1058 Record op current researches relating to 



would be. A prism or other object being simply moved round on a 

 sector so placed, will evidently remain unchanged in its distance fz-om 

 that central point. 



" In constructing the illuminating prism, it was to be recollected 

 that there was but one direction in which the light could be placed, 

 viz. in the plane of the object, or say one-tenth of an inch above the 

 plane of the stage, and vertically to the sector's plane ; and, secondly, 

 that the distance of the light from the stage must be assumed. The 

 prism, therefore, necessarily reflects the rays at a greater angle than 

 90°, and its reflecting surface usually requires silvering. This has 

 been assumed to be an objection ; but the light is still more than 

 ample, as well as beyond that given by most other illuminators, the 

 prism having (although a triple combination) only two uncemented 

 surfaces. I have, from my own experience, adopted a distance for the 

 source of light of about 15 inches, as most useful for general work ; 

 but should a distance of 2 feet or upwards be selected, then the prism 

 may be one of total reflection and its reflecting surface consequently 

 remain unsilvered. 



" The manner of using the instrument is, shortly, as follows : — 

 The microscopist will, of course, place it as he would any other 

 Microscope, conveniently on a table, and incline it to the desired angle 

 for work. The lamp, or other source of light, is to be placed directly 

 opposite, and in front of, the instrument, and at the proper distance of 

 height, the distance being always the same, and the height that which 

 brings the light into the plane of the upper plate of the stage. The 

 adjustment may be verified and corrected as follows : — Place a slider 

 with a grayed surface on the stage (grayed surface upwards) ; move 

 the prism to the lowest point of the sector (or to zero), and turn it 

 directly outwards, or towards the light ; adjust the distance of the 

 prism from the grayed surface, so that an image of the light is formed 

 upon the latter ; and looking through the tube of the instrument (the 

 lenses being removed), observe if the image formed on the grayed 

 glass be central with the tube ; if not, make it so by a slight altera- 

 tion in the inclination or azimuth of the instrument, without varying 

 its distance from the light. It is by no means necessary to make these 

 adjustments accurately ; but the more accurate they are, the more 

 perfectly will the image on the grayed surface only revolve, and without 

 changing place, on moving the prism on the sector. It is, perhaps, 

 unnecessary to observe that the Microscope, without making any of 

 these adjustments, may be used in the same manner as, and with all 

 the convenience of, an ordinary instrument, while, by making the 

 adjustments as described, we obtain the peculiar advantages sought for 

 in the construction. 



" These advantages may be shortly summed up as follows : — An . 

 object being placed upon the stage, and the focus adjusted, the observer 

 can examine it under every azimuth of illumination by revolving the 

 stage, and under every possible kind of illumination in each azimuth, 

 viz. direct transmitted light, oblique transmitted, dark-ground illu- 

 mination, and, finally, the illumination for opaque objects, by simply 

 moving the prism on the sector ; and he can do all this without once 



