MAGNETIC STAGE DIAPHRAGM-PLATE. 131 



place of the object by a steady and practised hand. Another ar- 

 rangement, which has some advantages over the preceding, has 

 been proposed by Mr. J. B. Spencer ; this consists in surround- 

 ing the aperture of the stage by a ring of soft iron, the surface 

 of which projects very slightly above the brass plate, the mag- 

 nets (cut out of sheet-steel) being attached to the under side of 

 the object-bearer. This method is perhaps more readily applica- 

 ble than Mr. Busk's to the stage of any microscope, and will pro- 

 bably interfere less with its other fittings. 1 



55. Diaphragm-Plate. No microscope stage should ever be 

 without a diaphragm-plate fitted to its under surface, for the sake 

 of restricting the amount of light reflected from the mirror, and 

 of limiting the angle at which its rays impinge on the object 

 (see Figs. 18 and 21). This plate should always be at least half 

 an inch below the object, since it is otherwise comparatively in- 

 operative ; and thus, whilst it may be fixed immediately beneath 

 a movable stage whose thickness serves to remove it sufficiently 

 far, it should be fixed on the end of a short tube forming a sort of 

 well on the under side of the stage, when this consists of but a 

 single fixed plate. The diaphragm-plate should be perforated 

 with holes of several different sizes, in the largest of which it is 

 convenient to fit a ground-glass (this, by means of a screw-socket, 

 may be made removable at pleasure), the use of which is to dif- 

 fuse a soft and equable light over the field, when large trans- 

 parent objects (such as sections of wood) are under examination ; 

 between the smallest and the largest aperture, there should be an 

 unperforated space, to serve as a dark background for opaque 

 objects. The diaphragm-plate itself, the "well" of the stage, in 

 fact every part through which light passes to the object from be- 

 neath, should be blackened, in order to avoid the interference 

 that would be occasioned by irregularly reflected rays. The edge 

 of the diaphragm-plate should be notched at certain intervals, 

 and a spring catch fitted so as to drop into the notches, in order 

 that each aperture may be brought into its proper central posi- 

 tion. This simple arrangement, in combination with the mirror 

 (which should be concave on one side and plane on the other) 

 and side-condenser ( 64), affords to the Microscopist all the 

 means of illuminating his objects, whether transparent or opaque, 

 which are ordinarily requisite : to bring out the highest powers 

 of the instrument, however, more refined methods of illumina- 

 tion are required ; and a far greater variety of treatment is needed 

 in the case of many objects, the determination of whose true 

 characters is a matter of difficulty, even under every advantage 

 which can be derived from assistance of this kind. 



56. Achromatic Condenser. In almost every case in which an 

 objective of 1-4 th inch or any shorter focus is employed, its per- 



1 For a more detailed description, with illustrative figures, of Mr. Busk's Magnetic 

 Stage, see " Quarterly Microsc. Journal," vol. ii, p. 280 ; and for Mr. Spencer's, vol 

 iii, p. 173. 



