DIAPHRAGM- PLATE. — ACHROMATIC CONDENSER. 



107 



' well ' of the Stage, in fact every part through, which light passes 

 to the object from beneath, must 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 position. — Another very simple and effective arrangement 

 for the same purpose, consists in the use of a single Diaphragm 

 having an aperture of about 3-16ths of an inch, which is fixed in 

 a tube that slides in the short tube screwed into the aperture of 

 the stage (Fig. 34) for carrying the Polariscope, &c. When this 

 diaphragm is pushed up so as to approach the Stage, it cuts off 

 only a small portion of the cone of rays reflected upwards from 

 the concave mirror ; but when drawn downwards, it cuts off more 

 and more of the peripheral portion of that cone, and thus gradually 

 reduces, the light. A small shutter for closing the aperture, so as to 

 give a black back-ground for Opaque objects, is generally supplied 

 with a diaphragm of this kind. — So great an advantage is often 

 derivable from a gradational reduction or augmentation of the 

 light, that the Microscopist who desires to avail himself of this 

 will do well to provide himself with one of the forms of Gra- 

 duating Diaphragm, which have been recently introduced. That 

 long a*go invented by Dollond for Telescopic purposes is equally 

 applicable to the Microscope ; 

 the circumstance that its aper- Fig. 57. 



ture is square, instead of round, 

 not constituting any practical 

 objection to its use. In another 

 form, introduced by Mr. Collins 

 (Fig. 57), four shutters are made, 

 by acting on a lever-handle, to 

 move inwards simultaneously, so 

 as to narrow the aperture, the 

 shape of which always remains 

 more nearly circular than square. 

 And in the ' Iris Diaphragm ' 

 recently devised by Mr. Brown,* 

 the multiplication of the num- 

 ber of shutters makes the aper- 

 ture practically circular. Either 

 of these may be advantageously 

 attached to the "Webster Condenser (§ SO). 



79. Achromatic Condenser. — In almost eveiy case in which an 

 Objective of l-4th inch or any shorter focus is employed, itsper- 



* " Transactions of the Microscopical Society," Vol. xv. p. 74. — Another 

 form of Graduating Diaphragm, in which the reduction of the aperture 

 is effected by twisting a tube of Vulcanized Caoutchouc, is described by 

 Mr. S. B. Kincaid in the "Trans, of Microsc. Soc." Vol. xiv. p. 75. 



Collins's Graduating Diaphragm. 



