Illumination and Illuminating Apparatus 101 



with the top lens removed become low-power condensers of 

 moderate aperture. 



If the advice given be followed and the condenser used 

 intelligently, reliance will be placed in the work performed 

 and far superior results secured than would be possible with 

 a badly corrected chromatic condenser. 



The Spot Lens. 



Before the sub- stage condenser came into general use the 

 spot lens and paraboloid were largely utilized for obtain- 

 ing dark-ground illumination. They have, however, been 

 to a considerable extent superseded, owing to the perfection 

 in which the same effect can now be obtained with the 

 condenser. They, are, nevertheless, often employed, and 

 preferred by some to the condenser. It must be understood 

 that they cannot take the place of the condenser for ordinary 

 direct illumination. The method of using is simple, the spot 

 lens being intended for low powers up to J inch, and the para- 

 boloid for higher ones. With both of them a plane mirror 

 and the flat of the wick of the lamp should be used. If the 

 spot lens be employed, the sub-stage that carries it should 

 be moved up and down until a perfectly black ground is 

 obtained ; if additional brilliancy is required on the object, 

 a stand condenser interposed between the lamp and the 

 mirror, with the convex side of the condenser towards 

 the mirror, will give a brighter effect. The paraboloid is 

 adjusted in a similar manner, but instead of having a fixed 

 black spot on the top of the lens it has an adjustable one, 

 and the pin carrying this black spot should be moved up 

 and down until the best effect is obtained. This latter is 

 far more expensive than the spot lens, and the advantage 

 gained is so slight that we can hardly recommend it. 



For one class of work the spot lens is especially advan- 

 tageous. Most sub-stage condensers have a very short 

 focus, and if organisms in water in a trough are being 

 examined, it is impossible to focus the condenser accurately 



