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of collimation is directed at the horizontal wire of the level.
Having pointed the collimator at this telescope, turn it half
round in azimuth, and it will be found directed either exactly
at the horizontal wire of the (adjusted) level, or at a small
angle above or below it. In the former case the instrument
must be perfect ; in the next it measures elevations in defect
by half that angle; and in the latter, in excess by the same
quantity.

On the Effect of Flexure in the Cylinder.

Were the object-glass fixed within one ring, and the cross
wires within the other, a slight deflection of the tube could
not sensibly affect the parallelism of the line of sight to an
axis passing through the centres of both rings. When the
stop is fixed, as in the sector, between the rings, the effect of
flexure must be to depress the horizontal wire below the level
it would otherwise maintain. The line of sight, although still



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334 Mr. J» Nixon on his Horizon-Sector.

capable of being adjusted to bisect the same object with the
tube direct and inverted, must be inclined, at an angle of ele-
vation, to the horizontal axis passing through the centre of the
rings. As this deflection evidently increases, or diminishes
the error arising from an inequality in the rings, accordingly
as the object-glass is fixed nearest to the wider or to the nar-
rower ring, its precise amount should be ascertained.

The error, as measured by the fourth method, being \$ U T>
or 5"'7, with the object-glass in the place of the eye-tube, the
difference of 14" will be the sum of the two flexures. As the
stop is fixed 6*3 inches from the narrower ring, and 10*8 inches
from the wider one, 5"'2 will be the deflection of the line of
sight in the former, and 8"*8 its value in the latter case.
Hence the measurements by the first method, which requires
the tube inflexible, will be in defect by 5"'2 ; whilst those by
the eleventh method, which are not only exclusive of flexure,
but also suppose the stop placed equidistant from the rings,
must be augmented by half the sum of the two flexures. Ihe
peculiar situation of the stop of the eye-tube, made use of in
the observations by the sixth method, renders the quantity of
deflection uncertain.

The following list contains the instrumental error, as given
by each of the different methods, corrected for flexure. The
mean of the whole, 21", cannot possibly deviate from the truth
by more than a second or two.

Method. No. of Obs.

I. Error 16"-5 + 5"'2 Flexure =2l"'7 2

II 26 2 1

IV 19 -8 4

■y # 9 ......... ••••••••• 21 *8 •••••• 8

VI ll"-5 + 5"-2(?) 16-7 2

VII 30*5 2

X 21 -3 18

XI 18"-7 + 7"'0 20-7 35

Arithmetical mean 22£; — rational mean 21j.

Were the terrestrial refraction unquestionably a- constant
ratio of the arc of distance, the error of the sector might be
ascertained by comparing the observed refraction on short
arcs, with its apparent value, for others of greater extent
This method applied to 50 arcs, amounting in the whole to
upwards of 8°, and ranging from S'*0" to 25' 34", determined

the true refraction to be wrr, and the error in question 17"*2.

Leeds, Feb. 8, 1833. John NlXON.



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[ 835 ]

L V, An Account of Test Objects for Microscopes. By Andrew
Pritchard, Esq.*.

[With a Plate.]

"PVERY important advance in our knowledge of those
■*-* bodies in the material universe, from which our earth ap-
pears as an atom, has been coeval with, and greatly dependent
upon, some augmentation of the powers and effectiveness of
telescopes. Before the discovery of the double stars and ne-
bulae, the goodness of these instruments was determined by
their capability of showing the planets and their satellites.
But, since our acquaintance with the former bodies, telescopes
have to undergo more severe tests, and greater accuracy in
their construction is required. What has been advanced in
regard to the telescope will be found applicable to the mi-
croscope ; and to the discovery of certain objects which may
be considered as tests of the penetrating and defining powers
of this instrument, we may justly attribute the grand and mag-
nificent improvements which the microscope has recently re-
ceived.

In the perusal of the works of Leeuwenhoek, Dr. Goring
met with a passage describing the dust, or imbricated scales,
which cover the wings of the silkworm \Phalena Mori), from
which he was led to suspect there were some peculiar pro-
perties in the lines on the feathers and scales of insects, ren-
dering them more difficult to be discerned than other micro-
scopic objects ; and the result of his investigation was the dis-
covery of their properties as tests — a description of object be-
fore unknown in the annals of microscopic science.

Now, as it is undoubtedly of the highest importance to the
naturalist that he should know the exact capabilities of rite
instrument, in order that he may not be led astray in his in-
vestigations, by placing undue confidence in it; and as these
tests offer the best means of accomplishing this end, I conceive
them to be of the greatest value and interest. As no complete
account of them is extant, I shall endeavour to supply this de-
ficiency in the present chapter, and illustrate the subject by
accurate drawings of them, greatly magnified. * * *

Having ascertained that different test-objects require dif-
ferent degrees of perfection in the instrument used to develope
their structure, it became an interesting pursuit to discover
those which are best adapted for this purpose, and the pecu-
liarities, in the illumination, &c. under which they are exhi-

* Abridged from the " Microscopic Cabinet." London : Whittaker, Treacher
and Co; 1832. An account of this work was given in the Lond. and Edinb.
Phil. Mag. and Journ. vol. i. p. 163.



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336 Mr. Pritchard's Account of Test Objects for Microscopes.

bited with the greatest perspicuity. In this investigation, it
was found that there were two distinct properties in a micro-
scope, and that the instrument might possess a very consider-
able approximation to perfection in the one, and fall short in
the other, or vice versa, or might be perfect in both. The
lines on the dust or feathers from the wings of the lepidoptera,
and those on the scales from the body and limbs of the thysa-
nuraeous insects, offered the means of determining their good-
ness in one particular, viz. their penetration, and the structure
of the hair of animals, certain mosses, &c. served to ascertain
their defining power.

The analogy between telescopes and microscopes is so great,
that I cannot be said to digress from my subject by stating
that the aforesaid observations apply also to the former of these
instruments, which seldom combines the two qualities of pene-
tration and definition to any great extent. Thus, a telescope
with a large aperture will frequently resolve clusters of stars,
and exhibit nebulae, while it will fail in defining the disc of a
planet, or the moon, with precision ; and, on the other hand,
one of moderate aperture accurately figured will define the
latter, but be wholly inert on the nebulae and clusters. So a
microscope with large aperture and high power will show the
" active molecules" and lined objects, while it will not define
a leaf of moss, or a mouse hair ; and another with a smaller
aperture will define the latter, but prove ineffective on the
former. This is very manifest in single lenses which require
different apertures for different objects*.

The penetration of a microscope has been shown to be de-

Eendent on its angle of aperture, and that whenever this was
$s than a certain quantity, the lined structure of the scales
cannot be rendered visible, however perfect the instrument
may be; and the defining power is inversely as the quantity of
spherical and chromatic aberration.

A proof, or test-object, may be defined to be one which re-
quires a certain degree of excellence or perfection in a micro-
scope or engiscope for the development either of the whole, or
some particular part of its structure.

Test-objects are separable into two great divisions ; but as
I intend only to treat on one of them, it is proper here to point
out their distinction. In the first division I place those which
operate out of focus, and tell us what the defects of an instru-
ment are. The second, those which, if exhibited by a micro-

* I have a very beautiful sapphire lens (plano-convex of one fifteenth
focus) that shows the lines on the long brassica very distinct and sharp,
when its aperture is large, but will not define a moss satisfactorily with
this aperture; but as stops behind the object have the effect of reducing
it, with them it shows the latter.



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Mr, Pritchard's Account of Test Objects for Microscopes. 337

scope, assure us that it possesses certain good qualities. The
first division, as artificial stars, enamel dial-plate, wire gauze,
&c.*, which inform us of the state of their aberration, achro-
matism, centering, adjustment, curves, &c, I shall pass over, —
as many persons are not disposed to enter into a scientific
scrutiny concerning the causes of their demerits, and because
they are more applicable to engiscopes, or compound micro-
scopes, than to single and compound magnifiers, — and shall
content myself by giving some simple means of determining
effectiveness by means of the second division.

(1.) Lepisma saccharina. — The insects of the families Le-
pismenas and Podurellae are comprehended in the order Thy-
sanoura of Cuvier and Latreille ; they are small, frequenting
damp places, and are of various colours; they leap like fleas.

The scales of these apterous insects must be taken from fresh
specimens, for, when long dead, they adhere so firmly to the
insect, that they cannot be detached without injury.

Their longitudinal lines slightly radiate from the point of
insertion ; they are readily seen, and appear flat or square,
like the indentations on some bivalve shells: these are the
prettiest scales I am acquainted with. There are other lines
in various directions, as shown in the drawing of a magnified
scale at fig. 1. Plate III. When the candle is so placed as
to bring out the latter strongest, and the scale is turned round
in the axis of the microscope in certain positions, they will
cease to appear connected. In this object it is the sharpness
and cleanness of the spaces that chiefly evince the goodness of
a microscope, for the longitudinal lines are easily developed.

(2.) The Morpho Menelaus. — This butterfly is indigenous
to America, the wings are indented, and their superior surface
of a highly-polished blue colour.

The imbricated scales from the centre of the superior side
of the wing are of a pale blue, mixt with others almost black.
The former are mostly broader than the latter, and are the test-
objects required ; they measure about one one-hundred and
twentieth or an inch in length. When viewed in a microscope,
they exhibit a series of longitudinal stripes or lines, as shown
in the magnified drawing, fig. 2. Plate III. Between these
lines are disposed cross striae, which, with the lines, give it
the appearance of brick-work.

The microscope or engiscope under examination should
be able to make out these markings, with the spaces between
them, clean and distinct. The cross striae, which give the

* For a particular account of these objects, see Dr. Goring's Memoirs
" Ou the Exact Method of, &c." p. 191. Mic. Cab.

Third Series. Vol. 2. No. 11. May 1833. 2 X



i. nur?i ... '. *« *"• •



338 Mr. Pritchard's Account of Test Objects for Microscopes.

brick-work appearance, are seldom to be seen all over the
feather at once. The tissue that covers this scale or feather
contains the largest portion of colouring matter, and is often
destroyed in removing them from the wing, and along with it
the cross striae. In such cases, the longitudinal lines only
can be visible. The damaged specimens are easily known by
their paleness.

(3.) (Alucita pentadactylusy and hexadactylus.)— -The ten
and twenty Plumed Moths. — The structure of the wings, or,
more properly, plumes of these insects, is so peculiar, that few
persons acquainted with entomology are strangers to it.

The twenty-plumed moth is more delicate in its form than
the other. The feathers or scales, employed as proof objects,
must be taken from the body of the insect, and not from the
plumes or wings. Their breadth is generally greater than
their length, and their form is never symmetrical. They are
transparent, and about one one-hundred and eightieth of an
inch long. The scale is often partially covered by a delicate,
uneven, membranous film, which obliterates the lines on those
parts. The longitudinal lines are not difficult to resolve, but
their proximity is such, that they require a considerable power
and careful illumination to separate them distinctly. They are
elegant microscopic objects, but rather scarce. * * *

(5.) The Clothes Moth.— {Tinea vestianella.) — These small
brown moths possess very delicate and unique scales, requiring
some tact in the management of the illumination, to resolve
their lines distinctly. I should observe, that it is the small
feathers only, from the under side of the wing, that must be
considered as tests ; the others are easy. A magnified view
of a small one, about one four-hundredth of an inch long, is
given in fig. 3. of Plate III. They are readily made out
under the single and doublet magnifiers. This is a favourite
object with some, who exhibit it as the standard of excellence.
I do not consider it very difficult; though it must be admitted,
to bring out the lines sharp and clean, requires an excellent
instrument.

(6.) PontiaBrassica* (Leach.)^— The pale slender double-
headed feathers, about one eightieth of an inch long, having
brush-like appendages at their insertion, obtained from some
portions of the wing of this large cabbage butterfly, afford an
excellent criterion of the goodness of a microscope. Some
connoisseurs prefer them to all others, and form an accurate
judgement of an instrument by the manner in which it demon-
strates this single object. They are easily detached from the



• This is the Pieris Brawca of Latreille.



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Mr. Pritchard's Account of Test Objects for Microscopes. 339

wing by the point of a quill, but must be gently handled, for,,
like many others, they are soon mutilated; indeed I have sel-
dom seen them perfect in the ordinary sliders. Those speci-
mens which are easily resolved are readily distinguished, being
short, broad, and more opake. There are also found, on the
same wing, two or three other sorts, but they are unworthy of
notice as proof objects.

In Plate III, at fig. 4*. is represented a sample of the re-
gular proof feather. It is very transparent, and has a yellowish
tint; the surface is seldom smooth, as indicated at the part a.
In the engiscope these inequalities are not so observable, and
therefore, when the lines appear strong, the surface is more
uniform than in the microscope. This object requires the
light more oblique than any other of the lined kind. On this
account I have seldom been able to see the lines satisfactorily
with Dr. Wollaston's illumination, unless the magnifier was
much out of the axis of the perforation. If we throw the light
of a candle (placed a few inches behind the stage) obliquely
pn them, they can be seen very sharp. I have seen them in
this way with a simple jewel lens, of only one fifteenth of an
inch focus.

, (7.) The Podura plumbea. — (Lead-coloured Spring-tail.) —
The body and limbs of these insects are covered with scales,
which, from their extreme delicacy, require great care in re-
moving. They are also very soft, and easily wounded. The
fluid which exudes from the injury so completely adheres to
the scales as to obliterate all their markings. Hence they
must be cautiously handled. Those who are desirous of pre-
serving these insects, should keep camphor along with them ;
through, omitting this, I once had a large collection of them
consumed by a species of mite (Acarus) 9 which had insinuated
itself into the box.

. I have never been able to see the lines on them with a power
much below 250 (that is, one twenty-fifth of an inch ibcus),
and therefore microscopes of a lower power cannot be expected
to show them, except of very superior quality ; for it must con-
stantly be kept in mind, that that instrument is the best which
exhibits an object with the least amplification, all other things
being equal.

It is affirmed, by a very acute experimenter, of these scales,
that " all are difficult, and some seem to defy all power of de-
finition." The latter part of this quotation is perfectly accu-
rate; but I differ in the former, because many specimens,

* The reader should examine this and the other figures with a hand
magnifier.

2X2



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S40 Mr. Pritchard's Account of Test Objects for Microscopes.

especially the French ones, are very easy, and unworthy the
title of proofs ; and, as they might be substituted for those I
am describing, and thus a common instrument might pass for
one of superior excellence, I feel justified in giving this cau-
tion *.

The size of these scales varies from one nine hundredth to
one hundred and sixtieth of an inch in length, and, as they
decrease in size, become more transparent They are of dif-
ferent forms, but possess a general character, easily recognised,
by the want of any sharp angles. Under a microscope not
having sufficient penetration, the tissue appears devoid of
structure or markings ; but, when placed in a superior one,
and the illumination properly made, they show a series of
lines or cords on their surface, and present a much greater
variety in their arrangement than the scales of any other
species of insect. Some have the lines straight, as shown in
the magnified scales, Plate III. fiff. 5. and 6, and have two
sets of oblique lines on them, similar to fig. 8f; others are
waved and curved, as shown in fig. 7, 9, and 10, while on
some of the small ones, as fig. 11, nothing satisfactory has
yet been developed. In these figures I have endeavoured to
give the appearances which the objects present under the mi.
croscope ; and it will be observed, on a careful inspection of
them, that the lines on fig. 9, 10, and IS, (which are only
portions of scales,) are very different from those on fig. 5.
and 6, the former ones not being so sharp and defined as the
latter.

As a genera] rule, it will be found that the smaller the
scales the more difficult the test; those in fig. 6, however,
cannot be included as tests, as they are very easily resolved.
I must not omit to notice, also, that the cords on these scales
are loosely attached to the tissue, and are often rubbed off in
mounting. Of course it will be fruitless to examine such spe-
cimens. Those on which the greatest reliance may be placed
are similar to fig. 5, though the same scale will assume all the
appearances of fig. 8, 9, 10, and 13.

Before leaving the subject of the lined objects, I should no-
tice, that all objects of similar structure are more or less tests,
as the lines on the scales of some beetles, one of which, from

* It should be remembered that the exhibition of the lines on these
scales is only proof of the penetration of a microscope; and unless the out-
line of the scale is sharp, the instrument is defective in definition.

f As in the scales of the PonHa Brastica, only one system of oblique lines
can be seen at once; the other system is similar to those in fig. 8, but
running in a direction at right angles to them.



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Mr. Pritchard's Account of Test Objects/or Microscopes. 341

the diamond beetle (Curculio imperialis) is shown at fig. 12;
Plate III.* The lines and markings on certain vegetable tis-
sues, and many others too numerous to name, may also be
employed as proof objects. The reason for making a selection
of those above described, has been in order to render the task
of judging of the merits of an instrument by different indivi-
duals more simple and satisfactory, so that by the assistance
of the drawings, and a sample of the objects, they may ascer-
tain the quality of an instrument without the trouble of com-
paring it with others, which are often difficult, and sometimes
impossible, to procure.

Definition.

The defining power of microscopes and engiscopes depends
on their capability of collecting together all the rays from any
one point of the object, or, in other words, their freedom from
aberration, and is independent of their penetration ; for, if we
take an engiscope and view a lined object with the aperture of
the objective, as it is usually sold in the shops, its defining
power may be very fair ; but if we enlarge the aperture so as
to enable us to develope the lines which it will then accom-
plish, the defining power of the instrument will be injured to
such an extent as to render the outline quite confused. The
great desideratum, then, in microscopes and engiscopes, is to
obtain these two qualities combined, which, however, is only
rarely attained.

Cylindrical or spherical bodies appear the best suited for
ascertaining the goodness of an instrument, as regards defini-
tion ; and the following examples, which are prefaced by re-
marks on the method of illuminating them, I deem sufficient
for this end.

In the preceding class of objects, oblique diverging rays ap-

* The scales from the body of the diamond beetle, either as transparent
or opake objects, are by far the most brilliant, in point of colour, of any
of the lined class. In viewing them as opake objects, with single lenses,
in order to exhibit the lines, the scale must be brought a little within the
focus, and the illumination carefully arranged. As you cannot exhibit them
with single lenses of a one twentieth or one thirtieth of an inch focus with-
out using silver cups, it is difficult to procure oblique light. As transparent
objects, they are much easier managed. They present a mottled sort of
colour, composed of the brightest carmine, mixed with purple, blue, and
yellow, and their lines are distinctly seen, as shown in fig. 12. As the
lines on some of these scales are of easy resolution, it will not be advisable
to trust every specimen as a test. The small ones from the legs of the
Brazilian beetle are the most difficult, and many of these require the most
rigid adjustment of the focus and illumination to resolve the lines, and the
slightest tremor, though not enough to occasion any sensible dancing (as a
carriage at a distance), is sufficient to render them invisible. :



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342 Mr. Pritehard's Account of Test Object* for Microscope*

pear to be essential for the development of their structure, the
degree of obliquity varying, however, with different specimens
of scales* The extremes of this variation are the Podura
plumbea and Pieris Brassica> the delicacy of the former re-
quiring almost central light, while the latter requires it very
oblique. From this cause artificial illumination is to be pre-
ferred to day-light for this class of objects, as the light of a
lamp or candle gives the rays diverging without any apparatus
whatever. The same effect, however, may be produced in
day-light, either with Dr. Wollaston's or Dr. Goring's illu-
minator, where the rays, after meeting at the focus of their
illuminating lens, are permitted to diverge, and, by placing
the object out of the centre, oblique vision is obtained. In the
investigation of the class of objects now to be described, direct
parallel rays are preferable, and, indeed, in most cases are es-
sential ; and on this account they are scarcely ever well defined
by candle- or lamp-light. In these, therefore, clear day-light,
directed through the axis of the instrument, should be em-
ployed.

1. The hairs of the common mouse (Mus domesticus) differ
both in size and form ; the principal varieties, with their rela-
tive diameters, are shown in Plate III. fig. 14, 15, and 16,
These are drawn, as seen by transmitted light, and as proof-
objects should have their transparent parts clearly and dis-
tinctly separated from the darker portions. This remark
holds good for the whole tribe of hairs and mosses, and it is
from the sharpness with which the parts are separated that a
correct opinion of the goodness of an instrument can be ob-



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