302 ACCESSORY APPARATUS 



tor low powers. \Vhen the highest class of work li;is to be done it 

 ;.s- ,-,-,//'/// tn have condensers nnif<"f 1<> tli<> power of the objective used. 



\ ilrv apitchromatic condenser of merit is made by Swift and 

 Son; it. has a X.A.of O95 and an aplanatic cone approximating 



n-'.i and works witli ease through any object- 

 slide, but is corrected to do this by thinning 

 The front lens and setting the front and back 

 < liiibinat ions further a part than would be the 

 case if they were used as an objective. The 

 lower combination lias a large, clear aperture. 

 The optical part of this instrument is shown in 

 lig. '2 H ; we have used it. and find it a tho- 

 roughly practical and serviceable condenser. 



Ki. -'ii -Swift's- Before the introduction of the homogeneous 



rlmmiiitio ilMii'.i) con- system, and the production of such great aper- 

 1 x A 0'95. turns by Powell and Lealand as a 1 ;"> in a ^-th, 



a /..tli, and a .,',,th 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 snl isequent introduction of the apochromatic system of lenses, 

 much larger cones were required. To meet this necessity Powell 

 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 

 mi 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, having an 

 aperture of 1 - 40, and will work through a mounting slip of - ()7, and 

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

 unapproachei 1 . 



Messrs. Powell and Lealand have produced an entirely ne\\ 

 condenser, strictly apochromatic, employing a Huorite lens in the 

 combination, and presenting features in the highest degree desirable. 

 We find its K.A. to lie (>'.).>, its focal length long enough for a thick 

 slip, its aplanatic aperture '.). We haw found it of the utmost 

 pra-.'tical value in critical work, and this valuable apparatus has 

 been greatly increased in efficiency by the application of a device 

 ly Mr. E. .M. Nelson, providing it \\ith n correction cnlltir, which 

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

 microscope may lie. It is similar in practice to the correct ion 

 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, sn that it may lie conveniently reached by the finger. The 

 whole condenser is represented in lig. iU 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 onlv the back 

 Icn^ of i he combination, lea\ ing the mount riyfid. 



The object of this correct ional movement is primarily to increase 



the maximum aplanatic aperture of the condenser ; this is ell'ected 



fating the lenses. 1 1' the hack of a wide angled objective be 



