80 JOURNAL OF THE [April, 



their angle and by the apochromatic system, tends to make visible 

 by lamplight what before could only be seen by sun, we should 

 expect thac 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 justi- 

 fies a cautious and scientific scepticism in receiving a new expla- 

 nation until we have repeated the experiment often enougli and 

 under such varying conditions as to exclude doubt. 



As we increase or reduce the obliquity of the light in examin- 

 ing Fleurosig/na formosiim, we know that the alveoli are distorted 

 (or may be) in varying ways and directions. So ne 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 maybe made little short of kaleidoscopic. Hex- 

 agonal 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, mak- 

 ing 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 

 or.ly 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 investiga- 

 tions under similar circumstances and with similar objectives. 



An examination of his print No. 12 with a hand lens will illus- 

 trate 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 formosiim j but 

 if we use the lens we see at once that, in the patch referred to, 

 the dots are twice as numerous as the alveoli of the shell. The 



