338 . PROCEEDINGS OF THE ACADEMY OF [DeC, 



produce color by this means, merely giving them a more or less 

 opaque or sometimes metallic appearance, but in such forms as 

 A. oregonus, in which the structure is almost as fine as on the gold 

 film described, fairly bright colors appear, generally a bronzy green 

 by transmitted light, and the complementary color, pink, when 

 illuminated from above, which is as it should be, if the color is due to 

 optical resonance from the granules of the secondary structure. 



But the diatoms that show by far the brightest colors by trans- 

 mitted light, are certain species of Actinocyclus, notably ralfsii, in 

 which, as far as I am aware, no secondary structure has hitherto been 

 noted, yet if this theory was correct they should contain such a 

 structure and, to account for the brighter colors, it should be finer 

 than in any species of Aulacodiscus. On examination, this structure 

 was readily found to be present and particular^ visible in the central 

 blank space and wedge shaped radial segments between the coarser 

 cellular structure of A. ralfsii. It is extremely delicate and will afford 

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

 after finding it with an oil immersion and selecting a well-marked 

 valve, was able to see it distinctly with a well-corrected water immer- 

 sion of about 1.00 N. A., and even with an exceptionally fine dry 

 objective of slightly less aperture. 



If it should prove that this structure in Actinocyclus has been 

 previously seen by others (as is usually the case with all supposedly 

 new observations with the microscope) it will not make the slightest 

 difference in this connection, as it was entirely unsuspected and new 

 to me, and being found, not by accident, but because its presence 

 was indicated theoretically, it seems to furnish rather strong evidence 

 in favor of the correctness of the theory. As in the case of Aulaco- 

 discus, Actinocyclus shows complementary colors with transmitted 

 and reflected light, and fairly bright polarizing effects when mounted 

 in balsam. On examination in a fluid of about 1.43 refractive index, 

 it remained visible and showed bright colors by transmitted light. 



All the diatoms with this peculiar secondary structure which I 

 have as yet examined in media of various refractive indices, show 

 brighter, higher order colors as the refractive index is increased, 

 although the structure itself shows no change in visibility, indicating 

 that the internal structure is combined with surface irregularities 

 that permit the index of the medium to modify the color in accord- 

 ance with the change in wave length of light transmitted through 

 it. f A hypothetical view would be that the structure corresponds 

 to that of a foam, both with respect to the internal closed cavities, 

 and the external surface. 



