52 PKACTICAL PHOTO-MICROGKAPHY 



through a narrow slit which in effect is the condition that 

 holds in objects with fine structure illuminated by narrow 

 beams then the amount of diffraction becomes much larger in 

 relation to the total amount of light actually passing. 



It will be found in practice that the numerical aperture of 

 the condenser should not exceed from two-thirds to three- 

 quarters of that of the objective ; usually a two-thirds cone will 

 be sufficient. It will be understood that this does not mean that 

 the whole aperture of the objective is not being utilised, for each 

 detail of the object is producing its own diffraction image, and 

 throwing broad cones or fans of light which are picked up and 

 utilised by the objective. Consequently the effective working 

 aperture is, in reality, greater than the aperture of the sub- 

 stage condenser itself. 



The proportion of the aperture of the objective which is 

 being filled by the condenser may be approximately determined 

 by removing the ocular from the microscope and looking down 

 the body-tube. If the aperture of the condenser equals or 

 exceeds that of the objective, then the back lens of the latter 

 will appear to be filled with light ; on the other hand, if the 

 condenser aperture is less than the objective, there will be 

 a greyish-black marginal zone, and in the centre of this a 

 brightly illuminated area. The relation of the diameter of 

 this bright area to the whole objective aperture indicates 

 what proportion of the total available aperture is being used. 



Achromatic condensers are now made either dry or oil- 

 immersion in the same way and for the same purpose that 

 objectives are so made. It will be advisable for the photo- 

 micrographer to obtain a condenser of the highest possible 

 aperture, since, as already pointed out, he can always reduce 

 it with the iris -diaphragm ; or he can for low-power work 

 remove the top lens of the condenser-combination, and so 

 obtain one which will illuminate a larger field, and which is 

 therefore suitable for use with medium or low powers. 



For work of the most critical nature the image of the radiant 

 may be projected and carefully focussed in the plane of the 

 object ; but for many purposes, especially when photographing 

 with radiants of small area, this particular method of obtaining 

 critical illumination may be departed from. In focussing the 

 image of the radiant, however, care should be taken that where 



