represented at g, and the brass disc or button, 

 with its shank, at h. 



This is an exceedingly elegant and most convenient method 

 of mounting injected and other opaque objects, and will be 

 found very useful whenever it is required to demonstrate to 

 classes any particular set of organs, as cells mounted in this 

 way can easily be removed, thus enabling the teacher to ar- 

 range the objects on the disc in the order in which he may 

 wish to present them, and the rapidity with which any one of 

 them can be brought under the object-glass, will assist the 

 student in ascertaining the distinctive differences existing 

 between them. 



When not in use the discs may be kept in drawers, each 

 drawer having a number of pins, similar to that fixed on the 

 slab, screwed on the bottom of it, to receive them. Mr. Hett 

 employs mahogany boxes, about the size of those used to hold 

 twelve slides, but shallower, a brass pin being fixed into the 

 centre of the bottom of each box : this pin is somewhat longer 

 than that fastened to the microscope, so that it projects beyond 

 the boss on the disc, the projecting portion is furnished with a 

 thread, the disc being dropped into the box is secured by means 

 of a nut. 



These discs may be used with a microscope of the usual 

 form, the only additional apparatus required being a brass clip, 

 furnished with a pin to receive them; this clip should befitted 

 to the stage in the same manner as an ordinary object-plate, 

 or the object-plate itself may have a pin attached to it for the 

 reception of the discs. 



MR. boss's IMPEOVBX) ACHROMATIC MICROSCOPE. 



This instrument is represented in plate xi. ; it diflTers in 

 many respects from that shown in plate i. ; firstly, in having 

 a flattened bar, C, instead of a triangular one ; secondly, in 

 the stage being thin and of a new form, with rectangular 

 movements, and capable of being revolved one-third part 



