14 Structure of the Scales of Lepisma Saccharina. 



matic condenser with and without central stops, and with Wen- 

 ham's paraboloid. All these methods point to the same conclusions. 

 Following up the line of observations described by the late Kichard 

 Beck, in his most valuable contribution to our knowledge of this 

 subject, the same results were arrived at in regard to the appearance 

 of coarse beading, &c., viz. " that the interrupted appearance is 

 produced by two sets of uninterrupted lines on different surfaces."* 

 That the longitudinal and the oblique lines are on different sides of 

 the scale is also plainly seen by their lying in different focal planes 

 under a sVth objective. And further, while examining a scale in 

 fluid I have repeatedly observed air bubbles on one surface of it 

 confined by the longitudinal ribs, and on the other side others 

 bounded by the obhque ridges ; and on moving the slow adjustment 

 up and down, with the movement of the bubbles under control, 

 they never interfere or mix with each other.! Nothing further is 

 required to prove that these markings are actually ridges, and that 

 they project from different surfaces of the object. The experiments 

 of Mr. Beck settle this question. 



As microscopical definition advanced the very feeble radiating 

 lines were noticed in the spaces between the ribs, formerly thought 

 to be smooth. In the central portion of the test these lines are 

 parallel with the main ribbing. They in their turn were seen to 

 be uneven and pronounced to be " beaded striae."^ Must this fine 

 beading like its shadowy predecessors be also extinguished by 

 intersecting cross lines, and so add one more to the long list of 

 illusory appearances ? To attempt to throw some light upon this 

 question is the principal object of the present article. 



In the fu'st place, it is far from being a difficult feat to see this 

 beading. Any first-class lens, from a ^th upward, when properly 

 handled, will display it or something very like it. The writer 

 has found it an easy task with Wales' -xz^ immersion, or even with 

 a Beck ith and deep eye-piece. With Tolles' J^th immersion the 

 fine transverse structure indicated above is brought out, and it be- 

 comes at once evident that the small beads are indeed spurious like 

 their big brothers, and for a similar reason. 



The fine transverse markings seem to branch from the faint 

 radiating ones, and have the appearance of a network of minute 

 capillaries. Beside these there are coarser transverse waves or 

 corrugations of the membrane. In numerous instances, air bub- 

 bles have been observed imprisoned between the heavy ribs on one 

 or two sides, and by these corrugations on the other sides. There- 

 fore the corrugations may safely be said to be on the same surface 

 of the scale with the longitudinal ridges, and the branching vein- 



* ' The Achromatic Microscope,' Beck, p. 50. 



t See ' Micrographic Dictionary,' 2nd ed., p. 34, fig. 3, pi. xxvii, 



X See ' M. M. Journal,' March, 1873, PL XI., Figs. 3 and 4. 



