90 PEACTICAL TREATISE ON THE MICROSCOPE. 



MESSES. SMITH AKD BECk'S SMALLER ACHROMATIC 

 COMPOTJND MICROSCOPE. 



This instrument is represented in Plate 4, fig. 1 ; it is 

 mounted on three feet, AAA, capable of being closed to- 

 gether ; into a circular plate attached to these feet is screwed 

 a pillar, B, having a cradle joint, C, at its upper part ; to the 

 joint is attached a bent arm, D, grooved like that of the larger 

 instrument, and supporting in a sicnilar manner the compound 

 body, G, the triangular guide and rack being seen at E. The 

 milled heads, F F, are for the coarse adjustment. To the 

 bottom of the compound body there is attached a small tube, 

 into the lower end of which the object-glasses are screwed, 

 and to its upper end a lever, the short extremity of whose 

 long end, I, is capable of being moved up and down by the 

 nut, K, working on the screw, H, and so forming the fine ad- 

 justment ; the tube. is kept tight against the nut by a spring, 

 which, when the object-glass is accidentally brought in contact 

 with any object on the stage, allows of its retreating for a 

 short distance, and in most cases, prevents either the object 

 or the object-glass itself from being fractured, hence it has 

 obtained the na,me of the safety-tube. To the lower end of 

 the arm, D, the stage, K, is screwed; this consists of two 

 plates, which are capable of being moved in two directions at 

 right angles to each other, by racks and pinions connected 

 with the milled heads, M M. The object-plate, L, is pre- 

 cisely similar to that in the preceding instrument, and to the 

 under side of the stage aU the usual apparatus may be fixed. 

 The mirror, N, is mounted in the ordinary way upon a semi- 

 circular frame, O, having a pin passing through a piece of 

 cork in the end of the tube, P, on this it can be turned hori- 

 zontally. To render this a cheaper instrument, the stage 

 shown at fig. 2 may be substituted for the adjustable one. A 

 represents part of the arm supporting the compound body, B 

 a plate of brass attached by screws to the lower end of the 

 same arm, C the joint at the upper part of the pillar. Upon 

 the plate B is supported the object-plate, D, capable only of 

 being moved by the fingers in two directions, the one verti- 



