114 



PRACTICAL TREATISE ON THE MICROSCOPE. 



other, to the outer one, b, is attached a flat plate, a, which 

 slides underneath the stage, and is adjusted for distance by 

 the screw, f; at c is seen a milled head, which is connected 

 to a pinion, and by means of a rack attached, the inner tube, 

 carrying the achromatic combination, d, is raised or depressed ; 

 the upper part of the outer tube, b, is larger than that at c, 

 this is for the purpose of allowing the miUed ridge of the 

 achromatic combination to pass up and down freely. For the 

 low powers, such as the half and quarter of an inch, the com- 

 bination, d, only is used ; but with the higher powers, the 

 second part, e, may be slipped over d, whereby the focal point 

 of the illuminating rays will be materially lessened in dia- 

 meter, although increased in brilliancy. The flat mirror 

 is generally used as the reflector or the prism described in 

 page 118. 



Wlien the compound body can be turned away from the 

 stage, the adjustment of the axes of the illuminator and ob- 

 ject-glass is a very simple matter, the only movement required 



in the condenser is that of 

 either increasing or di- 

 minishing the distance the 

 flat plate, a, has to slide 

 through ; this is done either 

 by screwing or unscrewing 

 the screw, f, until the spot 

 of light formed on the ob- 

 ject by the illuminator is in 

 the centre of the field of 

 the object-glass. But when 

 the compound body is a 

 fixture, then it is neces- 

 sary that the condenser 

 should have two adjust- 

 ments ; a section of such a 

 condenser is represented 

 by fig. 59, as constructed 

 by Mr. Ross, a a exhibit 

 Fig. 59. the plate by which it is 



