STRUCTURE OF COSCINODISCUS ASTEROMPHALUS, ETC. 159 



the new Zeiss N.A. 1*6 objective, and the attendant para- 

 phernalia, seems to show that after all the black dot is more 

 ^correct than the white one. As before stated, the white one 

 is the easiest to photograph, for the black dot seems never to 

 be sufficiently defined to look as sharp as we should like it." 



For the sake of the aforesaid tyro I will here quote a little 

 advice which Mr. Nelson gave me eight years ago (I began 

 photomicrography very late in life) on the white dot. Among 

 several black-dot photographs which I sent him, and which he 

 was kind enough to praise, there was a white-dot Isthmia 

 nervosa of which he said : ' : I think the Isthmia would be better 

 with black-dot focus ; this white-dot focus is an out-of -focus 

 ghost. It is much easier to get than a correct picture, and on 

 that account it seems to be a favourite with some photo- 

 graphers ; but any one really interested in the work should aim 

 At something higher. Of course, with very fine structures, a 

 white dot is all that can be obtained with our present lenses." 

 I thought then, and still think, that this was the kind of 

 mentor who would always command and receive the highest 

 respect. 



We come now to Pleurosigma angulatum, of which a black-dot 

 image x 3,700 is thrown on the screen. The next picture on the 

 screen shows a fractured valve which has been denuded of a part 

 of its outer membrane. The next image shows this outer mem- 

 brane x 2,000 broken up into fragments so minute that the 

 particles of silex have in some instances only one, two, three or 

 four holes shown as black round dots ; this outer membrane is so 

 thin that the silex is almost invisible, and in this respect differs 

 very much from the inner membrane, whose image x 2,000 is 

 now thrown on the screen. I do not, however, discern any 

 difference between the holes in the two membranes. 



Finally, we have to consider the structure of Pleurosigma 

 balticum. This, because of its convexity and thickness, is difficult 

 to photograph, and yet more difficult to understand. I am 

 illustrating its structure by showing you fifteen different photo- 

 graphs, each of which I must describe very briefly ; but before 

 doing so, let me define the word " fibril " : a fine filament of silex 

 which contains holes in a row like a string of beads ; it may 

 be long or short. This definition differs altogether from that 

 which Mr. T. F. Smith gives to the same word. 



The first slide shows the ordinary valve with Van Heurck's 

 canaliculi x 1,500. 



