GO The Advancing Powers of Microscopie Definition. 



this slide were the most plainly marked of the series at my disposal, 

 partly because, as Mr. Beck collected the scales himself from a care- 

 fully-identified insect, there could be no question as to the species 

 from which they were derived. A brief account of the results of 

 these studies must now be presented." 



" 1. The Begeeria domestka (Seira domestica of Sir J. Lubbock). 

 .... Their markings were very bold, and the object was in every way 

 a beautiful one With carefully-centred illumination the mark- 

 ings resembled the notes of exclamation on the Lepidocyrtus scale. 

 .... When suitable obliquity was given to the light, an appearance 

 readily started into view, which recalled at once the description of 

 Lepidocyrtus scale given by Dr. Pigott. In focussing upon the 

 upper surface of the scale, varicose ribs, which might almost be described 

 as rows of beads, were seen, with similar ones belonging to the lower 

 surface ajipearing less distinctly in the intercostal spaces. By de- 

 pressing the objective still further the lower varicose or beaded ribs 

 came into view, the upper ones ai:)pearing less distinctly in the inter- 

 costal spaces. These varicose ribs were alternately reddish and 

 greenish, their colour varying with changes in the illumination. 

 When the upper ones were reddish, the lower were greenish, and 

 vice versa. The words varicose ribs would seem to describe the ap- 

 pearance more accurately than ' rows of beads.' Certainly no such 

 distinct isolated bosses were seen, as may be observed, in so many 

 diatoms. It is worthy of note, moreover, that on most of the scales, 

 when the objective, after having been carefully focussed, first upon 

 the upper ribs and then on the lower, was still further depressed, the 



exclamation marks started into view When monochromatic 



sunlight was used to illuminate the object, the form of the varicose 

 ribbing appeared unaltered, if anything, rather more sharply defined 

 than before ; simply the colour phenomena disappeared, the object 

 appearing like a black engraving on a ground of the tint employed. 

 .... The photographs were taken with the immersion 1-1 6th of 

 Powell and Lealand. 



" 2. The Lepidocyrtus curvicollis. With the l-8th and l-16th 

 Powell and Lealand, and oblique light thrown lengthways along the 

 scale, appearances were observed which corresponded essentially with 

 what was seen on the Degeeria. The varicose ribs, however, were 

 finer, and as it were more difficult to obtain appearances, which would 

 suggest the epithet beaded. Nevertheless the structure in question 

 was observed in a tolerably satisfactory manner with all the slides of 

 Lepidocyrtus belonging to the Museum. A well-marked scale, on the 

 slide presented by Mr. Joseph Beck, 1-1 60th of an inch long, and 

 l-700th broad at its widest part, was selected for the purpose of 

 preparing the photographs which accompany this memorandum. One 

 of these shows the exclamation marks as seen with central illumina- 

 tion, the other shows the varicose or beaded aspect as displayed by 

 oblique illumination." 



" The general conclusions arrived at during the investigation may 

 be stated as follows : — 



" The scales of Degeeria (Seira) domestica and Lepidocyrtus cxirvi- 



