377 



actinocyclus r a lps ii 



By Edward M. Nelson, F.E.M.S. 



{Read June loth, 1900.) 



This interesting diatom, especially when viewed under a low 

 power, is so transcendentally beautiful that it will attract the 

 attention of even those who, like Gallio, " care for none of these 

 things." The charm in this diatom consists not only in its 

 remarkable system of rays, from which it derives its name, but 

 also in its exquisite colouring. When, however, this diatom is 

 viewed in a critical manner with a wide-angled oil-immersion 

 lens all its lovely colour vanishes and its beautiful rays become 

 so inconspicuous as to be hardly noticeable ; in spite of this, 

 however, its interest to a scientist will be rather increased than 

 diminished. It is not difficult to account for the loss of the rays, 

 for when the diatom is examined under a low power the dots, or 

 more accurately the minute perforations in the silex, are so 

 closely approximated to one another that they appear to run 

 together and form rays, but when this structure is examined 

 under a higher power of greater aperture, these dots are so 

 widely separated that they cease to give this appearance of 

 lines or rays. 



The reason for the loss of the i colour is not quite so obvious, 

 for colour may be produced in a variety of ways e.g. by 

 polarisation, by the unequal refraction of light, by diffraction, 

 by the varying thickness of transparent thin plates, and lastly 

 by pigments. Now we know that exceedingly minute objects, 

 such as bacteria and micrococci, when stained by pigments do 

 not lose their colour when examined by high powers ; but on the 

 other hand objects such as diatoms, which owe their colour to 

 the diffraction of light by their minute structure, change their 

 colour from violet to red and finally lose it altogether as the 

 power, or rather the aperture, of the objective is increased. It 

 is an instructive experiment to examine w^ith dark ground 

 illumination and a low-power objective, say one inch or | inch 

 of aperture '25 to "3 N.A., a slide containing various species of 

 Pleurosigma that have different degrees of fineness of structure : 

 the coarser forms will appear ruddy, those a little finer greenish, 



