82 PRACTICAL TREATISE ON THE MICROSCOPE. 



provements made in the microscope for the last twelve years. 

 The general arrangement, which is properly the province of 

 the mechanic, has been contrived to obtain the utmost freedom 

 from tremor, and to afford the greatest facUity in using the 

 various movements, while the extent, direction, and number 

 of these have been collected from the experience of the most 

 indefatigable observers in all the various branches of micro- 

 scopic inquiry. Nearly five hundred instruments have been 

 made on the plan here represented, and as no alteration or 

 addition has been found necessary for the accomplishment of all 

 the modes of microscopic investigation at present employed, 

 the mechanical structure of the microscope stand may be con- 

 sidered thus far established. 



" The optical part also has arrived at such perfection, that 

 points or lines, whose distance is such that their separation is 

 bordering on interfering with the physical constitution of 

 light, can be distinctly separated; thus ensuring a reality in 

 the appearance of objects, where the minuteness of their 

 detail approaches the natural limit of microscopic vision. 



"Description of the Instrument, (Plate 1.) 



"A A are two uprights, strengthened by internal but- 

 tresses, mounted on a strong tripod, B, at the upper part, and 

 between the uprights is an axis, C, upon which the whole of 

 the upper part of the instrument turns, so as to enable it to 

 take a horizontal or vertical position, or any intermediate 

 inclination, such, for instance, as that shown in the plate. 

 This moveable part is fixed to the axis near its centre of 

 gravity, and consists of the stage, D D, the triangular bar 

 and its socket, E and F, the arm, Gr, which carries the 

 microscope tube, H, and the mirror, I. The stage, D D, has 

 rectangular movements, one inch in extent, on the racked 

 cylinders, a a, and are moved by pinions connected with the 

 milled heads, b b' ; it also has the usual appendages of forceps 

 to hold minute objects, and lens to condense the light upon 

 them. The triangular bar, together with the arm and micro- 

 scope tube, is moved by the milled heads, e e, and a more 

 delicate adjustment of this optical part is effected by the 



