Accessories. 515 



phragms, as by their means results are more easily and 

 uniformly obtained in inexperienced hands. 



The Webster achromatic condenser is another pattern 

 supplied by nearly all microscope- dealers, and at one time 

 it was very popularly used; but it is rather antiquated 

 now, and is suitable only for comparatively low powers on 

 account of its small aperture. Another method often 

 adopted is to have a fitting in the sub-stage, with a screw 

 to take an objective, and low-power glasses fitting this way 

 will make a very efficient form of condenser. Swift and 

 Sons make a very excellent achromatic condenser with an 

 air angle of 150, and for the most critical work of the 

 highest character the firm of Powell and Lealand make a 

 condenser of very large aperture of the apochromatic form. 

 This is very expensive, and requires skill in management, 

 but if the very best is desired this is unequalled. It is espe- 

 cially appropriate for use with the apochromatic objectives. 



Now as to the practical working with the condenser. It 

 will always be found best to employ the edge of the lamp- 

 liarne, as its image can then be projected into the field of 

 view; and we would here mention that it is a most im- 

 portant element in microscopy that a suitable lamp should 

 be employed. (For notes on lamps see page 59.) A plane 

 mirror should be used. The first step is to centre the 

 condenser accurately with the objective, and herein is 

 advantage of the centring sub-stage. Where no sub-stage 

 is fitted to an instrument, it is understood that the under- 

 fitting which receives the condenser is central ; but this 

 cannot be always perfectly accurate, for if a condenser be 

 rotated in the sub-stage it will probably be found that it 

 will be central in one position only. It is, therefore, essen- 

 tial that before starting any work this should be accurately 

 centred. Some condensers have fitted on top of them a 

 removable cap with a very small pin-hole. This pin-hole 

 should be focussed with, say, an inch or a half-inch objective, 

 the condenser centred by it ; then the cap should be removed 

 from the condenser for working. Condensers not having 



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