108 



APPENDAGES TO THE MICROSCOPE. 



formance is greatly improved by the interposition of an Achromatic 

 combination between the Mirror and the Object, in such a manner 

 that the rays reflected from the former shall be brought to a focus 

 in the spot to which the Objective is directed. A distinct picture 

 of the source of light is thus thrown on the subject, from which 

 its rays emanate again as if the object were self-luminous. The 

 Achromatic combination, which (at least in all First-class Micro- 

 scopes) is one specially adapted to the purpose, is furnished with a 

 Diaphragm -plate (as first suggested by Mr. Gillett) immediately 

 behind its lenses; and this is pierced with holes of such a 

 form and size, as to be adapted to cut off in various degrees, not 

 merely the peripheral, but also the central part, of the illumin- 

 ating pencil. The former of these purposes is of course accom- 

 plished by merely narrowing the aperture which limits the passage 

 of the rays through the central part of the lens ; the latter, on the 

 other hand, requires an aperture as large as that of the lens, having 



its central part more or 

 less completely occupied 

 by a solid disk, which may 

 so nearly fill the circle 

 as to leave but a mere 

 ring through which the 

 light may pass. Such 

 apertures are shown in 

 the Diaphragm-plate in 

 Fig. 58. — The Condenser 

 thus completed is con- 

 structed on different plans 

 by the three principal 

 Makers, in accordance with 

 the different arrangements 

 of their respective stages. 

 The thinness of the Stage 

 in Messrs. Smith and Beck's Microscope allows the diaphragm- 

 plate to be made upon the ordinary plan (Fig. 58), since it 

 can be brought sufficiently near to the lenses of the Condenser, 

 without coming into too close contiguity with the Stage ; and 

 this is obviously the simpler arrangement. In Messrs. Powell 

 and Lealand's smaller Microscope, of which the stage is too thick 

 to allow of the diaphragm-plate being placed beneath it, without 

 removing that plate from its proper position behind the lenses of 

 the condenser, the diaphragm -plate is made so small that it can be 

 received into the interior of the stage (Fig. 59), and is rotated by 

 a milled-head beneath ; the edge of this is stamped with figures, 

 each signifying a particular aperture, arid thus marking by its 

 position which aperture is in use. As, however, the smallness of 

 the Diaphragm -plate so limits the number of apertures that the 

 desirable variety could not be afforded by it alone, a second plate is 



Smith and Beck's Achromatic Condenser. 



