194 Transactions of the 



powers, and I soon made out the appearances I have endeavoured to 

 set down in Fig. 1, where the ribs seem irregularly beaded, but 

 where is also seen over the whole surface of the scale a series of 

 lines in the interspaces of the ribs, which would seem to correspond 

 with longitudinal markings seen on an after -examination with 

 transmitted light. A good ^th objective showed these lines very 

 easily, even sharper than I have ventured to draw them, and the 

 ith and -reth Powell and Lealand objectives enlarged, but did not 

 otherwise modify these appearances. 



Looked at by transmitted light, and after most careful attempts 

 to avoid sources of error, the scale showed appearances such as I 

 have endeavoured to represent in Fig. 2 — appearances principally 

 made out by the aid of a very fine modern ith and iVth of Powell 

 and Lealand, with the light made studiously as central as possible, 

 and stops both of condenser and iris diaphragm, not too small, in 

 order to avoid the risk of spectra. The markings to which I wish 

 to call particular attention, it will be seen, bear some resemblance 

 to the markings on some of the more delicate Podura scales, and I 

 think appeared more distinctly on the surface of the uncovered scale : 

 they seemed to me to be quasi-surface markings, and to be nearly on 

 the same absolute plane as the parallel ribs — markings in or upon 

 the surface of the membrane in the interspace between the one set 

 of ribs, and not to be in any way in the substance of the scale. 



The first appearance got in the examination of these markings 

 is a mottled or patchy efiect in the interspaces of the ribs, like ill- 

 defined corrugations of certain scales of the Lepidoptera ; and this 

 efiect was all that certain objectives, and particularly those of an 

 ancient date, would show ; but with my good modern ith and rVth 

 these blur-like appearances were resolved into markings seen quite 

 as vividly as 1 have drawn them — markings which I could trace all 

 over the scale, strictly parallel in every part to the quasi-parallel 

 ribs — markings as on or contiguous to the surface, and visible even 

 on that portion of the scale towards the edge where the " watered 

 silk " appearance obtains, and where the obhquity of the radiating 

 ribs is at a maximum ; showing, I think, clearly that these mark- 

 ings have no connection whatever with the system of radiating ribs, 

 but that whatever they may be, they belong to the surface of the 

 scale alone. If we speculate on the nature of these appearances, of 

 course they may be longitudinal plications of the membrane between 

 the ribs, but they certainly bear a curious resemblance to some of 

 the finer Podura markings. I do not think I manufactured the 

 appearances I have endeavoured to describe by any efiect of what is 

 called " cooking " the illumination ; for I carefully kept away from 

 any obhquity of light, which I generally find more or less the 

 source of most microscopic errors ; and, as I have said, in addition 

 to this precaution, I made use of quite a medium aperture both 



