OP THE MICROSCOPE 161 



The device is represented in the annexed figure, where P is 

 the basilar plate of the whole traverse system, having a circular 

 groove and track, in which the carriage, C, moves. On a pro- 

 jecting arm, A, of the carriage, C, are mounted whatever ap- 

 pliances are to be used to modify or direct the light upon the 

 traverse lens, T, in the direction of the object at the centre of 

 the system. 



In the figure the concave lens, N, is shown in position on the 

 arm. Thus situated, the interior convex and concave surfaces 

 being of no effect, the two exterior plane surfaces of the traverse 



Fig. 51. TOLLE'S ILLUMINATING TRAVERSE LENS. 



system constitute it a prism, and every slightest movement of 

 this concave facet lens on the traverse lens, T, would would give 

 a different prism to infinite variety. In this arrangement, the 

 concave mirror can be used in the ordinary manner and con- 

 dense light enough upon the object for all ordinary purposes. 

 The full interior aperture of a dry objective would be reached 

 at the very convenient obliquity of 41, i.e., at less than the 

 critical angle, or angle of total internal reflection between 

 crown-glass and air. L is a double-convex condensing lens, 

 that may be placed at about its principal focal distance from 

 the object. 



For a condenser, with the size of apparatus as drawn in the 

 figure, a simple lens of l inch focus, and about ten degrees 

 (10) of aperture is convenient, and if the lens is movable 

 along the arm, A, it can be focussed readily on the object, the 

 position being fixed by inspection, This would be well for 

 parallel rays. If diverging rays are used, another lens of two 



