662 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
bluish-white. On the inner side of the shell the thin circles, or “ eye- 
spots,” are usually smaller than on the outer side ; the diffraction effect 
by transmission of light will straighten the edges of the tessellated 
outline ; the squares will each have half the area of, and will be diagonal 
Fig. 74. 
to, the original squares ; and with their alternate colours we shall have 
exactly the appearance which Mr. Smith describes, and which is very 
well shown in prints * Nos. 1 and 2, compared with No. 6. 
The peculiarity of the quincuncial arrangement of alveoli is that 
when the circles crowd upon each other so as to become polygons 
bounded by straight lines, they form hexagons instead of squares, and 
even when they are circles in a continuous plate of silex the hexagonal 
outline is a persistent ocular illusion. We should expect, therefore, 
that the tessellated appearance with equal squares of red and blue would 
be a mark of P. formosum as distinguished from P. angulation, under 
proper conditions of illumination and examination. 
We are justified in concluding, therefore, that the phenomena of 
colour 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. angu- 
latum be discriminated. As he recognizes some photographs made by 
me, and deposited with the Eoyal Microscopical Society in 1884, as 
showing this discrimination, it is due to scientific accuracy to say that 
they were made with a Wales 1/15 water-immersion objective of about 
1*0 N.A. aperture, and with a narrow cone of light coming from a 
Webster condenser under the stage having a diaphragm with a 1/4-in. 
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 consider- 
ably less.f 
* These prints are given in an article by Mr. Smith in the journal here quoted 
(pp. 61-72). 
t In my note-book, June 3rd, 1884, I find that I entered my observation of one 
of the broken shells which I photographed, as follows : “ A remarkably interesting 
