304 



ACCESSORY APPARATUS 



FIG. 245. Watson's parachromatic condenser. 



powers, for we know of no similar instrument that yields so large a 

 ' solid cone ' of illumination. The construction is an unusual one, 

 the corrections for both spherical and chromatic aberrations being 

 effected by means of a cemented triple back lens, as is shown in the 

 illustration of the optical system in fig. 244. The only flint glass 

 used in it is the middle of the triple back. The total numerical 

 aperture is 1'33, the aplanatic aperture being in excess of 1'25. 

 The magnifying power is J inch, and the clear aperture at the back 

 of the lens is y%ths inch, and it works through a slip of '073 

 thick. 



With the front lens removed it is an efficient dry condenser for 

 medium powers, magnifying fths inch, with a total N.A. of -56, the 



aplanatic aperture being 

 over '5. It is mounted 

 like their ' Parachro- 

 matic condenser ' shown 

 in fig. 245, which is also 

 a very useful instru- 

 ment, with a total N. A. 

 of I/O, a power of fths 

 inch. It is shown here 

 principally for the 

 mounting, which is 

 identical with that used 

 with fig. 244. The 



collar into which the optical part fits carries an iris diaphragm ; on 

 the diametrical edge of this is engraved a scale showing the NJL at 

 which the condenser is working when the iris diaphragm is in a 

 given position. We have used this condenser with much pleasure 

 and profit, and can commend it as a truly valuable instrument and 

 yet remarkably low in price. 



A. condenser satisfying modern necessities has also recently been 



made by Messrs. R. and J. 

 Beck, which w r e illustrate in fig. 

 246 in its complete condition. 

 The optical combination consists 

 of four systems of lenses, the 

 front of which is a hemisphere, 

 with three combinations behind, 

 and the whole is constructed on 

 the principle of an oil-immersion 

 objective. The N".A. varies from 

 1'35 to 1*4, and the aplanatic 

 cone is about 1*3 N.A., the 

 working distance being fully "06. 

 We can speak highly of this 



instrument ; it is in our judg- 

 FiG.246.-Beck's new achromatic ment the begt condenser eyer 

 coiiaenbei. . . 



made by this firm. 



Another condenser has been made recently by the same firm, 

 with K.A. of 1*0, the maximum obtainable without immersion 



