THE OPTICAL EQUIPMENT 49 



solid cone may be of widely varying dimensions ; that is, it 

 may have a narrow or broad base, the latter being as wide as 

 the condenser itself is capable of transmitting. The cone is 

 narrowed by closing the iris-diaphragm below the condenser, so 

 that the light entering the condenser is reduced and confined 

 to a greater or less proportion of its area. To obtain what is 

 known as a hollow cone, the central rays are stopped out by 

 introducing below the condenser a central stop varying in size 

 according to the objective being used. This results in the 

 illumination of the object by an annular cone of rays, no direct 

 light, but only that reflected or refracted by the object, entering 

 the objective. 



In cases where critical illumination is required, the image of 

 the radiant then being projected on to the object, it is of the 

 first importance that the illuminating cone should be aplanatic. 

 A short table is given later showing the relation between the 

 N.A. and the size of the aplanatic cone in well-known types of 

 condensers. 



The deficiencies of the chromatic condenser are not only its 

 want of spherical correction, but also, as its name shows, that 

 it is not chromatically corrected. An achromatic condenser, 

 therefore, gives an%H!age of any radiant that may be in use 

 which is of a very much higher order than the simpler type. 

 Achromatic condensers are to be obtained both dry and oil- 

 immersion. The dry ones have total apertures, usually of 1-0, 

 and an aplanatic aperture varying from 0-9 to 0-95. They 

 also vary in focal length ; in fact, to obtain such a high aperture 

 it has been found necessary to make them of short focus. The 

 average focal length of a condenser of this type is one-fifth of 

 an inch. The result is that the image of the radiant as seen 

 in the microscope is small, and is not usually sufficiently large 

 to illuminate fully the field with any other than a high-power 

 objective. To meet this difficulty most achromatic condensers 

 are made so that the top combination may be removed, the 

 effect being that the focal length is increased. The image of 

 the radiant is therefore spread over a larger area, and conse- 

 quently will evenly illuminate an object with a medium-power 

 objective. This is a perfectly satisfactory method with most 

 sub-stage condensers either dry or oil-immersion, and their 

 efficiency is not substantially lowered thereby. 



