1/0 MICROSCOPIC ILLUSTRATIONS. 



the motions of both of which are governed by the milled 

 head, r. The triangular gun-metal bar, b, is truncated at 

 its edges, and is therefore strictly speaking a hexagon, 

 tl us form giving greater strength in proportion to the space 

 occupied by it than a perfect triangle would, while it still 

 possesses the property of being locked fast when pinched 

 on only one side : it is evident that even an equilateral 

 hexagon would, when pinched on one side, always cause 

 two others to bear close against a triangular containing 

 tube. This bar slides up and down in the external cir- 

 cular one, c, between two plates of metal, pierced with 

 triangular apertures to receive it: one is soldered in at 

 the summit of the socket, h, the other below the milled 

 head, s ; and springs are placed on the same side as the 

 rack, both above and below the pinion, to steady the 

 bar, and give the friction necessary to prevent it from 

 falling from its position by gravitation. If the present 

 stand was made to operate solely with an engiscopic refract- 

 ing body, it would be better to omit the rack-work of the 

 upright bar altogether, and substitute a pinching screw 

 for it, giving a fine movement to the body within the neck 

 of the arm, by a screw on Ramsden's principle. In 

 its anterior side is sunk the rack which adjusts the focus 

 by means of the milled head, s, which is repeated on the 

 opposite side. Rack-work is not perhaps the best sort 

 of movement which might be contrived, but it is good 

 enough, when well executed, for all practical purposes. 

 On the top of this triangular bar is fixed the gun-metal 

 wheel,/, within the teeth of which works an endless 



