SMITH AND BECKS LARGE MICROSCOPE. 



113 



spondence being secured by the action of the planing machine 

 that ploughs both grooves), the " secondary" body always has 

 its axis so perfectly continuous with that of the first, that no 

 special adjustment is need- P IG 2 9. 



ed to " centre" the greater 

 part of the illuminating 

 apparatus ; and, secondly, 

 that the tube will carry the 

 achromatic condenser at 

 its upper end, the polaris- 

 ing prism at its lower, and 

 the selenite plates between 

 the two, a combination 

 that cannot be made in 

 any other instrument ( 

 63). Moreover, as all 

 these fittings are received 

 into a tube of which the 

 exact size and position are 

 assured, the makers of this 

 instrument can supply ad- 

 ditional apparatus at any 

 time, with the certainty of 

 its accurate adjustment. 

 This "secondary body," 

 however, has not the rota- 

 tory movement possessed 

 by Mr. Ross's " secondary 

 stage ;" and to the limited 

 class of purposes, there- 

 fore, which that move- 

 ment is adapted to serve, 



it Cannot be adapted. Smith and Beck's Large Compound Microscope. 



The mirror is hung in the usual way between two centres; 

 but the semicircle that carries these, instead of being at once 

 pivoted to the tube which slides upon the cylindrical stem, 

 is attached to an intermediate arm ; and by means of this 

 it may be placed in such a position as to reflect light very ob- 

 liquely upon the object, and thus to bring out a new set of ap- 

 pearances, with which it is very important in certain cases to be 

 acquainted. ^ In regard to weight and complexity, this instrument 

 holds a position intermediate between the two last described. 

 The mode in which the body is supported, appears to the author 

 decidedly preferable to that adopted by the other makers ; and 

 though it has the disadvantage of separating the focal adjust- 

 ments from each other and from the stage motions more widely 

 than is the case in the two preceding instruments, yet the differ- 

 ence is scarcely perceptible in practice. The milled heads acting 

 on the former are both of them in positions in which they are 



