332 SUMMARY OF CURRENT RESEARCHES. 



MICROSCOPY. 



A. Instruments, Accessories, etc.* 

 (3) Illuminating- and other Apparatus. 



Polarization and Color Effects exhibited by certain Diatoms.^ 

 F. J. Keeley {Proc. Acad. Nat. Sci. Philadelphia, 1918, 69, 334-8). 

 The author discusses E. M. Nelson's paper reprinted (p. 340) in the 

 Society's Journal for June, 1917, in which Nelson suggests that "it may- 

 be that silex deposited by animals does not exhibit polariscope effects, 

 while that deposited by plants, such as diatoms, will do so." The author 

 considers that the existence of the innumerable surfaces, in diatoms and 

 other minute silicious organisms capable of reflecting light, amply 

 account for the polarizing effects, and that, when suitably illuminated, 

 all diatoms, radiolaria, sponge spicules, powdered 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 balsam, or other media approximating to their own 

 index, the reflections are nearly or quite eliminated, and most such 

 objects become invisible. Nelson, however, calls attention to the 

 interesting fact that there are certain species of diatoms that show 

 almost equally brilliant polarizing effects when mounted in balsam, 

 and Keeley points out that in all these there is a secondary structure 

 which the medium has failed to penetrate, and has thus left a number of 

 reflecting surfaces sufficient for polarization. Among the diatoms which 

 show by far the brightest colours by transmitted light are certain species 

 of Actimocyctus, notably raJfsii, in which, so far as the author knew, 

 no secondary structure had hitherto been observed. His theory led him 

 to a special investigation, and a secondary stracture was readily found 

 to be present, and particularly visible in the central blank space and 

 wedge-shaped radial segments between the coarser cellular structure of 

 A. ralfsii. This secondary structure is exceedingly delicate, and will 

 afford an excellent test for the definition of wide aperture objectives, 

 though, after finding it with oil-immersion and after selecting a well- 

 marked valve, the author was able to see it distinctly with a well-corrected 

 water-immersion of about 1 • 00 N.A., and even with an exceptionally fine 

 dry objective of slightly less aperture. 



The author has also found that interesting polarizing effects can 

 be obtained from the fine granules of a gold film on a glass surface. 

 Such films reflect various colours according to the size of the particles, 

 and transmit the complementary colour. The films may be obtained by 

 means of electrical discharges from the end of a gold wire, or by 

 deposition from solution of gold chloride in collodion. A. N. D. 



* This subdivision contains (1) Stands ; (2) Eye-pieces and Objectives ; (3) 

 Illuminating and other Apparatus ; (4) Photomicrography ; (5) Microscopical 

 Optics and Manipulation ; (6) Miscellaneous. 



