262 On the Advancing Aplanatic Power  [Meurna, Nov 1 kero, 
certainly administered the cowp de grice to all researches in this 
direction by the following final verdict :—“ It is most easy to pro- 
duce the beading (as described post facto in July) with the worst 
of three +';th objectives in his possession, by means of a deep eye- 
piece and long draw-tube.” Mr. Wenham also produces his 
September’ knotted cord beading (described by the President and 
now disclaimed) by his 35th. 
These few spherules, such as can be described with the 5th 
can readily be shown with a fine half-inch and a power of 800, by 
spoiling the definition of the crowd of beading I have described, 
z.e. by totally disarranging the corrections. 
The effect as thus produced may be explained roughly as 
follows :—When a brilliantly refracting bead is seen in the “clear- 
ing” peeping through the single structure, it swells out as all bright 
points do, as an effect of the error of the glasses, and the false ap- 
pearance is produced as figured. Shillalahs are then rendered 
instead of spines equally false. 
The optical difficulties of defining rouleaus of transparent bead- 
ing, arranged in close parallel lines and crossed by similar rouleaus 
at a small inclination, cannot be overrated ; the free beads at the 
intersections of the lattice-work are of course most readily developed. 
In the Degeeria kindly presented to me by Mr. MacIntire, the 
rouleaus cross at a greater angle than those in Test-Podura curvi- 
collis. 'The upper set may be plainly made out in the smallest and 
most transparent rounded scales, lying like straightened necklaces 
of beads or strings of pearls, of a faintly tinted rose colour. 
When the adjustments are made for the plane of vision inter- 
vening between the two sets, on the large scales both sets may be 
seen at once. But in all similar cases the beading next the source 
of light is of a brighter and paler colour than the second set nearer 
the eye, just as described in D note, December 1869, and as is seen 
in Lepisma, note C. 
Where Mr. Reade and Mr. Wenham have described beads 
isolated and separated by at least six diameters, the beads I have 
discovered and described a month before these papers appeared, lie 
in close contact, and are at least six times more numerous. 
It is to be regretted that large coarse Podura scales of the best 
sort are exceedingly scarce. It is earnestly to be hoped some enter- 
prising entomologist will rediscover a colony of these interesting 
insects, which doubtless still exist in abundance. 
Mr. Slack, F.G*S., gives another instance of advancing power in 
the beading into which he has lately resolved the costae of Pinnu- 
laria major by means of Powell and Lealand’s new immersion 4th, 
which he eulogizes as a triumph of optical art. 
I lately communicated to this gentleman an interesting ob- 
servation made upon the P. Formoswm. Each spherule appeared 
