HARD WICKE'S SCIENCE - G OSSIP. 



59 



tually demolish the ' ' beaded structure " theory ; or, 

 as Dr. Anthony pertinently puts it, "strike at the 

 root of a vast number of descriptions of (jnasi- 

 beaded tissue, said to have been seen in many objects 

 when examined under high power lenses."* 



By a simple contrivance, the arrangement of a few 

 thin glass rods placed at right angles to each other, 

 and made to revolve in contrary directions, the late 

 Mr. Hennah demonstrated how easily illusory ap- 

 pearances can be produced and photographically 

 depicted. As the light is made to strike the rods 

 in a more or less oblique direction, a variety of very 

 curious spectral phenomena will appear. These can 

 be considerably varied and rendered more deceptive, 

 as the foci of objects placed behind the rods ai'e 

 made to fall within or upon their front surfaces, f I 

 have repeatedly amused myself by producing 

 " beaded " and other phantoms with a series of rods 

 so arranged. I have also by using a very oblique 

 pencil of light, observed "beads" in many of the 

 scales of Lepidoptera. The drawing fig. 46 was 

 made whilst I was engaged on the paper published 

 in the Monthly Microscopical jfoicrnal, 1872, "On 

 Gnats' Scales"; fig. 47, a large scale from the wing of 

 one of the diurnal Lepidoptera, was drawn at the 

 same time ; both of which convey some idea of "rows 

 of beads," and have since been proved by Dr. Wood- 

 ward to be illusory or spectral. A thorough investi- 

 gation of all the circumstances and facts at our com- 

 mand in connection with this question can only lead 

 to one conclusion — that nothing new has either been 

 made out or discovered with regard to the structure 

 of i3odura or gnats' scales by the aid of the 

 "aplanatic searcher," or "amplifier"; and further, 

 that rows of red, green, and blue "(^^a^z-beads," 

 when seen in them, are solely due to diffraction 

 phenomena and increase of chromatic aberration, 

 produced by the introduction of a series of lenses into 

 the body of the microscope. Moi'eover, the charm 

 of novelty cannot be claimed for these beaded appear- 

 ances : they were described and figured in the third 

 volume of the "Transactions of the Microscopical 

 Society of London," 1848, by Mr. Warren De la Rue, 

 who described cross strim on the scales of Amathusia 

 Ho7'sJic'ldii with "^beaded lines or protuberances." 

 When these were focussed at their summits, they ap- 

 peared as "brown dots. "J When these scales were 

 exhibited, they were pronounced by competent ob- 

 servers to be "the overlying pigment-cells " between 

 the two layers of membrane. The interference of 

 "overlying pigment" in a corrugated tissue might 

 be expected to produce'a delusive appearance, and it 

 scarcely admits of a doubt that the generally received 

 description of the structure of the podura-scale is the 



* JMonthly Microscopical jfournal, vol. xv. p. 256. 



t BTonthly Microscopical Journal, vol. v. p. 195. 



% Warren De la Rue, F. R. S. &c. "On the Markings on 

 the Scales of Amathubia Horsfieldii," Micros. Soc. Trans., 

 December, 1848. 



more correct one. The bead-like swellings or dots 

 observed in these scales, and in those of many of the 

 Lepidoptera, are simply aggregations of minute 

 particles retained between layers of membrane thrown 

 into wavy longitudinal folds, again crossed by ribs or 

 strics. These, when slightly out of focus, appear as 

 varicose dots or beads.* This is fully confirmed by 

 careful focussing ; when first the upper series of 

 varicose ribs come into view, then, by slightly depress- 

 ing the objective a lower set is seen, the upper set 

 almost disappearing. By another slight movement 

 of the fine adjustment, the true ribs are lost sight of, 

 and the "exclamation dots " come into view, or the 

 object assumes a variety of colours. This train of 

 phenomena is frequently reversed, in consequence of 

 the relative differences between the upper and lower 

 series of strice. In the darker-coloured scales or 

 scales slightly charred and broken, the stria and pig- 

 ment ai-e best defined. 



The footstalk of the scales of the larger Lepi- 

 doptera is sometimes seen filled with colouring matter, 

 of an albuminoid or fatty nature. Scales examined 

 before the insect is dead, or whilst it is under the 

 influence of chloroform, and still attached to the wing, 

 are seen to terminate, not as a simple stalk, but as a 

 series of diverging rootlets, as represented in the 

 accompanying drawing. I infer, then, that all scales 

 and wings are nourished in a somewhat similar 

 manner as hairs and the epidermal coverings of 

 animals. The colouring matter of the scales heightens 

 the iridescent effects, and aids in imparting beauty and 

 variety to the gossamer wings of the insect tribe. 

 Besides the charm of colour, the scales of Lepidoptera 

 are exceedingly variable in form ; being oval, oblong, 

 cordate, curvate, filifonn, or capillary ; with free 

 ends, rounded, truncate, toothed, &c. These, and 

 other peculiarities, have a certain value for the ento- 

 mologist in the study of the laws of evolution, varia- 

 tion, and distribution of species, while to the micro- 

 scopist, all modifications tending towards persistency 

 materially assist in the differentiation of the fauna. 

 In the Lepidoptera especially, changes in form and 

 colour eventuate in that interesting phenomenon, now 

 known as " protective mimicry." I have no hope of 

 presenting an exhaustive examination of these 

 interesting objects, and must content myself on the 

 present occasion with a few brief remarks on a few of 

 the more curious forms of scales, and which, I trust, 

 will induce other workers with more time at command 

 to follow up the subject. Among the diurnal Lepi- 

 doptera the scales of the genus Papilio terminate in a 

 double footstalk, and somewhat resemble an ancient 

 weapon, a "bipennis." On the anterior wing of 

 Papilio Polydamzis, two curious scales are found; 

 one leaf-shaped, and very pointed, the other not 



* See Dr. Woodward, " On the Structure of the Podura 

 Scale," jW. M. J., vol. v. p. 158 ; also, Dr. Maddox, " On the 

 Structure of the Scales of some of the Lepidoptera,'' M. M. jf., 

 vol. V. p. 247. 



