THE OPTICAL EQUIPMENT 



57 



rays by a fixed stop in the objective itself. As, however, this 

 method is to a large extent superseded by the paraboloidal and 

 similar sub-stage condensers lately introduced, it would not 

 appear to be advisable to adopt it, for it involves a permanent 

 alteration of the objectives. One portion of the optical 

 combination of the objective requires to be ground away, or a 

 permanent dark stop fixed ; so that the lens as it stands is 

 useless for ordinary observation. To render it useful for 

 ordinary work, a special front -lens must be provided which 



FIG. 15. Diagram of Path of Bays (a) in a Paraboloidal Condenser; (6) in 

 Dark-ground Illuminator with Spherical Surface. 



is interchangeable with that suitable for the dark-ground 

 illumination method. 



The best modern appliances which are particularly suited 

 for use with high-power dry lenses, and which may be used 

 for the illumination of living bacteria or similar unstained 

 objects, are the Zeiss paraboloidal condenser, or one by Leitz, 

 similar in purpose but different in design. 



The Zeiss arrangement (shown diagrammatically in Fig. 15 a) 

 is an improved oil -immersion paraboloid, practically the same in 

 design as has been in use for many years, but mathematically 

 calculated to yield a perfect image, which the earlier paraboloids 

 certainly did not produce. The apparatus is calculated to 

 eliminate both spherical and chromatic aberration. The illu- 

 mination is effected by internal reflection of the light within the 

 paraboloid, and relies but to a very small extent on refraction, 



