Monthly Mi ical y 
one. of the Microscope. 255 
It would be difficult to assign a reason for the great advance 
now made in the performance of objectives since these lines were 
written,* unless it be in the manner here indicated. 
It is a self-evident truism that where no errors exist, correction 
is unnecessary. 
Still, a difficulty arises, which may be thus stated; if the globule 
test detects the least fault of any optical kind, it seems passing 
strange that it did not suffice to test inferior glasses up to 1869. 
Either it failed, or the erroneous indications were ignored or hushed 
away. If the glasses were absolutely corrected, no further advance 
was possible. But the advance has been made, the new jth to 
witness, therefore improvement was possible; and therefore resi- 
duary error remained and may still remain, requiring amelioration 
or extinction. 
The above statement, as regards aberration, appears tolerably 
well fortified by this argument, though it has excited an unusual 
amount of opposition. A distinguished writer, however, in the 
‘Student’ observes— Microscopical observers have been very 
plainly told two startling and unwelcome truths, for such we must 
pronounce them. Firstly, Dr. Pigott says that they have not seen 
their favourite test-object, the Podura scale (Lepidocyrtus curvi- 
collis), properly ; secondly, that their best object-glasses are aftlicted 
with sufficient spherical aberration to have rendered the structure 
what he describes invisible.” ..... 
“So early as 1848, Mr. De la Rue, V.P.R.S., distinguished the 
beadings of the scale Amathusia Hosfieldii; and he says I have 
convinced myself (speaking of the cross-stria) by careful and re- 
peated examinations that the striz themselves are really beaded.” 
In this paper in the ‘Student’ a variety of instances are given 
and figured, in which Lepidopterous scales are distinctly beaded. 
There is nothing improbable that the same beaded law of structure 
should take effect in the Podura scale. The only real objection 
appears to lie in the difficulty experienced by microscopists in de- 
fining the beads themselves. 
In the same paper Mr. Gosse’s ‘ Evenings with the Microscope’ 
is quoted to this effect :—‘ The ribs of the Lepisma scale diverge at 
different angles, those of the upper surfaces being more divergent, 
divaricating from the foot-stalk, while those of the lower membrane 
are coarser, more nearly parallel, their bones ranging along the hind 
edge of the scale. The effect of the intersection of the sets of lines 
at_ so acute an angle is to convey the optical impression that the 
scale is covered with short irregular dashes.” 
This scale is beautifully figured in Beck’s plate with transparent 
structureless ribs and fine dashes. 
I beg again to call attention to my description of the beaded 
* May, 1869. 
VOL. IV. T 
