VISUAL WORK WITH THE MICROSCOPE. 495 



as to be a little higher than the mirror. Having then roughly 

 focused the object, we proceed as follows : 



1. Centre the edge of the flame on the field, partly by means 

 of the mirror, and partly by moving either the lamp itself or the 

 microscope laterally. 



2. Centre the substage condenser, by bringing the substage 

 iris into view in the field, and centring it with the substage 

 centring screws. 



3. Having then opened the substage iris, focus the image of 

 the flame again on the field, and re-centre it (see No. 1). 



4. Adjust the collecting lens in front of the lamp, and focus it, 

 so as to throw a nearly-sharp enlarged image of the edge of the 

 flame on the surface of the mirror, keeping the collecting lens 

 rather within than without the focus. 



5. Close the collecting-lens iris, so as to let it show on the field, 

 focus it, and centre it by moving the collecting lens from side to 

 side and by raising or lowering it taking care not to move the 

 mirror or the microscope After which, the collecting-lens iris 

 may be opened up. 



6. If the field is not then fully illuminated, even to the extreme 

 edge, the lamp is too far off, and must be brought a little nearer, 

 or until the whole field is fully illuminated. 



7. The ocular should now be temporarily removed, and the 

 back of the objective examined, the substage iris being fully opened 

 up for the purpose. If crossed by a streak form of illumination, 

 the lamp and collecting lens are too near. If illuminated by a 

 circular image, but not quite to the edge, the N.A. of the con- 

 denser is too small for that of the objective. If very bright in 

 the centre and fading away in brilliancy towards the edge, the 

 condenser is not aplanatic, or the collecting lens is not properly 

 adjusted in relation to the lamp. 



(Here I may remark that the objectives which are most 

 difiicult to illuminate, as described, are low powers with an 

 unusually large N.A.) 



Having adjusted the illuminating system, it will possibly be 

 found that the object is flooded with light, and this must be 

 corrected by cutting down the substage iris, as previously 

 described. 



Suitable complementary^olour screens may now be introduced, 

 and if the object to be examined only occupies a part of the 



