240 CORRESPONDENCE. 



which I could not distinguish from Figs. 2, 3, and 4, except the note 

 of admiration shown in Fig. 2. That I was not able even to glimpse 

 through the whole series of observations, with all the apparatus em- 

 ployed. Then substituting first a Powell and Lealand -gth dry, 1870, 

 the appearances were all a little more distinct by reason of the in- 

 creased amplification, and so when the Powell and Lealand ^th dry, 

 1870, was substituted for the ^th. The illumination was by the St. 

 Germain students' lamp, with coal oil, and a J-inch condenser in all 

 the observations. The different appearances were produced by slightly 

 altering the focus, or the cone adjustment, and other appearances, 

 bearing an equal appearance of truth, were produced in the same way. 

 There can be no doubt that all the appearances shown in the figures in 

 Plate XI. may be produced by proper (or improper ?) manipulation. 

 But, like Dr. Pigott, I only saw them near the margin, when the 

 oblique and longitudinal ribs cross each other. In the central 

 portion of the scale I did not find them. But the question is not 

 what may be seen ? but what is ? Which of the various appearances, 

 if any, represents correctly the structure of the scale. 



"Whilst using the y^th I removed the dry front and substituted the 

 " immersion" front, when — "presto — change" — all my pretty beads 

 were in a moment gone. I had forgotten that the cone glass of 

 the slide was cracked. The water flowed in, and in place of the heads 

 were only the oblique ribs as plain, palpable, distinct, and as real as 

 the longitudinal ones. No coaxing could bring back the beads, with 

 any apparatus. The minute transverse, irregular wrinkles or corru- 

 gations of the membrane were there, and the water formed little 

 lacunai between them and the ribs. At separated points, when the 

 wrinkles crossed the ribs, they formed little elevations, which, when 

 not properly focussed, formed little isolated bead-like points, as 

 different from the beads before seen, as they were from the large ones 

 in Fig. 1, which surely no one would ever mistake for those of 

 Fig. 4. 



It would be difficult to persuade the late President of the E. M. S. 

 that a pig's head is formed upon a type altogether different from that 

 of other Vertebrata. And it would be none the less so to persuade an 

 entomologist, who is familiar with the appearances and structure of 

 the scales of Diptera, Coleoptera, and Lepidoptera, that those scales are 

 of a typical structure, different from that of the order Thysanura ; 

 and that in place of the ribs' wrinkles and corrugation of the mem- 

 brane in the three first-named orders, there is substituted in the last- 

 named order an internal framework of beads. 



However, Mr. Editor, I do not profess to be knowing as to these 

 matters, nor capable of throwing light upon them. But like, no 

 doubt, hundreds of others, I want to know what the truth is when I 

 see it through the microscope, and whether there is any infallible rule 

 by which false appearances may be distinguished from the true ones ; 

 and I shall have attained my object if some one who knows more and 

 sees better is able to shed some light upon this subject in the mind of 

 a benighted 



Tyro. 



