132 ACCESSORY APPAEATUS. 



formance ia greatly improved by tlie interposition of an achro- 

 matic 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. This 

 may be accomplished sufficiently well for ordinary purposes, by 

 adapting a French triple combination of about l-4th inch focus, 

 to the end of a tube IJ inch long, which shall slide within an- 

 other tube fitted to the opening in the stage, by the bayonet 

 catch or any similar connection that gives attachment to the 

 diaphragm-plate. If this be correctly centred in the first in- 

 stance, and the workmanship of the microscope be good, no more 

 expensive arrangement will be required, by such at least as may 

 be satisfied with that degree of perfection, which suffices for the 

 clear discernment of all but the most difficult objects. The slid- 

 ing movement of the tube, especially if it be accomplished by a 

 lever-action (as suggested by Mr. Quekett), is quite sufficient for 

 the adjustment of the focus ; and the removal of the outer lens 

 adapts it for use with objectives below l-4th inch, to whose per- 

 formance it often affords important assistance. In the most per- 

 fect arrangement of the Achromatic Condenser, however, such 

 as is now adapted to all first-class instruments made in this 

 country, the achromatic combination is one specially adapted to 

 the purpose; and is so mounted as to insure the greatest ac- 

 curacy of its adjustments. By Mr. Ross it is supported by what 

 he terms the " secondary stage" (§ 37) ; and by Messrs. Smith 

 and Beck it is carried upon the summit of the " cylindrical fit- 

 ting" which answers the same purpose (§ 39) ; whilst by Messrs. 

 Powell and Lealand, it is attached by a bayonet-catch to the 

 under side of the fixed stage-plate (§ 38). In either case it is 

 provided with a pair of milled-headed screws (Fig. 36), which give 

 it a slight degree of horizontal motion in transverse directions, 

 for the purpose of procuring an accurate centring ; and where, 

 as in Messrs. Powell and Lealand's instrument, the focal adjust- 

 ment is not given by the movement of the carriage which bears 

 it, a rack and pinion is attached for this purpose to the tube of 

 the condenser itself In order that the Achromatic Condenser 

 should be made to afford the greatest possible variety of modifi- 

 cations of the illuminating pencil, it requires to be furnished 

 with a diaphragm-plate (as first suggested by Mr. Gillett) imme- 

 diately behind its lenses ; and this should be 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 parts of 

 the illuminating pencil. The former of these purposes is of 

 course accomplished, 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 



