304 



ACCESSORY APPARATUS 



power>. 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 bv means <>f a cemented triple back lens, as is shown in the 

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

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

 aperture is I'.'!.'!, the aplanatie aperture being in excess of T25. 

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

 of the lens is ,'^ths inch, and it works through a slip of '()7'.\ 

 t hick. 



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

 medium powers, magnifying Sths 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 5-ths 

 inch. It is shown here 



principally 



PIG. 245. Watson's parachromatic condenser. 



mounting, 



for the 

 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 1ST. A. 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 we illustrate in fig. 

 iMii 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 

 objectiv< . The X.A. varies from 

 I '!"> to 1-4, and the aplanatic 

 cone is about I'.'i 1ST. A., the 

 \\orking distance being fully ()('>. 

 We can speak highly of this 



instrumenl : it is in our iudir- 



" 16 Beck - new acl Mt.ic , ' 



,,,,. ment the best condenser ever 



made by tin's firm. 



Another condenser lias been made recently by the same firm. 

 A. of l-ii. the maximum obtainable without immersion 



