302 ACCESSORY APPARATUS 



for low powers. When the highest class of work has to be done it 

 is -needful to have condensers suited to the power of the objective used. 



A dry apochromatic condenser -of merit is made by Swift and 

 Son ; it has a N.A. of 0'95 and an aplanatic cone approximating 

 O92. and works with ease through any object- 

 slide, but is corrected to do this by thinning 

 the front lens and setting the front and back 

 combinations further apart than would be the 

 case if they were used as an objective. The 

 lower combination has a large, clear aperture. 

 The optical part of this instrument is shown in 

 fig. 241 ; we have used it, and find it a tho- 

 roughly practical and serviceable condenser. 

 FIG 241 Swift's apo- Before the introduction of the homogeneous 



chromatic (1899) con- system, and the production of such great aper- 

 denser, N.A. 0-95. tures by Powell and Lealand as a 1/5 in a th, 



a ^Vth, and a .>Vfch of an inch focus, the cone 



transmitted by Powell's dry achromatic condenser was as large as 

 could be utilised. But with apertures such as these, and because 

 of the subsequent introduction of the apochromatic system of lenses, 

 much larger cones were required. To meet this necessity Pow T ell 

 and Lealand, at the urgent suggestion of English experts, made first 

 a chromatic condenser on the homogeneous system ; but this was 

 subsequently succeeded by an achromatic instrument of great value 

 on the same system. This combination consisted of a duplex front 

 with two doublet backs ; it is nearly of the same power as their 

 dry achromatic condenser, but is of much greater aperture. It was 

 brought afterwards to a very high state of perfection, Imving an 

 aperture of 1'40, and will work through a mounting slip of *07, and 

 for aperture and working distance is, like its dry predecessor, quite 

 unappr cached . 



Messrs. Powell and Lealand have produced an entirely new 

 condenser, strictly apochromatic, employing a fluorite lens in the 

 combination, and presenting features in the highest degree desirable. 

 We find its N.A. to be 0*95, its focal length long enough for a thick 

 slip, its aplanatic aperture '9. We have found it of the utmost 

 practical value in critical work, and this valuable apparatus has 

 been greatly increased in efficiency by the application of a device 

 by Mr. E. M. Nelson, providing it with a correction collar, which 

 can be used with the utmost ease, no matter in what position the 

 microscope may be. It is similar in practice to the correction 

 collar of an ordinary objective; it has a steeper spiral slot, and 

 only half a revolution of movement ; a long arm is fixed to the 

 collar, so that it may be conveniently reached by the finger. The 

 whole condenser is represented in fig. 242, and the arm for moving 

 the correction collar is seen on the right of the optical tube : it 

 turns at the slightest touch, and the collar moves only the back 

 lens of the combination, leaving the mount rigid. 



The object of this correctional movement is primarily to increase 

 the maximum aplanatic aperture of the condenser ; this is effected 

 by separating the lenses. If the back of a wide -angled objective be 



