62 PRACTICAL TREATISE ON THE MICROSCOPE. 



down by means of two small handles projecting from the 

 cell containing the lens. Two small tubes, i, into which 

 either a condensing lens or a pair of forceps may be fitted, 

 are attached to the under side of the stage. 



The magnifiers employed in this instrument were either 

 single lenses or doublets, and Mr. Valentine, who is so well 

 known as a most skilful vegetable anatomist, has managed to 

 dissect under a lens of one-twentieth of an inch focus. 



To make it a compound microscope, the arm carrying the 

 lenses can be removed, and a compound body, supported on 

 a bent arm and provided with a conical pin, at its end, 

 can be substituted, and the coarse and fine adjustments in 

 the piUar will answer the purpose of focussing the compound 

 instrument, as well as the simple magnifiers. 



This microscope, the first of the kind ever made by Mr. 

 Eoss, was remarkable for the excellence of its workmanship, 

 and may be said to have paved the way for a new era in the 

 forms of these instruments. 



A very useful microscope for dissecting is that made by 

 Messrs. Smith and Beck, represented by fig. 39 ; it may be 

 supported upon a heavy circular brass foot, or be screwed to 

 the cover of a box, or block of hard wood. The central 

 pillar is circular, about six inches long and three-fourths of 

 an inch in diameter, and from it may be raised a triangular 

 bar, by a rack and pinion, turned by two large milled heads. 

 The lens-holder has two movements like that of Mr. Ross, 

 the one by a pin fitting into the top of the triangular bar, 

 the other by a rack and pinion. The mirror is of the usual 

 construction. The stage is of a circular figure, three inches 

 in diameter; into it may be fitted dissecting troughs, com- 

 posed of a ring of brass, with a glass bottom, or a similar ring 

 with an ebony bottom, and others equally useful, which are 

 covered or lined with cork. This instrument is suppUed with 

 single lenses and with doublets, and has proved a very useful 

 working tool in the hands of Mr. Darwin, who suggested 

 many ingenious pieces of apparatus to fit into the hole in the 

 stage for holding subjects under examination. Besides this 

 single microscope of Messrs. Smith and Beck, and those 



