SUBSTAGE CONDENSERS, ETC. 99 



fill the lens to a greater amount will cause two dots of black to appear on each side 

 of the lamp flame, which then becomes immediately recognisable. The last point 

 before the appearance of these black dots (really due to spherical aberration) indicates 

 the largest aplanatic cone of the condenser. Slowly and cautiously close the iris 

 diaphragm until it is just visible and measure the size of the aperture with compasses 

 as before. If the diameter is slightly greater than the previous measure for the 0-95 

 lens, the aplanatic cone is, of course, just above 0-95. A little experience and thought 

 will soon render these operations quite easy, and the microscopist will be able to 

 readily compare the largest aplanatic cone of the condenser he is testing with its 

 advertised N. A., and the performance of one condenser with another. 



The difference which so often exists in these two measures is very striking, and is 

 said to be mostly due to errors in spherical aberration, most condensers being more 

 or less under-corrected Fig. 52, and consequently focus their central rays at a greater 

 distance than their jperipheral ones. If a condenser be well corrected the lamp-flame 

 image, as seen on looking down the microscope with the eye-piece in situ, should be, 

 when accurately focussed, intensely bright, whilst the field is commensurately dark ; 

 but very frequently this darkness is conspicuous by its absence. 



It may be here asked what does it matter even if the condenser should be badly 

 corrected and possess a small aplanatic cone ? It is this. The object of a condenser 

 is to bring as much of the light of the illuminant as possible to a focus on the object. 

 If now all the rays do not come to the same focus, all of those which come to another 

 are so much lost, and only serve to scatter light into the field, and besides this, when 

 using a broad illuminant (such as is produced after obtaining critical light with the 

 edge of the flame, by turning the lamp-flame broad side on), not only is there an 

 unequal illumination of the field which is immediately apparent, but no critical light 

 is obtainable at the margins of the field without losing it in the centre, and vice versa. 

 As now the best of definition will only lie over the area where critical light exists, so a 

 critical image cannot be obtained over the wliole field at one and the same time, and 

 appearances may thus be produced in that part where it is absent which do not in- 

 reality exist. Again, in using oblique light, the loss at the margins of field is a serious 

 matter. Two small pictures are shown in Figs. 3 and 4 , Plate III, one giving the effect 

 of the condenser out of focus over the whole field, and the other with true critical 

 light. When we come to the photography of bacteria and diatoms, and other minute 

 objects, it will be seen that neglecting to obtain this actual critical light over the 

 whole field may be a source of positive evil. Compare Figs. 2 and 5, Pla,te IK. 



4. The definition given by a condenser is important so far as it goes m one respect 



