348 On the Illumination of Objects under the Microscope. 



first sight quite hopeless to get results in the short space 

 between the microscope-objective and the object, and I was 

 much surprised to find how well the method worked, even 

 under these unfavorable conditions. On looking over tho 

 ' Beiblatter' of the Annalen I found, after finishing my obser- 

 vations, that a similar attempt had been made by W. Seibert, 

 and described in the Zeitschrift fur ln*trume?itenk uncle, ii. 

 p. 92 (1892) . With what success the work was attended I 

 do not know, since, at the present moment, I am unable to 

 get access to the early numbers of this journal. 



With this preliminary statement I feel warranted in 

 presenting to the Society the results of my observations, as I 

 feel sure that in certain cases the method can be used to 

 advantage in the examination of microscopical objects, in 

 which the detail is lost owing to an almost uniform optical 

 density. 



I chose as an object some powdered glass, immersed in 

 cedar oil of the same mean refractive index. The glass 

 particles were almost invisible under ordinary conditions of 

 illumination, as by the use of the substage condenser and 

 diaphragm it was just possible to see the outline of some 

 of the larger particles, but nothing whatever could be seen of 

 their actual form. 



I then arranged the illuminating system as follows : — 

 A screen bounded by a straight edge is placed in front of an 

 incandescent gas-lamp, so as to cut off half of the mantle and 

 give a source of light bounded by one perfectly straight side. 

 (A gas-flame with a horizontal screen does about as well.) 

 A small lens of very short focus was placed on the stage, or 

 rather below the stage, as close as possible to the object. 

 The lamp was at a distance of six feet, and the light reflected 

 from the mirror was brought to a focus by this lens, passing 

 through the object on its way. An image of the lamp is 

 formed in space by the lens, and by raising the tube of the 

 microscope the image can be seen. A f-inch objective was 

 used, the difficulties increasing as we use objectives of shorter 

 focus. A little strip of thin brass, with a carefully-cut 

 straight edge, was fastened to the stand carrying the " bull's- 

 eye " condenser with a bit of wax, and moved into position 

 between the objective and object, so as to cut off the flame- 

 image with the exception of a narrow thread of light along 

 the straight edge. It is important to have the brass screen 

 set accurately in the plane of the flame- image with its edge 

 parallel to the straight edge of the flame. This adjustment 

 must be made with the microscope focussed on the flame- 

 image, the brass strip being raised and lowered until it is also 



