306 



ACCESSORY APPARATUS 



I-'K.. -2 111. I'uker's new achromatic condenser 

 N.A. 1-0. 



iris diaphragm supplied with this condenser is graduated to show 

 tin- N.A. \\hen greater accuracy is required, but the still more 

 accurate method of employing fittings with separate discs with their 

 N.A. marked on them is also supplied by the makers. 



A very complete achromatic condenser is now made by Baker of 

 Eolborn. This condenser is a modification of the well-known Abbe 

 form, in that the diameter of the component lenses is considerably 

 smallei : this reduction in the size of the lenses, allowing, as it does, of 



greater freedom of move- 

 ment of the mechanical 

 stage, has been effected 

 without in any way de- 

 creasing its optical effi- 

 ciency ; on the contrary the 

 aplanatic aperture lias been 



increased, thus rendering 

 it especially suitable for use 

 with high powers. The 

 total aperture is N.A. I'O, 

 of which N.A. 0-90 is 

 aplanatic : the diameter 

 of the back lens is '2'2 mm. 



(]-.', in.) and the power of the condenser as a whole is 10 mm. ( p, in.) 

 with a working distance of 2 - 5 mm. d 1 ,, in.): with the front lens 

 removed for low-power work the power is reduced to 20 mm. ( f lt - in.), 

 and the working distance, which is calculated with the lamp liame 

 at ten inches, is increased to 1O5 mm. (i in.). 



The above is mounted in the usual sub-stage fitting of universal 

 gauge with iris diaphragm and carrier with dark-ground stops, as 

 shown in the Illustration of it in fig. 249. 



it is essential for ideal illumination with transmitted light 

 (l)that the illuminating axial cone should be approximately equal 

 to the aperture of the objective used ; (2) that the object should be 

 placed at the apex of this cone. 



If an objective breaks down with this ideal illumination, which 

 is very probable. \\ e must be content to sacrifice the ideal; or, as 

 is also exceedingly probable, if the object under examination lacks 

 contrast, the ideal method must be modified, lint if we have a 

 suitable object and a perfect objective, it is the strong conviction 

 of some leading expert- bhat, as we increase the cone in aperture, we 

 increase the perfect rendering of the image, until the point is reached 

 \\here the cone from t he condenser is equal to the aperture of the 

 objective. This ideal can be realised with fine apo- and seini- 

 apocliromatics up to '.'> to- I N.A. With the most perfect objectives 

 of the present day of ."> N.A. and upwards we iind in practice that 

 iie.M results are obtained uhei i a cone of ] ight is used which, on 



the removal of the eye piece, is found to occupy three-quarters of 



t he area of t he back lens of t he object ive. 



No condenser is sufficiently free from spherical aberration to 



equal to its own aperiure. ( 'ondeiisers are all more 



or less under corrected, and consequent Iv focus their central rays at 



