108 CHAPMAN JONES ON THE SECONDARIES OR 



other work on this subject, the nature of the dotted structure 

 and the reason for its elusive character do not appear to be 

 known. Having among my slides of Pinnulariae one in which 

 the valves have suffered much both mechanically and from 

 corrosion (I suppose in cleaning the material) I have searched for 

 information on these points. 



I think that the photographs that I have the honour of laying 

 before the Club, as well as the examination of many other speci- 

 mens on other slides as well as this particular one, lead to the 

 following conclusions : 



1, That finding the dots is not a matter of resolution as 

 ordinarily understood, but rather of rendering them more 

 conspicuous, except perhaps in some of the smaller valves. 

 The dots in Mr. O'Donohoe's photograph are of about the 

 same order of coarseness as those of Surirella gemma, and this 

 diatom cannot be called a very difficult one to resolve. I have 

 measured them as well as I can from the reproduction of his 

 photograph and find that their distance apart is equivalent to 

 from 60 to 70 thousand to the inch. The dots in the specimens 

 I have examined are not uniformly distanced in any one case. 

 In figure No. 1, which represents a part of a large valve, they 

 range from 32 to 40 thousand to the inch ; in figure No. 2, which 

 is a rather small valve, 50 to 70 thousand to the inch ; in the 

 other figures from 43 to 46 thousand. High-power dry objectives 

 of ordinary apertures have therefore quite sufficient " resolving " 

 power. 



2. That the dots are not immediately connected with the 

 costae as Mr. Slack and others appear to have assumed. 



3. When viewing a valve in the ordinary way, that is, with its 

 outside or convex side uppermost, it appears that there is a 

 dotted membrane under the costae, and a non-dotted if not 

 structureless membrane over the costae, and that the dots are 

 generally hidden by this upper membrane, unless special means 

 are taken to render the dots conspicuous. 



4. This dotted membrane appears to be attached, at least in 

 some cases, to the girdle. This accounts for its absence from 

 some valves. On the other hand, it is sometimes attached to the 

 costae. Perhaps there is a strip of dotted membrane attached 



