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; ^. 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 



