po ae PROCEEDINGS OF SOCIETIES. 323 
imposed strata of the cylinders of spun glass, he would let it be 
assumed for the occasion that the Podura scale was really formed of 
cylindrical or semi-cylindrical corrugations ; it did not matter whether 
there were two membranes or only one. Wherever these cylinders 
intersected they would develop the waviness which produced the 
black markings so often referred to; and with a low power the undula- 
tions were to be seen, as he himself had seen them twenty-five years 
ago with a 43-inch on the Podura scale, and at that time thought them 
wonderful. By putting on more power a variety of effects would be 
obtained. The intersections of the hairs of the antennze of the male 
gnat were similar in effect, and produced a beautiful test. Looking at 
the fact that these effects were dependent on three or four conditions ; 
on the symmetrical equality of the corrections at points on both sides 
and equidistant from the focal point ; on the variation of these correc- 
tions; on over-correction or under-correction of the object-glass; could 
anyone predict what would be seen in an unknown object? The 
practical point was whether the observer could define a point ; for the 
false images above and below the focus when an object consisted of 
two strata only mystified, and indeed often misled him, to select that 
which was false because more captivating than the true one. If three 
observers, A, B, C, were each exhibiting a different object, in all 
probability each would declare his own method of focussing the 
favourite appearance was the right one ; yet all being different, which 
would be the right one? He appealed to photography in confirmation 
of the principle of judging of definition by comparing a known object 
with its image, for the photographer was never satisfied until he could 
develop an image true to nature. Then only could the definition be 
ascertained. He thought they were only at the very beginning of 
this matter; and regarding the progress that had been made during 
the last thirty years, he looked forward to a more than corresponding 
improvement in the future powers of the microscope in the course of 
a future generation. No sane person would venture to deny the 
probability of such an advancement being made hereafter. 
Mr. Slack observed that Mr. McIntire confirmed the existence of 
beads in some cases, and it seemed to him that analogy intimated 
their reality in others, where without such aid the optical difficulties 
would lead to hesitation and doubt. Lepidoptera and Thysanura scales 
seemed constructed upon the same plan of two membranes with cor- 
rugations and dotted deposits between them. He did not like calling 
these deposits pigments, because in many cases they were colourless, 
or nearly so. 
With regard to the scales of Urania Leilus referred to by Mr. 
McIntire, he found it beaded in all positions when the scale was 
rotated and the illumination was unilateral from a single radial slot 
inacondenser. Observations made with torn scales showed that when 
no crushing occurred, the severance took place between the bead rows, 
the ends of which exhibited distinct terminal beads. If such scales as 
those of Morpho Menelaus were softened in a drop of warm caustic soda 
solution, and then torn by rubbing a covering glass over them, the 
same appearances would be seen. In his examinations of Lepidocyrtus 
VOL. IV. 2k 
