TOLLES" TRAVERSE LENS. 181 



and concave surfaces being of no effect, the two exterior 

 plane surfaces of the traverse system constitute it a prism, 

 and every slightest movement of this concave facet lens 

 on the traverse lens, T, would give a different prism to 

 infinite variety. In this arrangement the concave mir- 

 ror can be used in the ordinary manner and condense 

 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 1J inch focus, and about 

 ten (10) degrees 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 inspec- 

 tion. This would be well for parallel rays. If diverg- 

 ing rays are used another lens of two or three inches 

 focus, mounted on the arm, A, will conveniently take 



