ACHROMATIC CONDENSER. 



133 



Fio. 35. 



Ross's Achromatic Condenser. 



Fig. 36. 



apertures are shown in the diaphragm-plates in Figs. 35 and 36. 

 The Condenser thus completed is constructed on three different 

 plans by the three principal makers, in accordance with the differ- 

 ent arrangements of their respective stages. By Mr. Eoss, who ori- 

 ginally carried Mr. Gillett's plans into operation, the diaphragm- 

 plate has the shape of a short frustrum of a cone (Eig. 35), so at- 

 tached to the condenser, that the portion of the plate which passes 

 through it shall cut it transversely ; each 

 aperture is indicated hy a number on the 

 dial ; and a spring-catch is so arranged, as 

 to mark when any one of the apertures is 

 in its right place, and to show its number. 

 The thinness of the stage in Messrs. Smith 

 and Beck's microscope, allows the dia- 

 phragm-plate to be made upon the ordinary 

 plan (Fig. 36), since it can be brought 

 sufficiently near to the lenses of the con- 

 denser, without coming into too close con- 

 tiguity with the stage ; and this is obvi- 

 ously the simplest and most convenient 

 arrangement. By Messrs. Powell and Lea- 

 land, again, — ^their stage being too thick to 

 allow of the diaphragm-plate being placed 

 beneath it, without removing that 

 plate from its proper position be- 

 hind the lenses of the • condenser, 

 ^the diaphragm-plate is made so 

 small that it can be received into 

 the interior of the stage (Fig. 37), 

 but is rotated by a milled head be- 

 neath; and the edge of this is 

 marked by numbers, each signify- 

 ing a particular aperture, and thus 

 marking by its position which aper- 

 ture 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 made to ro- 

 tate immediately beneath it upon the same axis (like the hour 

 andwninute hands of a watch), by means of a second milled 

 head, numbered at its edge like the first ; and the apertures 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. 



57. Reflecting Prisms. — Every mirror composed of glass silvered 

 at the back, gives, as is well known, a double reflection ; namely, 

 a principal image from the metallic surface, and a secondary im- 



Smilh and Beck's Achromatic Condenser. 



