660 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
which is fainter because it proceeds from alveoli not so closely con- 
nected in rows. It may be called a secondary striation. With still 
more effort we may get a much finer and fainter striation, parallel to the 
midrib, by throwing light at right angles to it, or nearly so. By lamp- 
light, and with objectives net apochromatic, and not exceeding the 
aperture of 1*0 N. A., these lines are usually in patches, upon spots 
here and there, longer (in the length of the shell) than they are wide. 
But with sunlight this tertiary diffraction striation may be made to 
cover the whole surface of Pleurosigma angulatum by an exquisitely fine 
longitudinal grating over its whole surface, as was demonstrated by 
Dr. Woodward in one of the most striking of his photomicrographs in 
what is called “ the Abbe experiment.” * As the improvement in our 
lenses, both by increasing their angle, and by the apochromatic system, 
tends to make visible by lamplight what before could only be seen by 
the sun, we should expect that something like the fibrillae shown in 
Mr. Smith’s photographs would be visible. Finding it would not prove 
that it is purely the result of known laws of diffraction ; but it justifies 
a cautious and scientific scepticism in receiving a new explanation until 
we have repeated the experiment often enough, and under such varying 
conditions as to exclude doubt. 
As we increase or reduce the obliquity of the light in examining 
Pleurosigma formosum , we know that the alveoli are distorted (or may 
be) in varying ways and directions. Some of these are figured in 
‘ Carpenter on the Microscope,’ but they are only a few of a numerous 
series. Whoever will experiment a little may satisfy himself that 
the permutations and transmutations of the diatom markings may be 
made little short of kaleidoscopic. Hexagonal markings may become 
square, and may have short lines running off from one angle. These 
lines may be lengthened, and the square or hexagon reduced to a dot, so 
that the appearance of the surface may be that of oblique series of 
parallel dashes. The direction of these lines depends on the direction 
of the light, making a series of gratings, of which the prevalent 
character may be oblique in either of two directions, transverse or 
longitudinal. The so-called intercostal points may be enlarged and 
brightened until they become the most prominent marking, and the 
alveoli proper may be diminished to insignificance. These appearances 
are so like many of those in Mr. Smith’s series that we, who can only 
see the print and cannot get our fingers upon the fine-adjustment of the 
Microscope and note for ourselves the effect of a change of focus, are 
necessarily made cautious in accepting his interpretations; but there 
should be caution in rejecting as well as in accepting, and he fairly 
challenges us to repeat his investigations under similar circumstances, 
and with similar objectives. 
An examination of his print No. 12 with a hand-lens will illustrate 
what I am saying. When looked at with the naked eye, this print 
shows a long patch of longitudinal striation on the lower side of the 
valve. Immediately below the midrib we see the coarse, oblique dotting 
peculiar to Pleurosigma formosum ; but if we use the lens we see at once 
that, in the patch referred to, the dots are twice as numerous as the 
* See Roy. Micr. Soc. Journ., ii. (1879) p. 675 ; also Mon. Mier. Journ., xvii. p. 82. 
