PKOCEEDINGS OF THE SOCIETY. 985 



or trite apertures, and the best form of apertometer or means of 

 measurement, that is a dilierent branch of the qiaestion not as yet 

 concluded. 



It will save much confusion if we take Colonel "Woodward's recent 

 suggestion* for the nomenclature of the angle of immersion lenses, 

 and instead of water, glycerine, oil, or balsam angles to take the 

 degrees of the primary angle, or that imtliin the body of the front 

 lens, of immersion object-glasses, and call it " interior angle." The 

 varied exterior angles in the above fluids will then follow and be 

 known by their relative refractive indices. The stumbling-block of 

 180° (which may mean 82° only in the front lens) will then be en- 

 tirely cleared away. 



Mr. J. Beck read a paper, " Note on the Structure of the Scales of 

 a species of Mormo" slides of which were exhibited under the Micro- 

 scope (see p. 810). 



Mr. Crisp expressed a hope that the Fellows would look at the 

 scales at the close of the meeting, as they seemed to furnish a con- 

 vincing proof of the truth of Mr. Beck's theory. 



Dr. Edmunds said that some time ago he had given attention to 

 the Podura scale, and found that he could not determine on which 

 side of it the minute plumules seemed to grow, the scale being so 

 extremely thin and so transparent. He could not, however, help 

 thinking that if they w^atched the scale very carefully with some 

 kinds of dark-ground illumination, dry, and with the cover hard down 

 upon the scale, they would get appearances which were wholly 

 irreconcilable with the theory put forward by Mr. Beck. In this 

 way they got the " featherlets " projected out of the membrane, giving 

 a beautiful brush-like appearance. Then, if they had the scale 

 mounted in balsam and illuminated by highly oblique light, they 

 would get an appearance almost identical with that which was seen in 

 the former instance ; they lost all the hyaline appearance of the scale 

 itself, and saw these featherlets becoming an independent source of 

 light, standing up illuminated from the surface of the scale. 



Mr, Beck said he based the statements in his paper entirely in- 

 dependently of what anyone could see, because they might see any- 

 thing according to tlie way they looked at it. They must rather 

 reason according to the nature of the structure they were viewing. 

 In this case they were looking at two membranes united together, 

 but which could be taken apart. They could run moisture i;p and 

 down on the lower external surface, but could not do so on the upper 

 surface, which was thus shown to be almost smooth. They therefore 

 had a complex substance to deal with, and it was necessary to reason 

 as to what that structure was comjiosed of. Now, considering 

 the whole class of these scales from those of Lepidocijrtus down to 

 those of Lepisma and others, if they could satisfactorily connect the 

 one with the other, and could show that the appearances could be pro- 

 duced by regular corrugations, then it was much more consistent to 



* 'Am. Quart. Micr. Journ.,' i. (1879) p. 277; this Journal, ante, p. 783. 



