.1891.] NEW-YORK MICROSCOPICAL SOCIETY. 83 



equal squares of red and blue would be a mark of P. fonnosuin 

 as distinguished from P. angulati/in, under proper conditions 

 of illumination and examination. 



We are justified in concluding, therefore, that the phenomena 

 of color and form thus examined are not only consistent with, 

 but strongly confirm, the generally received theory of diatom- 

 structure, and cannot be said to indicate anything new in that 

 direction. 



Mr. Smith also expresses the opinion that only by means of a 

 wide-angled objective, and illumination by a wide cone of light 

 from the sub-stage condenser, can the upper and lower films of a 

 shell like P. angulatitm be discriminated. As he recognizes some 

 photographs made by me, and deposited with the Royal Microsco- 

 pical Society in 1884, as showing this discrimination, it is due to 

 scientific accuracy to say that they were made with a Wales y'- 

 water-immersion objective of about i.o N. A. aperture, and with 

 a narrow cone of light coming from a Webster condenser under 

 the stage having a diaphragm with a ^-inch opening behind it. 

 Mr. Smith's own objects photographed could not be illuminated 

 with a very wide cone of light, as they were mounted dry and he 

 tells us he used his condenser dry. There was therefore a 

 stratum of air both above and below the slide on which the object 

 was mounted, and the illumination could not exceed the "critical 

 angle," 82°, in passing through the cover glass, and must in fact 

 have been considerably less.* 



In my own experience I have found a broad cone of illumina- 

 tion unsatisfactory, for the same reason that I have found oblique 

 light in one direction unsatisfactory. It is almost impossible to 

 centre the sub-stage condenser so accurately that a wide cone 

 can be trusted to be central. If you centre it by examination 

 with a low power, it is almost certain that it will not be centred 

 for a high power, for two objectives are rarely centred alike. 

 The field, under a magnification of 1,750 which Mr. Smith has 

 commonly used, is so small that the least decentring will illumi- 



* In my note book, June 3d, 1884, I find that I entered my observation of one of the 

 broken shells which I photographed, as follows: "A remarkably interesting frag- 

 ment of P. angulatum, showing partial removal of one film, and fracture through dots 

 over a large space." In preparing this paper I have repeated the examination with 

 the objective named, and find the distance between upper and lower film easily 

 appreciable in focussing. 



