380 



INVERTED MICROSCOPE. 



description of it that will be readily understood by referring* 

 to the figure. The most important part is a four-sided prism, 



with the angles a, b, c, d 7 respectively 55°, 107^ 



521° , 



145 c 



the angles being of such dimensions that a ray of light passing 

 into the prism in the directions shown by the arrows, and per- 

 pendicular to the surface a d, after undergoing total reflection 

 from the inner surfaces a b and b c (on both of which the light 

 strikes at an angle much less than forty-five degrees), will pass 

 out perpendicular to the surface c d. If the line c be followed, 

 it will be readily seen how a ray of light passing through the 

 object-glass B descends into the prism and passes out of it up- 

 ward through the eye-glass D, the tube of which is inclined to 

 the perpendicular 35°. The other parts of the instrument are 

 understood by simply looking at the figure. E is a heavy sup- 

 port that revolves on another support H, which carries a column 

 I, on which are placed the mirror, diaphragm, etc. The prism 

 used has each side nearly an inch in length, and little less in 

 width, which is about the most convenient size. The arrange- 

 ment for adjusting the focal distances is somewhat peculiar, and 

 is readily understood by refer- 

 ence to fig. 4. There are three 

 tubes (the outer one of which is 

 F) that slide on each other; the 

 inner is fastened to the plate O ; 

 the second tube has a projecting 

 collar, on the under surface of 

 which rest the extremities of two 

 springs y, and on the upper sur- 

 face two points of the lever X, 

 which is moved by means of the screw T. The plate O is 

 fastened on to the top of the prism by the binding-screw L 

 (fig. 1), that readily allows of the plate being detached at 

 pleasure, which it is necessary to do at times in order to wipe 

 the upper surface of the prism. The way in which the observer 

 operates is to screw one or other of the object-glasses to a small 

 cap, K (fig. 4), that simply rests on the upper end of the outer 

 tube F, which is readily moved up and down by the finger for 

 the coarser adjustment, while the minute adjustment is obtained 

 by moving the screw T. 



This description suffices to make it clearly understood how 



Fig. 4. 



