102 THE . MICROSCOPE AND ITS REVELATIONS. 



made of two plates screwed together. A different arrangement may be 

 adopted with advantage, when the Stage is provided with a cylindrical 

 fitting for the reception of Illuminating and Polarizing apparatus. A 

 short tube sliding into this may carry a shoulder at- its upper end, upon 

 which may be fitted two or more caps with apertures of different sizes, so 

 that these perforated caps may be either pushed up flush with the sur- 

 face of the stage, or may be lowered to any distance beneath it, according 

 as the best effect is produced. A ground-glass for diffusing light may 

 also be adapted to lie on the shoulder in the place of the perforated 

 caps; and there should also be an wwperforated cap to serve as a back- 

 ground to opaque objects. Such great advantage is often derivable from 

 a gradational modification of the light, that the Microscopist who de- 

 sires to avail himself of this will do well to provide himself with one of 

 the forms of graduating diaphragm which have been recently ^introduced. 

 That long ago invented by Dollond for Telescopic purposes is equally 

 applicable to the Microscope; the circumstance that its aperture is square 

 instead of round, not constituting any practical objection to its use. In 

 another form, introduced by Mr. Collins (Fig. 72), four shutters are 



FlG - 72. TIG. 73. 



Collins's Graduating Diaphragm. Beck's Achromatic Condenser. 



made to move inwards simultaneously, by acting on a lever-handle, so as 

 to narrow the aperture, the shape of which always remains more nearly cir- 

 cular than square. And in the 'Iris Diaphragm 7 devised by Mr. J. H. 

 Brown, 1 the multiplication of the number of shutters makes the aperture 

 practically circular. The new construction of this, devised by Mr. Geo. 

 Wale, U. S., is so simple, inexpensive, and effectual, that its general adop- 

 tion in place of the Diaphragm-plate may be anticipated. 



99. Achromatic Condensers. In almost every case in which an Objec- 

 tive of l-4th inch or any shorter focus is employed, its performance is 

 greatly improved by the interposition of an Achromatic combination 

 between the mirror and the object, in such a manner that the rays re- 

 flected from the former shall be brought to a focus in the spot to which 

 the object is directed. A distinct picture of the source of light is thus 

 thrown on the object, from which the rays emanate again as if it were 

 self-luminous. The Achromatic combination, which (at least in all 

 First-class Microscopes) is one specially adapted to the purpose, is fur- 

 nished with a Diaphragm-plate immediately beneath its lowest lens (Fig. 

 73); and this is pierced with holes of such forms and sizes as to cut off 

 in various degrees, no merely the peripheral but also the central part of 



1 "Transactions of the Microscopical Society," Vol. xv , p. 74. 



