252 Transactions of the 



pieces instead of high ones. The depth of focus, in many ways, is 

 very valuable, yet when oblique light is employed, and the object 

 develops colour, is a constant source of danger to correct interpre- 

 tation of structure. This, it is believed, v/ill be acknowledged by 

 those accustomed to examine objects structurally. 



When more is learnt of its optical combinations, or some secure 

 method discovered for furnishing a combination as " corrector " and 

 " amplifier," we shall all feel deeply thankful to Dr. Pigott that the 

 employment, long since adopted and rejected by opticians, of inter- 

 vening lenses, has found a permanent place in the addenda to the 

 ordinary instrument. 



The object of this paper being to deal with the general and par- 

 ticular stmcture of Lepidoptera scales, and their relationship to the 

 structure of the " Test scale," the " aplanatic searcher " would not 

 have been alluded to, had not some of the observations and draw- 

 ings been made with the form found by " error and trial " conve- 

 nient, viz. a triple and double achromatic, adapted to the proper 

 searcher, racked tube, and employed with a Gundlach's immersion 

 No. 1" (iVth), Wales' ^th, and Smith and Beck's ^th, with a No. 1 

 eye-piece and the equilateral prism, proposed by your much- 

 esteemed late President, used with a bright north daylight, without 

 achromatic condenser, except for the central portion of the Fig. 26 

 — the object being to simplify as much as possible the mode of 

 observation. 



All extraneous light was shut off from the object by a limiting 

 aperture in a thin brass slide, on which the objects between two 

 thin covers, for reversal, were fixed by springs attached to the sup- 

 port ; or if the object were on one of the ordinary slides, it was 

 surrounded by an opaque ring or slit, besides other precautions 

 alluded to farther on. 



After describing the general appearance with low powers, the 

 waviness of the scale according to the illumination giving place to 

 ''ribbing, shaded very darkly," &c., Dr. Pigott states,* " with 1200 

 these ribs have divided themselves into a string of longitudinal 

 beads. But with 2300 they appear to lie in the same plane, and 

 terminate abruptly on the basic membrane. Upon focussing for the 

 strings of beads attached to the lower side, the beading appears in 

 the intercostal spaces." Also, with " more obhque, yet achromatic " 

 illumination, " the intervening spaces showing fine traces of inter- 

 secting lines." 



Now, if these remarks be understood correctly, the structure of 

 the " Test scale " would be as if composed of a (flat) membrane 

 having on each side rows of beads external to its surfaces ; rows of 

 beads longitudinal on the one surface and oblique on the other. 

 Again, " synthetically," the same observer finds proofs of his views 

 * P. 300, No. XII., 1869. 



