306 ACCESSOKY APPARATUS 



iris diaphragm supplied with this condenser is graduated to show 

 the NjL when 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 

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

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

 smaller : 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 has been 

 increased, thus rendering 

 it especially suitable for use 

 with high powers. The 

 total aperture is N.A. TO, 

 of which KA. 0-90 is 



FIG. '249. Bakers new achromatic condenser .. . , ... 



N.A. l-o. aplanatic : the diameter 



of the back lens is 22 mm. 



(-}-- in.) and the power of the condenser as a whole is 10 mm. ( ^ in.) 

 with a working distance of 2'5 mm. (^in.): with the front lens 

 removed for low-power work the power is reduced to 20 mm. (-J^ T in.), 

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

 at ten inches, is increased to 10'5 mm. (^f 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 

 (1) 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, w r hich 

 is very probable, we 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. But if we have a 

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

 of some leading experts that, as we increase the cone in aperture, we 

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

 where the cone from the condenser is equal to the aperture of the 

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

 apochromatics up to '3 to -4 N.A. With the most perfect objectives 

 of the present day of *5 N.A. and upwards we find in practice that 

 the best results are obtained when a cone of light is used which, on 

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

 the area of the back lens of the objective. 



No condenser is sufficiently free from spherical aberration to 

 transmit a cone equal to its own aperture. Condensers are all more 

 or less under-corrected, and consequently focus their central rays at 



