Ohjed-glasses and their Definition. 217 



retaining their clubbed form ; and from the greater distinctness 

 that is now given to the memhrane by the destruction of its trans- 

 parency, the ribs or markings can be plainly distinguished as surface 

 elevations. From the brittleness of the burnt scales (which have 

 been heated nearly to ignition), they are easily fractured. When 

 this happens longitudinally, it takes place close to the markings, and 

 follows their shape, showing that they form a ridge or material 

 barrier for the line of direction. When the fracture is transverse, 

 the ridges again offer an impediment as the parting is not a 

 straight one, part of the markings projecting over and leaving 

 a corresponding notch in the separated portion. In fact Podura 

 scales treated in this way leave so little doubt of their structure, 

 that I think that no one that will take the trouble to try this 

 simple experiment will admit Dr. Pigott's interpretation. The 

 scales can be seen in every possible position ; some of them even 

 curled up point and quill almost together, thus affording an end 

 view. 



That remarkable Podura scale, the Seira BusJcii, alone appears 

 to afford the key to their structure. The ribbings are somewhat 

 prominent, and the ! ! ! markings so much elongated that only 

 three or four of them occupy the length of the scale, which is also 

 very convex, and the intercostal spaces much wider than in other 

 specimens. By no means of illumination, or any stretch of imagina- 

 tion, can these markings appear to be caused by the oblique crossing 

 of ribs on opposite sides. There are some coarse transverse mem- 

 brane corrugations similar to those which in the finer specimens 

 give rise to illusory headings. 



Let us not forget how deeply we are indebted to Dr. Pigott for 

 promoting these researches, who, by the power of negative in- 

 ferences, has elicited so much practical information, and who first 

 discovered a waved structure in certain Podura scales, and repeatedly 

 demonstrated that this might be resolved into ghost beads ap- 

 pearing as substantial as " peas in a pod." 



" Illumination is a vexed question for the photographer, who 

 has never yet succeeded in displaying the spherules of the A7n- 

 'phi'pleura pelliieida, which so many English observers have seen 

 microscopically. Unless, therefore, the defining power of photo- 

 graphy at least equals the human eye armed with the microscope, 

 no rehable argument can be drawn from its failure or approximate 

 revelations." Thus again did the dictum go forth, and shortly 

 after its echo had crossed the Atlantic, the genius of truth was 

 invoked, and with the sun for a limner, the desired result achieved, 

 and so exquisitely sharp and distinct are the lines on the photo- 

 graph, thus skilfully obtained by Dr. Woodward, that I am tempted 



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