Royal Microscopical Society. 259 



with the parabolic condenser, but without apparent advantage in 

 any respect. 



It will be gathered from the foregoing and the figures that, 

 like some others (Drs. Pigott and Woodward, &c,), the spines 

 — " notes " ! ! are not regarded as having any existence as separable 

 bodies, but as due to the refractive nature of the rib at that part, 

 its thickness and solidity, and the direction of the incident light, 

 which opticians know so well how to arrange to the greatest advan- 

 tage in testing the general " performance " of their objectives — a 

 charming object, doubtless, yet not the whole truth ! To quote the 

 words of one of our most skilled observers, the late Eichard Beck, 

 when concluding his paper on the photographic apj)earance of the 

 section of the bossed tumbler, " How can any reasoning be sound 

 when based merely on the appearances of the markings under the 

 microscope, which, as I have endeavoured to show, may be entirely 

 due to the peculiarity of the illumination, and may give not the 

 least indication whatever of the true structure ? " * 



Nor are Dr. Pigott's views considered as any way indicative of 

 the correct structure of the scale. The beads are regarded as marked 

 optical effects of refraction and interference where the junctions of 

 the longitudinal " ribs " by transverse and vertical septa take place 

 with the opposite membrane, by which we obtain, with the assistance 

 of the obhque attachments, the joint effect of short rods applied to 

 the long ones, and hence illusory, an opinion largely shared by 

 others. In relation to which, I may cite the optical effects pro- 

 cured by crossed glass cylinders, shown so beautifully in Mr. 

 Hennah's photographs,! and which he very kindly sent me. The 

 pseudo-beads are ^o well represented as real beaded structure, that 

 the appearance would puzzle the most practised eye, and prove 

 how necessary to be guarded in the term beads as components or 

 definite paiis of minute structure. % 



Amongst other experiments, an attempt was made, in a rough 

 way, to try and spht the scales of the wing of Poniia hrassica. It 

 was cemented on one side to a glass shde by coloured turpentine 

 cement ; then the wing was suddenly torn from the glass, trusting 

 to split some of the scales into thin layers. The scales were left 

 entire. The adherent scales were now subjected to the same violent 

 treatment, by attaching a slide to their exposed surfaces, but from 

 the scale being rendered so transparent by the cement, it was un- 



* ' Intellectual Observer,' vol. vii., p. 95. 



t See Report of Proceedings of Societies, M. M. Joum.,' No. XXVIIL, p. 195. 



X Several years since I made photographs of spun-glass hair cut into lengtlis, 

 then laid on thin glass covers, and attached to them by heat only ; next arranged 

 so as to cross or rotate over each other, either with the little rods touching, or 

 with one set resting on the free surface of the other thin cover, or with the free 

 surfaces of both covers in contact. Tlie eftects occasioned by interference were 

 highly curious and instructive. Unfortunately, the prints have been mislaid, or 

 they should huve been added to the others. 



VOL. V. U 



