ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 691 



this method than can be attained by employing the fringes of Fizeau 

 and Foucault. Conversely, having measured the thickness by other 

 means, the author has applied the formula to determine the wave- 

 length of the ray D 2 , and finds a value identical with Angstrom. 



It has been pointed out * that the author is not justified in con- 

 cluding that the values found by Ditschreiner and Van der Willigen 

 are incorrect, because it is not known to what degree of accuracy the 

 thickness of the quartz plate had been measured. 



Dotted appearance on Pleurosigma ang-ulatum.j — Mr. J. B. 

 Dancer once found that the oblique markings of a damaged valve had 

 been removed by abrasion against the cover-glass ; by no modification 

 of the light could they be rendered visible. When, however, oblique 

 illumination was directed in a line with the length of the valve, the 

 transverse markings were distinctly visible and apparently uninjured. 

 At first he thought that moisture had obtained access through the 

 crack in the thin cover, and he dried the slide over the flame of a 

 spirit-lamp carefully and repeatedly, but could not make the oblique 

 lines visible, although they were distinctly visible on other broken 

 , valves contiguous to the special one under examination, and also on 

 some portions of this valve ; the oblique markings which had been 

 dislodged were lying beside the edge of the damaged valve. Season- 

 ing from what he had seen, he was led to imagine that the oblique 

 markings were on the upper convex surface of the valve, and that the 

 transverse markings were on the inside or concave surface. If we 

 assume that the section of these raised markings are semi-cylindrical 

 in form — that is, being rounded at the top — there would be an im- 

 perfect cylindrical lens formed wherever these pellucid ridges crossed 

 the lower or transverse markings. These would present focal points 

 of light and possibly images of objects, such as are seen in the eyes 

 of beetles under certain conditions of illumination ; if this be true, 

 the so-called beads have no existence. 



Mr. Dancer in a subsequent communication J writes as follows : — 

 " In my letter of the 28th May, I assume that the cross section of the 

 ridges or markings on P. angulation are semi-cylindrical, and also 

 state that the A. pellucida ridges would form imperfect cylindrical 

 lenses, where they cross the lower transverse markings. To render 

 my meaning more intelligible, I may say that I had in my mind the 

 lens introduced, I believe, by Chamblant, of Paris, about fifty years 

 ago. If two pieces of polished glass, semi-cylindrical in section, have 

 their flat surfaces placed one on the other with exactly their cylindrical 

 surfaces at right angles to each other, a perfect lens is formed, having 

 no spherical aberration. These lenses are much used in Paris, and 

 occasionally in England, for hand reading-glasses and spectacle eyes. 

 I have had such in use for both purposes for over forty years. Now, 

 if we conceive that a number of minute lenses of this form were 

 placed in juxtaposition, and examined under a Microscope, they 

 would show images of any objects placed between the mirror and the 



* Zeitschr. f. Instrumentenk., v. (1885) p. 325. 



t Engl. Mech., xliii. (1886) p. 283. J Ibid., p. 329. 



2 z 2 



