ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



691 



plate h soldered to the latter, is inserted between them and on this the 

 plate P moves (see fig. 81) ; screws/ and /i fix P to b, and by means of 

 two other screws the tubes and P can be firmly united. H is attached to 

 the column B which can be lengthened, thus raising the plane of the 

 tubes. The plate D is connected with the table T in a special manner 

 (movable by the hand or by screws) which needs no particular descrip- 

 tion here — the mechanism is shown at g, i, k, m, mj, n, q, v, and w. 



Fig. 81. 



In a later form Dr. Blix has simplified the instrument, principally 

 by the omission of the cylindrical axis A and the parts in connection 

 with it. 



Andrew Ross's Screw and Pinion Coarse- and Fine-Adjustment. — 

 Amongst an accumulation of pieces of experimental mechanism devised 

 in connection with the Microscope at various times during the past sixty 

 years, we recently found one of the earliest fine-adjustments designed 

 by the late Andrew Boss, which is shown in figs. 82 and 83, and which 

 comprises a coarse- and fine-adjustment in one piece of mechanism. 



In place of an ordinary rack for the coarse-adjustment there is a long 

 screw the thread of which serves as a rack. The screw is sunk in a 

 groove cut vertically in the back of the stem F supporting the cross-arm 

 and body- tube, leaving about one-third of its transverse section to be 

 acted upon by the pinion D (and milled head E) for the coarse-adjust- 

 ment. The upper end of the screw passes through the cross-arm B, 

 and a milled head A is applied on the top by which it can be turned, the 

 screw-thread then engaging the teeth of the pinion after the manner of 

 a tangent-screw, so that the screw, together with the stem and body-tube, 

 travels slowly up or down, forming a fine-adjustment. 



