ACHEOMATIC CONDENSER. 109 



made to rotate immediately beneath it upon the same axis (like 

 the hour and minute-hands of a watch) by means of a second 

 milled-head numbered at its edge like the first ; and the apertures 



Fig. 59. 



Powell and Lealand's Achromatic Condenser. 



in the diaphragm-plate being simple circles, the centres of these 

 are covered by stops of different sizes, supplied by the second or 

 Stop-plate ; by which very ingenious arrangement a great variety 

 of combinations may be obtained, all of them indicated by the 

 numbering on the two milled-heads. The Large Microscope of the 

 same makers (Plate vi.) is furnished with a Condenser said to have 

 an angular aperture of 170° ; and its stop-plate has its apertures 

 so arranged with reference to those of the diaphragm-plate, as to 

 give passage (when required) to only the extremely oblique rays 

 proceeding from a small portion of the periphery — the mode of 

 illumination most effective for the most difficult lined tests (§ 120). 

 — A similar arrangement is now adopted by Mr. Ross ; whose 

 latest form of Achromatic Condenser is represented in Fig. 60. 

 The combination of lenses has a focus of about 4-10ths of an inch, 

 and an angular aperture of about 110°; and whilst this aperture is 

 found, when used with appropriate diaphragms, to give rays of an 

 obliquity sufficient for the resolution of the most difficult tests, it 

 is obvious that the focal length of this instrument gives it an advan- 

 tage over Condensers of shorter focus, the illuminating pencils of 

 which cannot reach objects mounted on ordinary slips of glass. The 

 Diaphragm-plate, b, is furnished with a series of eight apertures, 

 which progressively bring down the angle of the illuminating pencil 

 from 110° to 20° ; whilst the Stop-plate, a, has three circular stops 



