1901] MICROSCOPICAL JOURNAL. 47 
golden tint may be caused by the contrast with the bril- 
liant blue close to it), but on a dark ground exhibits a 
blue-green tint ; this is a diffraction color, which like all 
diffraction colors, turns white on resolution,or more strict- 
ly speaking shortly before resolution. 
The tint of the diffraction color of a diatom depends upon 
the aperature of the objective used, and the obliquity of 
the illumination. By this means we may therefore rough- 
ly determine the fineness of any diatomic structure by 
matching the tint with one whose fineness of structure 
has been measured, or with a test plate or ruled bands. 
Of course it is necessary that the comparison be made 
with the same objective and under the same conditions of 
illumination. A suitable illumination for this purpose is 
daylight, and an achromatic condenser with a central 
opaque stop, just large enough to give a dark ground. 
The question then is: what is the cause of the color in 
Actinocyclus Ralfsii? Obviously it cannot be a diffrac- 
tion color arising from the ordinary primary structure 
forming the “rays,” which give the diatom its name, be- 
cause as we have seen above, when this structure is re- 
solved the color is still visible, and no color arising from 
diffraction is visible when the diffraction itself is resolv- 
ed. It cannot be due to pigment, for if it were it would 
remain visible when the aperture was increased beyond 
A5 N.A. It cannot be caused by thin plates, because it 
would require reflected and not transmitted light to rend- 
er it visible. Polarization and refraction seem quite out 
of the question ; and as there is no other theory at hand, 
the answer must for the present be left undetermined. 
It was pointed out in 1897 (Journ. Q. M. C., Vol. 6, ser. 
2, p. 431) that with an apochromatic % of 1.4 N.A., used 
in connection with a wide-angled oil-immersion conden- 
ser giving a large aplanatic cone, a very delicate perfor- 
ated veil could be seen covering the whole valve of an 
Actinocyclus Ralfsvi. This very delicate structure has ob- 
