PROCEEDINGS OF THE SOCIETY. 373 



The Death of Mr. F. A. Nobert, of Barth, ia Pomerania (of 

 "Nobert's liues" celebrity) was announced (see p. 364). 



Mr. T. Powell said that he had been requested to exhibit again 

 Amphipleura pellucida with the y\-inch wide-angled immersion objec- 

 tive of 1 ■ 43 num. ap. (142^ balsam-angle) and illiuninated by the vertical 

 illuminator (see p. 363). In place, however, of mounting the diatom 

 on a slide, he had attached it to the under side of a cover-glass, and 

 without any slide beneath. It appeared to raise a very interesting 

 question, and though he could not pretend to have investigated the 

 matter critically, he had ventured to adopt the description given to it, 

 at the last Scientific Evening, of " opaque " illumination. 



Mr. Crisp said that Mr. Powell's exhibition certainly raised a very 

 interesting optical problem. When he first exhibited it the diatom 

 was on a slide, and the objection to the "opaque" view had then been 

 grounded on the supposition that the light was reflected back from 

 the slide. Mr. Powell now showed it without a slide, so that the 

 original objection was disposed of. In recent discussions upon the 

 subject in America, it had been assumed that the illumination was 

 clearly " opaque," but he thought proof was still required that it was 

 really so, and he inclined to the view which had been suggested by 

 Mr, Stephenson — viz. that whilst so much of the illuminating pencil 

 (incident from above) as was within the critical angle passed through 

 the diatom into air, that part which was beyond the critical angle 

 could not similarly escape into air from the under surface of the 

 valve, but was reflected upwards, and so illuminated the upper surface 

 by transmitted light. 



Mr. Powell said that he did not desire to be understood as by any 

 means insisting that it was " opaque " illumination, but he might 

 mention that he had often shown a diamond-beetle's wing illuminated 

 as an opaque object by the " vertical illuminator." 



Mr. Crisp, on removing the eye-piece of the Microscope, said that 

 nothing could be seen at the back of the objective but portions of the 

 bright annulus which represented the surplus apertiu'e of the objec- 

 tive in excess of 180^ in air. 



Mr. Stephenson would like to know whether Mr. Powell had tried 

 the experiment of reducing the aperture of the objective to the equi- 

 valent of the air-angle of 180°, by excluding the bright annulus of 

 light. By such a proceeding, the direct light from the vertical illu- 

 minator above the objective would still fall upon the valve, and, if it 

 were capable of reflecting light, it should of course be still visible ; 

 but he ventured to say, entirely from theoretical considerations, that 

 beyond, perhaps, the mere outline, it would be practically invisible. 

 Or again : if it were examined under the full aperture without any 

 stop, but with an infinitely thin film of air between the diatom and 

 the cover-glass, the same result would be obtained. He had no hesi- 

 tation in saying that, in his opinion, it was not a case of opaque 

 illumination, but that the diatom, adhering to the cover-glass, allowed 

 the rays beyond the critical angle to penetrate the glass surface, and 

 the light, passing through to the under side of the valve, was there 



