1917.] NATURAL SCIENCES OF PHILADELPHIA. 335 



glass, or even the surface of a ground glass plate, will show bright 

 polarization, provided they are mounted dry or in a medium of high 

 refractive index such as realgar. When in l^alsam, or other media 

 approximating their own index, the reflections are nearly or quite 

 eliminated and most such objects become invisible, although some 

 of the thicker polycystina and diatoms may still appear faintly 

 contrasted with the black background, l)ut Mr. Nelson calls attention 

 to the interesting fact that there are certain species of diatoms that 

 show almost equally brilliant polarizing effects when mounted in 

 balsam. 



In 1901, there was published in these Proceedings, a short com- 

 munication by me, relating the results obtained from the examination 

 of a series of diatoms in media of various refractive indices, which 

 demonstrated that the secondary structure of certain species of 

 Aulacodiscus differed from that of most diatoms by remaining 

 equally distinct in all media, including a liquid with refractive index 

 1.42 to 1.13, in which diatoms generalh" became practically invisible. 

 This structure has the appearance of fine granules arranged uni- 

 formly, but not symmetrically, between the larger cells constituting 

 the primary structure, instead of over or under the latter as is the 

 case with the perforated plates forming the secondary structure of 

 most species of Coscinodiscus, Triceratium and many of Aulacodiscus. 

 It is exhibited most distinctly by Aulacodiscus formosus, on which 

 it can be well shown with a half inch of .60 N. A., but is present on 

 many species of this genus, including argus, rogersii, oregonus, etc.; 

 and is well displayed in a photograph of A . sturtii used as one of the 

 illustrations in recent editions of Carpenter, "The Microscope and 

 its Revelations." It may be accounted for by various hypotheses, 

 among which, by a process of elimination, the most satisfying is to 

 assume that the apparent granules are hermetically closed cavities, 

 impenetrable by any medium in which the diatom is immersed. 



It will be found that almost all the diatoms which exhibit bright 

 polarization when mounted in balsam, are included in the species 

 having this secondary structure, and, as it is not penetrated by the 

 balsam, the same explanation will apply as to dry mounted forms. 



Mr. Nelson specially mentions Auliscus oaniaruensis as giving a 

 most charming image with polarized light, and it is likewise one of 

 the best forms available for elucidating the subject. This was one 

 of the species which I formerly studied sectionally, and the markings 

 consist of slender tubes penetrating an unusually thick silicious 

 plate. It was not included among those examined in media of 



