4 Scientific Proceedings, Royal Dublin Society. 



The cover-glass reflector should be inclined 33^ degrees with the 

 optical axis of the microscope. 



In the figure the illuminator is lettered I. The cover-glass, 

 C, reflecting the light, is shown at 33° with the vertical axis of the 

 microscope. This insures that the ray falling vertically through 

 the object-glass is at the polarizing angle (57° nearly) with the 

 glass reflector. It would be convenient if this angle was marked 

 by the makers on the rotating head commanding the reflector. 

 Or the correct position for the source of light (if artificial) may be 

 found by arranging it to be 25 units of length above the plane 

 of the aperture and 76 units distant. This gives a beam reaching 

 the reflector at the proper angle ; and it only remains to rotate the 

 reflector till the eye, looking through the microscope, perceives the 

 speculum reflector illuminated. 



The larger part of the entering beam passes through the 

 cover-glass, and is absorbed in the blackened walls of the illumi- 

 nator. The reflected part is plane polarized, the vibration being 

 executed parallel to the plane of the mirror. Descending, it 

 passes first through the slip carrying the rock-section, it being 

 advisable to invert the slip from its usual position, placing it 

 face downwards, so as to bring the section nearer the speculum 

 reflector. Then passing through the section, it meets the reflector 

 of speculum metal (R) ; and is returned by it through the section, 

 and so back to the cover-glass C. The greater part of the beam 

 again passes through, reaching the eye, some part being returned 

 to the source of light. What passes through is almost unmixed 

 plane polarized light. 



A lens placed in front of the aperture may be used to intensify 

 the illumination. It has the effect of diluting with ordinary 

 light the polarized beam somewhat, as rays from it are converging, 

 and therefore do not all strike the glass at the polarizing angle. 

 The loss of intensity of coloration is but trifling, and does not 

 diminish the value of the arrangement as a means of diagnosis. 



The usefulness of the reflector R is increased if it is perforated 

 (I think preferably excentrically) with a conical hole, so that the 

 upper edge is as sharp as possible. This hole should be blackened 

 on the inside. The use of it is obvious. A small crystal can 

 be examined simultaneously by the reflected ray and by light 

 transmitted from the polarizer beneath the stage : the crystal 



