iq8 THE MICROSCOPE, 



Spencer, that it would resolve amphipleura pellucida in balsam by 

 central sunlight. We both, Professor Burrill and myself, ridiculed 

 the idea, and concluded Professor Forbes must have some diffrac- 

 tion lines, for we both knew that he was not then familiar with the 

 appearance of amphipleura. But the next day Professor Burrill 

 came with another letter from Professor Forbes, which stated, the 

 resolution had been effected with the mirror-bar central, but with 

 the light thrown a little to one side, so as to make it bearable to the 

 eye; also, that the field, thus illuminated, had presented a yellowish 

 tinge. After Professor Burrill had left I observed that the sun 

 shone in through the southern window of my room on my working 

 table, and I concluded to try the effect of the light as arranged by 

 Professor Forbes. So I took out my Bullock's biological stand, the 

 same kind of stand used by Professor Forbes, and put on my -fa 

 Tolles homogeneous immersion objective, made in 1879, (n. a. 

 about 1.33), and Moeller's balsam test plate on the stage. I focussed 

 an amphipleura and arranged the light according to Professor 

 Forbes' direction, that is, threw it sufficiently to one side to make it 

 bearable, or easy, to the eye. To my astonishment the well-known 

 lines presented themselves more distinctly and more beautifully than 

 I ever had seen them before. The appearance of the frustule, as 

 seen, may be compared with that of a new file. A one-inch eye- 

 piece, by Tolles, was at the upper end of the tube. At first, of 

 course, I was at a loss to explain such an unexpected result, and 

 momentarily forgetting that the angle of incidence and that of re- 

 flection are invariably the same, I thought of several impossible 

 things, for instance, a reflection. from the lower surface of the objec- 

 tive, or from another diatom, but soon found that such could not be 

 the case. Being well aware that I had extreme oblique illumination, 

 the thing became interesting, and I kept on experimenting till I 

 learned the true cause, and found that the resolving rays are reflected 

 from the {externally convex) internally concave surface of the edge of 

 the immersion fluid. The latter consisted of a concentrated solution 

 of cadmium chloride in glycerine. The accompanying drawing, 

 though poorly executed, may serve to illustrate. My attention was 

 drawn into the right direction by finding that the resolution disap- 

 peared whenever too much immersion fluid was used, or whenever 

 the immersion fluid became smeared all over the cover; in other 

 words, whenever the outline or edge of the immersion fluid failed to 



