254 Transactions of the Roijal Microscopical Society. 



me 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 xrfth 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 sm'face of 

 the scale. 



Tliis 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 opinioin 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 ]\Iarkings of Frustulia Saxonica " 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 

 Btart into view ; the number of spurious lines, and consequently 



