258 Transactions of the 



dark shadows on either side of the bright part of the rib ; these 

 have been noted by Dr. Pigott ((/, Fig. 26). The shghtest touch 

 of the fine adjustment or prism displaces them, and by a fair 

 apphance of patience, " toying " with the focus and the adjustment 

 of the prism, the image is converted into the appearance at i, when 

 the chromatic effects are at their highest ; as on either side we have 

 blue and yellow, faint purple and orange, interrupted spaces, crossed 

 by a fine striation, which I believe are the representatives of Dr. 

 Pigott's fine set of beads, if not those brought out on the upper 

 surface in my photographs,* yet caused by the same internal dispo- 

 sition of the parts as in the Lepidoptera waved scales, and giving 

 rise to similar illusory effects. Still, it occurred to me if this were 

 the case, some such structure which is so evident in the Lepidoptera 

 scale as a supporting framework ought to be present. Much time 

 was spent in seeking this, as a friend who seeing me at work 

 naively said, with the head in a bag, as a black velvet covering was 

 so arranged as to shut ofi" all extraneous hght from the stage of the 

 microscope, and made to envelop the head ; for without such help, 

 and wliich was constantly used in these examinations, I could not 

 satisfy myself of the reality of Fig, 26, h. Fearful prejudice towards 

 this view might influence the interpretation, after being well seen 

 the slide and prism were shifted, and the scale examined at another 

 time with similar success. If the daylight were not very bright, 

 the transverse markings or bars were not seen with sufficient clear- 

 ness to draw them with the camera lucida, which has been used for 

 all the figures. It is here that monochromatic light, employed 

 after the method of Count Castracane, would, it is deemed, be highly 

 advantageous. 



These are some of the appearances seen under varying arrange- 

 ments, and have been made the subject of the Fig. 26, as they are 

 not generally given by others, and may help towards a better 

 understanding of the nature of the scale, and its similarity to the 

 wavy scales of the Lepidoptera. The centre portion of the Fig. 26 

 represents the optician's view ; and here an achromatic condenser 

 was employed, because it brought out the (faintly seen) oblique 

 lines in many parts. This view, so valuable to the optician, I have 

 never regarded, since attempting to photograph the scale long since, 

 as any indication of the real structure ; and I should scarcely have 

 dared to figure it, after the beaiitiful drawings by the late Eichard 

 Beck, and the splendid photographs of Dr. Woodward, save, as they 

 are so well known to microscopists, such a figure might, if placed in 

 the centre of the others, tend to the knowledge of the general struc- 

 ture, and facilitate the recognition of the different views given. I 

 may here mention that the scales were examined as opaque objects 



* 'M. M. Joiu-nal,' No. XX., p. G7, and No. XXIII., p. 261. 



