THE MICROSCOPE. 203 



its surface, a bright spot may be seen from almost any direction. 

 This proves that the polish of the front is not perfect, and that 

 the light is radiated. That the spot of light could not be seen 

 when condensed upon a perfectly smooth surface, may be illustrated 

 by condensing upon a well silvered clean mirror, a beam of sunlight 

 and looking at it from various directions. "When the eye is in the 

 path of the reflected ray, it becomes visible but at no other posi- 

 tion. 



. Dr. Detmers states that it is necessary that the immersion fluid 

 should have an externally convex edge, the light being condensed 

 by its inner concave surface upon the diatom! 



In the first place, the resolution of a balsam mounted amphi- 

 pleura pellucida, as an opaque object, is not easily done; and in the 

 second place it is not necessary that the edge of the fluid be limited by 

 any particular form at all. There should be enough fluid to cover 

 that part of the front upon which the light is condensed and the 

 light should be condensed upon such a part of the front cell that a 

 line drawn from the spot of light to the lower surface of the slide 

 and then up through the object into the objective, will form an 

 angle of not less than 82 . 



With the drop of homogeneous fluid forming an externally con- 

 cave surface, the resolution can be easily obtained, and I have suc- 

 ceeded in obtaining excellent resolutions where the fluid was spread 

 over the entire cover-glass, and, being plentifully supplied, covered 

 nearly the whole surface of the front cell — the edges being exter- 

 nally concave and certainly not presenting the proper angle to 1 

 reflect the light upon the object. Even if it could do so it would 

 not be condensed at all and would therefore be of too low intensity 

 to give the required resolution. , 



Some time ago, while using a Spencer ■£%, it occurred to me to 

 try a little experiment which would prevent any light at all from 

 reaching the edge of the fluid. I selected a slide which had a heavy 

 circle of black cement. A little fluid was put upon the cover and 

 upon this was placed a thin piece of sheet rubber having a hole in 

 the center a little smaller than the edge of the front cell. This was 

 pressed down until the fluid extended upon the ring of cement, so 

 that it would be impossible for direct light from below to reach the 

 edge of the fluid. More fluid was then dropped upon the rubber 

 and the objective lowered. The edge of the fluid between the rub- 



