56 Modern Microscopy. 



Either of the Abb4 illuminators or achromatic condenser 

 1*0 N.A. will be found the most generally efficient and 

 useful. The former is cheaper than the latter, but the 

 advantages of achromatism make the latter more desirable. 



The Spot Lens. 



Other means are provided for obtaining dark-ground 

 illumination, and consist of the spot lens and the para- 

 boloid. Owing to the perfection with which this effect can 

 be now obtained with the sub-stage condenser, these have 

 been largely superseded. They are, however, often used, 

 and preferred by some to the condenser. It must be under- 

 stood that they cannot take the place of the condenser for 

 ordinary direct transparent illumination. The method of 

 using is simple, the spot lens being intended for low powers 

 and the paraboloid 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, it should be moved 

 up and down in the sub- stage fitting 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 lamp-flame, will give an additional 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 re- 

 commend it. 



The Polariscope. 



This consists of two parts, each composed of a Nicol 

 prism in a suitable mounting one called the polarizer, 

 which fits into the sub-stage, and the other the analyzer, 

 which is inserted between the nosepiece of the microscope 



