Royal Microscopical Society. 5 



This experiment is a very curious one, and has occupied a great 

 deal of my attention. Sometimes I have succeeded, without any 

 difficulty, in satisfactorily proving to myself by rapidly calling into 

 existence, and as rapidly dismissing these almost palpable beads, 

 that they were mere illusions; and at other times, when I have 

 wished to do so, I have found it impossible so readily to call them 

 up, or to make them disappear if already in view. These failures, 

 I believe, were due to a predisposition in my own mind at the time 

 to recognize one view more strongly than the other, rather than to 

 want of skill; though no one kno\YS better than myself that the 

 necessary adroitness in manipulating high powers is not always at 

 one's own command. 



Another curious example of breaking up the surface of a scale 

 into false beads at pleasure was afforded by the inflated plumule of 

 Pieris Agathina (Plate LXX.). (I have drawn it under conditions 

 which seem to me to yield a truthful view.) On raising and lowering 

 the achromatic condenser, and varying the illumination, some re- 

 markable effects were presented, — now the longitudinal ribs assumed 

 a beaded appearance, and directly afterwards the lateral ones resolved 

 themselves each into three or four similar false images. 



A similar experiment was tried with the scale of the English 

 mosquito, CuJex ]pi])iens. 



These and many other experiments, especially the examination 

 of scales which lie across each other at various angles, causing dis- 

 tinct and palpable beads where the ribs intersect (an optical effect 

 familiar to most of us), lead me to hold the opinion very strongly 

 that very much of the beaded appearance we see on scales {lioiv 

 much I would not venture to say) is illusive, and very often caused 

 by the " dodges " we resort to in illuminating them.* 



As regards an internal deposit in scales, suggested to be the 

 case in the article in ' The Student ' for February on the Scales of 

 the Lepidoptera, I venture to dissent from that conclusion; and 

 perhaps I may be permitted to state why, theoretically. 



Such a deposit would, I think, considering the purely protec- 

 tive, and sometimes only ornamental, functions of scales, be of 

 neither use nor ornament. It would detract from their elasticity 

 and lightness, while it would add to their weight. Its presence 

 could only, I think, be atoned for by some great necessity like that 

 of the bulb and meiulla of vertebrate hairs, which is a store of 

 growing materials, needed by the slow and gradual growth of these 

 structures. But insect hau's or scales — for no one doubts these are 



* A lurking suspicion exists in my mind that some of these " dodges " pro- 

 duce results approximating to those which Dr. Pigott obtains. For instance, for 

 the last six or seven years I have been acquainted with the so-called beads upon 

 the scales of Deegeria domcstka and Mncrotnina mnjor ; Init I have not hitherto 

 interpreted them to be other than corrugations, vide 'Science Gossip' for 1SG7, 

 Figs. 43, 44, and 50, where the scales are figured. 



