MICROSCOPICAL IMAGERY. 109 



A deeper focus showed a deep blue central haze, melting 

 with a fine red fringe (Circular Solar Spectra, Froceedmgs R. S., 

 JVo. 146). 



In precisely the same way the spherules of diatoms develop a 

 great variety of interesting phenomena, under solar illumination 

 especially. Here is an extract from my note-book, which fairly 

 describes solar phenomena, seen in that exquisite diatom, Aida- 

 codisciir Comberi : — "The sun shines. The frustule is glorious 

 with solar spectra. No condenser. Size of smallest visible disc 

 1-75, 000th (inch), of largest handsomely shown, i-2o,oooth. The 

 solar spectra are of a noble order of beauty and precision. A few 

 diffraction rings are vividly seen, but they are all more or less 

 broken by another lower stratum of minute beading. The smallest 

 brilliant disc is surrounded by a perfectly .formed circular black 

 ring, about the 200,000th of an inch thick. A dot appears either 

 above or below the disc according, as the observing objective is, 

 in a slight degree, positively or negatively corrected by its screw 

 collar. I shall now venture to describe one of the most startling 

 observations it has been my good fortune to witness : — 



On a sunny day, an iris diaphragm by Beck, capable of closing 

 up to i-iooth of an inch, having been screwed into the nose of the 

 microscope, a very fine one-sixteenth objective, made expressly by 

 Powell and Lealand^ was directed to examine P. Formosum by 

 transmitted sunbeams. This diatom was chosen as its spherical 

 beads were nearly the same size as the heliostatic image of the sun, 

 i-4o,oooth. Now according to optical theory, each spherule 

 should emit pencils of many coloured rays ; in fact, a complete 

 spectrum. Now for the result : on closing up nearly the iris, so 

 as to reduce the objective aperture to i-iooth of an inch, 

 solar rays being obliquely employed, I was amazed to see a brilliant 

 isolated disc glow forth with an intense coloured effulgence, and 

 still more was I astonished to see each spherule successively 

 glowed with a differently coloured effect, when the slightest 

 change was made in the obliquity of the illumination. But the 

 instant the iris diaphragm was opened, all this solar glory utterly 

 vanished. The experiment is significant, though very difficult of 

 attainment. (Read before the Royal Society.) 



In carrying out for some years researches on circular solar 



