OF THE MICROSCOPE. 77 



means of this arrangement, every kind of illuminating apparatus adapted 

 to the sub-stage can be made to act at any obliquity whatever; and as 

 the tail-piece may be swung round on the side opposite to that of the 

 milled-heads of the traversing stage, until it is brought considerably 

 above the stage, oblique illumination may be thrown by the condenser, 

 not only on the under but also on the upper surface of any object. It 

 is 1 one great advantage of this method, that condensers of large angle of 

 aperture are not required for the purpose of oblique illumination; the con- 

 verging pencils given by ordinary Objectives of 1 inch or 1 J inch focus, used 

 as condensers, being fully adequate. Further, the swinging tail-piece may 

 be used to measure the angular aperture of Objectives in the manner to 

 be hereafter described, its inclination to the optic axis being marked by 

 .a divided arc on its upper segment, which also enables the illuminating 

 angle at which any particular object is best seen to be observed and re- 

 corded. Altogether, it may be unhesitatingly affirmed, that the Zent- 

 mayer system enables the best results of oblique illumination to be ob- 

 tained with greater facility than any other of equal effectiveness; while 

 the simplicity of the construction of the whole instrument enables Messrs. 

 Eoss to reduce its cost considerably below that of the old Boss or Eoss- 

 Jackson models. 



73. Powell and Leland's Large Microscope. These eminent Makers 

 have not made any essential modification in the construction of their 

 large Microscope, represented in Plate vn. ; preferring to furnish the 

 very oblique illumination now in general demand by enlarging the angu- 

 lar aperture of their Achromatic Condenser ( 99). The chief peculiar- 

 ity of their model consists in the attachment both of the Stage and Sub- 

 stage to a large solid brass ring, which is firmly secured to the stem of 

 the instrument. The upper side of this ring bears a sort of carriage that 

 supports the stage; and to this carriage a rotatory movement around the 

 optic axis of the principal body is given by a milled-head, the amount of 

 this movement (which may be carried through an entire revolution) being 

 exactly measured by a graduated circle. The stage, which is furnished 

 with the usual traversing movements, worked by two milled-heads on the 

 same axis, is made thin enough to admit of the mirror being so placed, 

 by means of its extending arm, as to reflect light on the object from out- 

 sfde the large brass ring that supports the stage and sub-stage. Light 

 of the greatest obliquity, however, may be more conveniently obtained 

 by an Amici's prism ( 102) placed above the supporting ring. The 

 sub-stage is furnished with rotatory and rectangular, as well as with ver- 

 tical movements. The instrument is so well balanced on its horizontal 

 axis, that it remains perfectly steady withoutclamping, in whatever po- 

 sition it may be placed. 



74. Beck's First-class Microscope. It was by this Firm that the 

 Jackson model was first adopted, for which the Author has already ex- 

 pressed his preference ( 49). Besides the steadiness imparted to the 

 double body by the support given to it by the limb along the greater part 

 part of its length, it is an additional advantage of this construction, that 

 by continuing the limb beneath the stage, the secondary body or Sub- 

 stage (which carries the illuminating apparatus) is made to work in a 

 dovetailed groove that is ploughed-out in continuity with that in which 

 the rack of the principal body slides, an arrangement obviously favorable 

 to exactness of centering. The Stage has a nearly complete rotation in 

 the optic axis of the instrument, motion being given to it by a milled- 

 head beneath the stage, the pinion attached to which can be readily 



