226 THE MICROSCOPE AND ITS REVELATIONS. 



to the direction of the light, and in fig. 5 when they lie in the same direc- 

 tion as the light with their narrow ends pointing to it. When this last 

 direction is reversed, the light from the points is so slight, that the scales 

 appear to have lost their markings altogether. If moisture should insin- 

 uate itself between the scale and the covering-glass, the markings disap- 

 pear entirely, as shown in fig. 3; and this is true also of the scale of 

 Lepisma. A certain longitudinal continuity may be traced between the 

 ' exclamation-marks ' in the ordinary test-scale; but this is much more 

 apparent in other scales from the same species (Fig. 420), as well as in 

 the scales of various allied types, which were carefully studied by the late 

 Mr. R. Beck. 1 In certain other types, indeed, the scales have very dis- 

 tinct longitudinal parallel. ribs, sometimes with regularly disposed cross- 

 bars; these ribs, being confined to one surface only (that which is in con- 

 tact with the body), are not subject to any such interference with their 

 optical continuity as has been shown to occur in Lepisma; but more or 

 less distinct indications of radiating corrugations often present them- 

 selves. The appearance of the interrupted ' exclamation-marks ' Mr. J. 

 Beck (op. cit., p. 254) considers to be due "to irregular corrugations of 

 the outer surface of the under membrane, to slight undulations on the 

 outer surface of the upper membrane, and to structure between the super- 

 posed membranes." It has been recently stated by Mr. Joseph Beck, 

 that the scales of a Lepidopterous insect belonging to the genus Mormo, 

 which under a low power present the watered-silk appearance seen in the 

 Podura-scale, under a l-5th show the ' exclamation ' markings, whilst 

 under a l-10th they exhibit distinct ribs from pedicle to apex; thus show- 

 ing in one scale how the appearances run from one scale into the other. 2 

 On the other hand, we are assured by Dr. Royston-Pigott, not only that 

 what a lens most perfectly corrected for spherical aberration ought to show, 

 is a minute beaded structure, alike in the ' exclamation-markings ' and in 

 the spaces between them; but that the markings whose perfect definition 

 had been previously considered the aim of all constructors of high-power 

 Objectives, are altogether illusory, these markings representing nothing 

 else than the manner in which the rouleaux of beads lie with reference 

 to one another. 3 The Author has fully satisfied himself bv his own 

 study, under an oil-immersion l-25th of Messrs. Powell and Lealand, of 

 a Podura-scale illuminated by the < immersion paraboloid ' (which gives 

 a view of it entirely different than any that can be obtained either by 

 transmitted or reflected light), that the 'exclamation-markings' are as 

 maintained by the Messrs. Beck the optical expression of a corrugated 

 or ribbed arrangement of the lower membrane of the scale, slightly 

 modified by the internal structure of the upper membrane, and probably 

 also (as confirmed by Mr. Wenham) by a structure interposed between 

 the two membranes. 4 And this conclusion is borne out, in opposition to 

 the doctrine of Dr. Royston Pigott, by two unrivalled Photographs taken 

 of the Podura-scale by Col. Dr. Woodward. One of these, taken with a 

 magnifying power of 3200 diameters, central monochromatic light, im- 

 mersion l-16th, and amplifier, shows the ' exclamation-marks ' better 



1 "Trans. of.Microsc. Soc.," N.S., Vol. x. (1862), p. 83. See also Mr. Joseph 

 Beck, in the Appendix to Sir John Lubbock's " Monograph," and in "Monthly 

 Microsc. Journ.," Vol. iv., p. 253. 



12 "Journ. Roy. Microsc. Soc.," Vol. ii. (1879), p. 810. 



3 See his paper ' On High Power Definition,' in " Monthly Microscopical Jour- 

 nal," Vol. ii. (1869), p. 295, and several subsequent papers. 



4 " Monthly Journ. of Microsc. Sci.," Vol. xi. (1874), p. 75. 



