PKOCEEDINGS OF THE SOCIETY. 445 



4. Chance aberrations in the lens system as a whole are neutralized 

 by the method in which the glass plate micrometers are calibrated, since 

 these aberrations would alter the calibrative curves in the same way and 

 to the same extent as the spectral measurements alternatively obtained. 



5. Concentric beams of convergent parallel light are incident on the 

 diatom, and therefore pure spectra are formed in the upper focal plane 

 of the objective. 



Turning to another point, his reason for selecting Pleurosigma was 

 that when working out a new method it was not of the least use attempting 

 the most difficult problems straight away, and it seemed to him that the 

 first thing to do was to try and get evidence which agreed in regard to 

 a diatom which everyone knew. When one knew a diatom well one had 

 progressed a long way towards finding out things about diatoms one did 

 not know. Investigators had discovered nearly all there was to be 

 found by means of present day methods. 



With regard to his apparatus, it was not altogether untried, nor 

 was it altogether problematic as to whether it was going to work or 

 not ; if that had been the case he would not have brought it before 

 the Society in its present form. It was true that measurements were 

 not actually being made by it, but it had taken some time to work out, 

 and a large number of other measurements had been made by other 

 apparatus he had made which were not quite so neat as this, nor so 

 well constructed, and he thought he was justified in saying that he had 

 more than a mere surmise that measurements were going to be obtained. 



Finally, he wished to thank Mr. Gordon for drawing his attention 

 to the ambiguity in the use of the symbol d in the two formulae ; this 

 would be corrected. 



Mr. Rheinberg said he had been extremely interested in Mr. Hart- 

 ridge's proposed methods of investigation and in his instrument, and 

 had been struck with the great amount of ingenuity displayed, as far 

 as the various optical arrangements were concerned. The selective 

 colour filter, with its perforation isolating the smaller portion of the 

 diatom under investigation when the green mercury rays only are passed, 

 yet locating the portion by rendering the entire diatom visible when 

 the other mercury rays are allowed to pass, appealed to him as a 

 particularly neat device. It was not difficult to understand the way in 

 which the instrument was to be used, and although it seemed not un- 

 likely that considerable practical difficulties might be met with in 

 pursuing the investigations, interesting results might certainly be looked 

 for. He would like to draw attention to the fact that some of the 

 results as to thickness and contour which Mr. Hartridge might obtain 

 seemed to hmi susceptible of being tested by other optical means. 



About twelve years ago he was studying the origin of the colour 

 phenomena in Actinocyclus Ralfsii, a diatom which altered its brilliant 

 coloration in a remarkable way when the numerical aperture was 

 varied. He had at the time estabhshed the fact that these appearances 

 belonged to a class of colour phenomena originally termed by Thomas 

 Young, the colours of " mixed plates," in other words, the same class 

 as the colour phenomena presented by laminary gratings. And after 



