THE COMPOUND MICROSCOPE. 87 



right angles to the movements, or else as seen in Plate 1, 

 where they are both in the same plane with them; in the 

 portable instrument there is, however, a screw introduced 

 instead of a rack, by which the movement from side to side is 

 effected ; but the screw is a fixture, and the stage-plate, with 

 the miUed head attached, is moved backwards and forwards on 

 the screw. 



To all Mr. Eoss's instruments the achromatic condenser, 

 the polarizing prism, and other apparatus, are capable of being 

 adapted, but there is no draw-tube to the compound body in 

 either of them for the erecting-glass or micrometer eye-piece, 

 as in the microscopes of Messrs. Smith and Beck ; the form 

 of eye-piece employed by Mr. Ross not requiring such an 

 addition in the use of the micrometer. 



MESSES. SMITH AKD BECK'S LARGE ACHROMATIC COMPOXIND 

 MICROSCOPE. 



This instrument is represented by Plate 3, and consists of a 

 firm tripod base, AAA, upon which two strong pUlars, 

 B B, are screwed : these at their upper parts support the 

 trunnions, to which the beU-metal arm, C, and the stage, E, 

 are attached, and by means of which this part of the iastru- 

 ment can be inchned at any angle. The arm supports the 

 entire length of the compound body, F, on its inner edge, 

 which is ploughed out in such a manner as to receive two 

 brass rods or guides attached to the compound body ; one of 

 these, which is soldered to the whole length of the body, is of 

 a triangular figure, and to its apex is screwed a thin flat piece 

 of metal of corresponding length, about five-eighths of an inch 

 broad, and one-eighth of an inch thick, having a rack, or 

 sometimes, two racks, cut on its oiiter or unattached side; 

 the former guide fits into a triangular channel ploughed out 

 of the arm, and the latter slides into a channel of the same 

 shape as itself immediately at the back of the triangular one ; 

 the triangular gxiide forms a firm support for the body to rest 

 upon, and the flat guide answers the purpose of keeping the 

 first in close apposition with the channel, whilst by the rack 



