300 
Transactions of the 
pMonthly Microscopical 
L Joui-nal, Dec. 1, 1869. 
It is impossible within the limits of this paper to describe further 
in detail the arrangements foucd to be most successful. 
It is well known that under a low power, as 80 or 100, the 
Podura is remarkable for its wavy markings (these are a safe guide 
in selecting the scale), aptly compared to "watered silk." It is 
here that the secret of their cause and nature is to be sought for : 
hitherto one which has baftled the most famous glasses of modern 
times. As a simple fact sometimes leads to a suggestion; view 
carefully against the light two pieces of the silk woven with the 
finest weft and warp placed one over the other : accordingl|F as one 
is lightly stretched more than the other or as the weft of one is 
inclined more or less to the weft of the other, instantly an endless 
series of waves are developed by the lines of optical interference : 
mesh intersecting mesh with infinitely varied effect: but always 
waves. Can the waves of the Podura be similarly caused ? 
Eaising the power to 200 or 250 and using a side light upon 
our scale athwart its length, all waviness disappears, and in its 
place is seen a longitudinal ribbing, shaded very darkly ; with a 
less oblique side light, lucid rhomboid chequers glitter brightly : 
the rhomboidal sides, crossing at acute angles, may be seen with a 
low power of 500. With 1200 these ribs have divided themselves 
into a string of longitudinal beads. But with 2300 they appear to 
lie in the same plane, and terminate abruptly on the basic mem- 
brane : upon focussing for the strings of beads attached to the lower 
side the beading appears in the intercostal spaces. The upper 
beads are best seen either green upon a pink ground or pink upon a 
greenish ground, which phenomena may possibly arise from the 
different dispersive powers or refraction of the various structures or 
the correction of the glasses ; or even more recondite causes. 
When the light is much more oblique, yet achromatic, the beads 
appear shaded as roughly represented in the diagram, the inter- 
vening spaces showing fine traces of intersecting lines. 
Using now an adjusting -J-inch at 250, and rotating the scales, 
some of the most favourable positions, with oblique light, inclined 
about 15 degrees to the axis of the scale, show a double set of lon- 
gitudinal lines forming a lattice-work. These lines are the markings 
existing on the other side of the scale. 
With 300 to 500 the celebrated " spines " appear, according to 
the size of the scale, as very dark short tapering marks (like 
" notes of admiration " without the dots ! ! !). To see these clearly 
with 2500 has been considered the ne plus ultra of microscopical 
triumphs, and it is consequently with no little diffidence that the 
writer ventures to traverse the belief of twenty-five years. The 
object of this paper is to show that definition can be further im- 
proved under the use of high powers, and if he should succeed in 
accomplishing this, the leisure of some years will not have been 
