VERTICAL ILLUMINATORS. 



75 



interior illuminator. The best objects to show the method 

 of using these accessories the student will find to be the 

 Podura scale and the diatom Navicula rhomboides, or the 

 Amphipleura pellucida, which will be illustrated in the next 

 chapter. 



Messrs. Powell and Lealand's vertical illuminator con- 

 sists of a highly polished plate of glass, set at an angle of 

 45 in an adapter which is screwed on over the objective. 

 There is an aperture in the side furnished with a revolving 

 diaphragm, through which the illuminating rays are made 

 to pass. These rays are reflected downwards, through the 

 objective, upon the object, as may be more easily under- 

 stood on reference to Fig. 54 a and b. 



FIG. 54. FIG. 55. 



Beck's vertical illuminator is very similar to the above, 

 but the plate of glass is replaced by a thin glass disc or 

 thin cover attached to a milled head, as shown in Fig. 55. 

 This seems to be a more handy form than the first, and for 

 ordinary instruments is perhaps the best. Still the author 

 must admit that with Messrs. Powell and Lealand's work- 

 manship their form of vertical illuminator is all that can be 

 desired. In December 1880, Mr. Powell exhibited at a 

 "Scientific evening" of the Royal Microscopical Society 

 Amphipleura pellucida lighted up with this illuminator, 



