ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 817 



alternately, are scon running diagonally across the valve, ami an imago 

 of the larger intersjiaces, thrown there from the under layer as on a 

 screen, gives rise to the appearances which have i)roclucecl so much con- 

 troversy as to whether the "markings " are beads or perforations. As 

 a matter of fact they are neither, 

 but simply a collection of focal Pig^ jqq, 



images or ghosts, and you may __ ^ __ ^ ^ __ 



as well speak of the picture ^2 IZZ ^Z ZZ ^Z ^ 



thrown by the optical lantern on ^ z^ ZZ I^ ZZ Z^ ZZ. 

 a screen as the structure of that ^I ^I ^ ^I ^ ^ 



screen, as speak of these focal ZI — ~ ^ ZI ZI 13 

 images as the structure of the 

 diatom. I have a valve of Pleuro- 



sigma formosum under the Microscope here to-night which shows finely 

 the arrangement of the fibrils on the valve. In some parts of the valve 

 the fibrils are seen lying loose, in other parts close together, form- 

 ing regular structure, while in other parts they are wanting altogether. 

 A print of the same valve shows in parts a regular collection of white 

 " beads," which are ghosts and utterly wanting where the outer membrane 

 is torn away. The distance apart of the alternate squares from the 

 centre of each other on Pleurosigma formosum is double that of Pleuro- 

 sigma angulatum, and our difiiculty is enormously increasetl ^^hen we try 

 to determine the structure of the latter. To me it is sufficient that the 

 two images present the same characteristics to convince me that the 

 structure is the same, but I know that other observers want more positive 

 evidence, which for a long time I was unable to give. What was wanting 

 was corresponding torn structure, and at last I am able to put that in 

 evidence also — not, I confess, in Pleurosigma angulatum proper, but in 

 an allied species, which for our jmrpose is practically the same. The 

 strife are of the same fineness — 50 , 000 to the inch ; there is the same 

 arrangement of large perforations on each side of the median line and 

 the margins; and the finer structure shows the same focal images formed 

 in alternate squares. On one corner of the valve of which I show a 

 print, the outer layer is strij)ped ofi", leaving the uuder one intact — found 

 on focusing down— while on the lower corner the fibrils are lying in 

 strips, and are of exactly the same character as those we have seen in 

 Pleurosigma formosum. 



Disturbances of Vision consequent on Microscopic Observation.* — 

 M. C. J. A. Leroy has noted a peculiar disturbance of vision which 

 afiects exclusively the eye which has not been employed during micro- 

 scopic observation. Letters seen at the usual testing distance of 5 m. 

 were blurred, and this effect was not corrected by spherical glasses or by 

 efforts of accommodation. In the table of radiating lines used as 

 diagnostic for astigmatism, the horizontal lines were disturbed while 

 the vertical ones remained clear, and no cylindrical glasses modified 

 the difference : thus the disturbance was not due to defect in accom- 

 modation or to simple astigmatism. The author was led to the con- 

 clusion that it is a diplopia always produced in a vertical direction 

 by noticing the fact that the horizontal lines of the curtain traversing 

 the top of the machine gallery in the Paris Exhibition was distinctly 

 double. This diplopia has its origin in the dioptric apparatus (cornea or 



* Comptes Eendiis, cviii. (1889) pp. 1271-3. 

 1889. 3 L 



