64 



JOURNAL OF THE 



[April. 



different positior.s (Fig. 2), the "x" marking the red, and the "o" 

 the green. Taking note of the difference of the colors in the 

 alternate holes of a grating may seem trivial to a biologist mark- 

 ing out, say, the life history of some obscure organism, but in 

 reality it points out a valuable quality belonging to an apochro- 

 matic lens only and helping to call attention, in all branches of 

 microscopic research, to points which else might escape attention. 

 I say this advisedly, having tested some of the finest glasses of 

 our English makes, but not apochromatics, on this object; and 

 although the two sets were shown in their proper positions, they 

 were both of the same color, and thus the different positions 

 likely to escape notice. I have not yet been able to discover the 

 •cause of the two sets of images in this layer. There is no diffi- 

 culty about the outer red ones being simply the alternate holes 

 of the grating; but I can find no cause for the inner green 



" beads," although it may point to a second grating inside on 

 which they are received as on a screen. 



The difference of appearance between the two sides of the valve 

 is most characteristic and one not to be mistaken when once 

 seen and noted, and is well shown on slides Nos. 12 and 13 — 

 No. 12 being the inner and No 13 the outer side of the valve. 



As I have said before, the structure of the outer part of the 

 valve is really divided into two — Nos. 2 and 3 in Fig. i — but it 

 is with the outer one of the two, as offering numerous examples 

 of torn structure, we have chiefly to deal. I have been able, 

 fortunately, to determine its character with absolute certainty, and 

 also probably to obtain a key from it of the structure of the other 

 two layers of the valve; but the results I propose to lay before 

 you are so little in accordance with my theory of structure hith- 

 erto accepted by diatomists, that I can well understand any re- 

 luctance there may be to receive them as true. I certainly should 

 have hesitated to accept them myself, had they not been forced 



