178 THE MICROSCOPE. 



ber of the aperture as marked on the graduated circular 

 plate. Three of these apertures have central discs, for 

 circularly oblique illumination, allowing only the pas- 

 sage of a hollow cone of light to illuminate the object. 

 The illuminator above described is placed in the second- 

 ary stage i i, which is situated below the general stage 

 of the microscope, and consists of a cylindrical tube 

 having a rotatory motion, also a rectangular adjust- 

 ment, which is effected by means of two screws I m, 

 one in front, and the other on the left side of its frame. 

 This tube receives and supports all the various illumi- 

 nating and polarising apparatus, and other auxiliaries. 



Very many modifications 

 of Gillett's condenser are 

 known to microscopists, by 

 far too numerous to de- 

 scribe in detail. Boss's im- 

 proved form is made to slip 

 into the sub-stage in the 

 same way as his Improved 

 Achromatic Condenser (fig. 

 110), and when arranged 

 for oblique illumination, is 

 an extremely efficient instru- 

 ment. The optical part is 

 FIG. no. Ross's improved Achro- similar to a T 4 oths object- 



matic Gillett Condenser. glass. It has two Sets of 



revolving diaphragms with apertures and stops, for 

 showing surface markings in a brilliant manner. 



Directions for Using Gillett's Condenser. In the ad- 

 justment of the compound body of the microscope for 

 using with Gillett's illuminator, one or two important 

 points should be observed first, centricity, and second- 

 ly, the fittest conpensation of the light to be employed. 

 With regard to the first, place the illuminator in the 

 cylindrical tube, and press upwards the sliding bar k in 

 its place, until checked by the stop; move the microscope 

 body either vertically or inclined for convenient use ; 

 and with the rack and pinion which regulates the slid- 

 ing bar, bring the illuminating lens to a level with 

 the upper surface of the object-stage ; then move the 



