TRANSACTIONS. 
On the Scales of Lepidocyrtus ? hitherto termed 
PoDURA-scALES_, and their value as Tests for the Micro- 
scope. By Richard Beck. 
(Read March 12tli, 1862.) 
The subject of the following remarks was cursorily alluded 
to in an abstract published in the last number of this Journal 
(April, 1862, p. 122). It is here intended, with the help of 
illustrations, to give in detail the proofs of the value of the 
Podura-scale as a test ; also some observations upon the struc- 
ture of the scales, a description of the insect, its habitat, and 
some remarks upon the best method of obtaining the speci- 
mens. 
The very mention of the Podura-scale as a test is often 
met with the immediate reply — Why, its markings can be 
seen with an inch and of this there is no doubt. It is not, 
however, a matter of seeing the markings, but of the way in 
which the markings are shown. I shall, indeed, only allude 
to the examination of the higher powers by this test, select-^ 
ing, in the first instance, chromatic and spherical aberrations, 
and bad workmanship, as the errors that these object-glasses 
are more especially liable to. 
PI. X, fig. 1, is a camera lucida drawing of part of one of 
the best scales, as seen by a one-eighth, with No. 3 eye-piece, 
the magnifying power being about 1300 linear; the markings 
are wedge-shaped, very black and brilliant, but with a narrow 
bright space towards the top; they are consecutive in the 
longitudinal direction of the scale^ and no transverse mark- 
ings whatever are present. 
In correcting a lens for colour, a secondary spectrum 
cannot be avoided, and a light green, resulting from the union 
of blue and yellow, is more generally approved, as being in- 
offensive, and quite immaterial. This or any stronger colour 
is instantly seen on the edge of all the dark markings, or on 
the outline of the scale itself. 
The test for spherical aberration is still more severe. If 
the object-glass be well corrected, and the object in focus, 
VOL. X. . g 
