254 Transactions of the Royal Microscopical Society. 
rue to the opinion that this description applies to the gnat scale as 
well as to the mosquito. Nevertheless, on examining a gnat scale, 
as requested by the donor of the slide, with the immersion yVth of 
Powell and Lealand, by central illumination, I succeeded, after 
some trying with the right-angled screws of the achromatic con- 
denser, in obtaining, as I suppose, the very appearance Dr. 
Anthony’s drawing is intended to represent, and the three parallel 
rows of minute intercostal beads started out suddenly into view 
between each pair of longitudinal ribs over the whole surface of 
the scale. 
This appearance was so realistic that at first I inclined to the 
opinion that it represented truly the actual markings of the scale, 
and accordingly I endeavoured to photograph it as requested, with 
the objective named, by monochromatic sunlight, and after several 
failures succeeded in obtaining a fair representation of what I saw. 
I send herewith a print (marked A) from the resulting negative. 
I have, however, since then been led to form the opinion that 
these clearly seen beads are a spurious appearance, produced by 
longitudinal diffraction lines, conditioned by the longitudinal ribs 
and parallel to them, which cross the true transverse markings 
at right angles, and thus give rise to the optical appearance of 
beads at the point of intersection ; the whole series of phenomena 
being similar in character and origin to the diffraction phenomena 
observable in many diatoms, &c., as described by me in my “ Note 
on the Markings of Frustulia Sctxonica ” in this Journal, December 
1875, p. 274. 
My chief reasons for this opinion in the present case are — 
firstly, that the longitudinal diffraction lines are clearly seen, both 
in the microscope illuminated by lamp or sunlight, and in the 
photographs (as, for example, in the print A) to extend into empty 
space beyond the contour of the scales almost as far as the ends of 
the bristles in which the parallel ribs terminate ; and secondly, that 
they vary in number with varying obliquity of illumination, so that 
in the same scale two, three, four, or five rows of beads can be seen, 
and photographed at pleasure, in each intercostal space. 
Since arriving at this conclusion I have had no difficulty in 
producing at will, either the beaded appearance, or that which I 
conceive to represent correctly the surface markings, on any scale 
I have tried, whether of the gnat or mosquito. 
If the selected scale is illuminated with the light thrown per- 
pendicularly to the transverse markings, by means of an Abraham s 
prism, the beaded ribs and smooth transverse markings will be 
clearly shown ; and if now the stage be rotated so as to turn the 
long diameter of the scale more and more obliquely to the illumi- 
nating pencil, the spurious lines, and with them the beads, will 
start into view ; the number of spurious lines, and consequently 
