Illumination and Illuminating Apparatus 95 



consistent ivith its outstanding spherical aberration not too much 

 interfering with the highest results, and is the limit of the 

 condenser for critical work. Any further advance of the 

 condenser gives merely annular illumination, which, of 

 course, is to be avoided, excepting when stops are used.* 



How to use the Condenser. 



The condenser requires as much care and skill in adjusting 

 as the objective, for if it be improperly set up it will give 

 rise to ' false images.' For an objective to work at its 

 best, it will be necessary to focus the image of the edge of 

 the lamp-flame sharply upon the object on the stage, and 

 this, with the modern condensers of large aperture, requires 

 to be as accurately performed as the focusing of the objec- 

 tive upon the object ; hence the value of the fine adjustment 

 to the sub-stage (see page 17). The following will be the 

 procedure : 



A proper microscope lamp, as described on page 107, 

 should be set in front of the instrument with the edge of 

 the wick towards the microscope, and the light from the 

 lamp may be allowed to fall directly upon the condenser, 

 or a plane mirror may be used. The sub-stage condenser 

 should now be centred, first having been placed in about the 

 position that it will occupy when focused. The centring 

 cannot, of course, be properly effected without a centring 

 sub-stage ; but where there is only a fixed under-fitting, it 

 is well to set the condenser at the position where it is most 

 central. It is understood that the under-fitting is centred 

 with the optical axis of the microscope when sent out by 

 the makers ; but owing to the fitting-tubes being more or 

 less elliptical, it often happens, if the condenser is rotated 

 in the under-fitting, that it will be central in one position 

 only, and at this position it should be placed for working 

 when there are no centring screws. Some condensers have 



* E. M. Nelson, English Mechanic, November 16, 1888 (vol. xlviii. 

 No. 1,234), and 'The Microscope and its Revelations,' by Dr. Dallinger. 



