neighbours, the French, ever foremost in the pursuit of glory,
both in arts and arms, have been doing in the affair of achromatic
object-glasses for microscopes. With the highest satisfaction I find
myself enabled to state, that Messieurs Chevalier, (ainé et fils,)
No. 69, Quai de l’Horloge, Paris, have rivalled our own artists, in
this branch of the manufacture of optical instruments.
Mr. J. Lister, actuated by a most laudable zeal for the prosecution
and advancement of optical science, as it concerns microscopes,
caused me to order for him one of Messrs. Chevalier’s instruments,
_in Mr. W. Tulley’s name_; for, as Mr. L. wished that Messrs. C.’s
pretensions should be fairly and thoroughly scrutinized, it was but
fair that the latter gentlemen should be stimulated to do their
utmost, by a consideration of the science of their customer. A
critical examination of the object-glasses of this instrument (for
making which every facility was afforded me by Mr. L.), forms the
subject of the present paper. [p249]
I here, then, enter upon the discussion of the merits and demerits
of the objectives of the said instrument[41], these being much more
perfected than those of another, of previous make, which I saw in the
possession of Mr. Howship, of Great George-street, Hanover-square,
to whom I received a letter of recommendation from Mr. Spilsbury, of
Ball-Haye. To the signal politeness of these gentlemen, in furthering
my views, I am greatly indebted.
Four object-glasses accompany Messrs. Chevaliers’ instruments (at
least those marked _perfectionnés_,) usually rated at the following
foci: 14 French lines, 10 ditto, 4 and 4: the two latter combine
together at will, and give a focus of two lines.
14.) Focus about 1.42 of an English inch, clear aperture 0.31,
original aperture as reduced by a stop, 0.10.
It is perfectly achromatic with its clear aperture, and may be used
without a stop on most transparent objects; requires to be cut off
to 0.23, to give the necessary distinctness for opaque ones.—(When
I speak of the apertures which C.’s lenses will bear, I must be
understood, here and elsewhere, only with regard to the _middle of
the field of view_, or rather that part of it where the distinctness
is greatest[42], for double object-glasses give the central rays only
correct, and confuse the oblique ones very much, for which reason,
conjoined with the small apertures they admit of, they were abandoned
by Mr. Tulley, for the triple construction, the true and regular form
for the microscope.)—There is an excess of spherical aberration in
convex lenses; neither are the glasses well ground, or centered, or
duly adjusted. The concave of this object-glass is tarnished, and
there are traces of seediness in the cement, which is, indeed, to be
seen more or less in the whole of them.
10.) Focus about 0.91, clear aperture 0.23, original stop 0.09.
[p250]
This object-glass is under corrected in point of colour, and wants
to be made longer in the focus to be achromatic. The excess of
uncorrected spherical aberration is in the convex lens; the glasses
are not well ground, centered, or adjusted; the same appearance of
tarnish as in 14; bears its clear aperture for the middle of the
field on most transparent objects, but must be cut off to 0.2 for
opaque ones.
Both of these object-glasses are ineffective upon test objects, from
want of sufficient power and aperture.
4.) Focus about 0.43, clear aperture 0.23, original stop 0.09,
perfectly achromatic. The uncorrected spherical aberration is in
the concave; centering and grinding very fine, but in very bad
adjustment; shows some transparent test-objects pretty well with its
clear aperture, and, cut off to about 0.16, performs well on many
opaque ones.
2.) Focus about 4.7, clear aperture 0.21, no stop, perfectly
achromatic, surplus of spherical aberration in the concave as
before; centering and grinding very fine; adjustment tolerable; in
other respects very similar to 4. This object-glass being adjusted,
does more singly on test-objects than any other, and carries an
aperture of 0.16 well on opaque bodies, showing the lines on the
diamond-beetle’s scales strong and well cut out.
Combination of 4 and 2—(quadruple.)
I am happy to be able to speak in terms of almost unqualified
approbation of this composition. It, of course, surpasses the
performance of any single triple-glass, _on those test-objects which
require extravagant angles of aperture_. The field also is good all
over; or at least would be, if the glasses were in adjustment, which
is the only drawback upon it. The focus of the combination is only
0.26, yet it performs admirably on transparent test-objects with its
naked aperture of 0.23, and is very fine on opaque ones with 0.16,
and doubtless would carry 0.2, if the adjustment was duly carried
into effect.
Messrs. C. have, I think, most assuredly here hit upon one of the
very best _compositions_ for the object-glass of a microscope; all
the imperfections of double object-glasses, _taken singly_, are here
done away, while their thinness and agglutination into one mass
allows of their combining together almost as if they were simple
plano-convex lenses, leaving moreover abundance of space for the
illumination of opaque bodies. [p251]
I must here state, that Messrs. C.’s object-glasses are all stuck
together, I believe, with fused gum-mastic, or, perhaps, with very
thick mastic varnish. This practice seems, in theory, to be bad, most
especially if the curves united together are not of the same radius;
nevertheless, practically speaking, the process of soldering seems
to me to do more good, by the obliteration of two surfaces, and by
keeping the glasses immovably adjusted, than harm in any other way. I
cannot, in fact, discover any very sensible difference in the optical
performance of these small achromatics, whether stuck together or
not. I _fancy_ that they have a little more light and clearness
when cemented, (as they certainly should have,) but cannot be very
positive. I hold it as a maxim in practical optics, as in our common
law, “de rebus non _apparentibus_ et non existentibus eadem est
ratio.”
I may observe, that Mr. Lister has combined that marked 10, with 4,
and finds the performance proportional to that of 4 and 2.
It will be remarked that Messrs. C., from an apparent ignorance of
the value of aperture; and perhaps impressed with the too common
and prevalent idea, that, having once obtained distinctness and
achromatism in their object-glasses, every thing else might be
accomplished by a condensation of artificial light, have reduced
their apertures to such a degree, as to render their instrument as
ineffective upon test-objects as a common compound; for when the
opening of an aplanatic glass is cut off to the same diameter as a
common one, it shows _nothing more_, though it will certainly exhibit
objects far _more satisfactorily_. Upon the apertures of microscopic
lenses their effects entirely depend, as was remarked a long time
ago by the great Huygens. An achromatic glass is more valuable than
another, merely on account of the larger aperture it will bear,
without causing aberration, and consequent indistinctness. Those who
are in possession of Messrs. C.’s microscopes should get the stop
behind the object-glasses turned out, and procure others to be used
ad libitum, according to what the goodness of the object-glasses will
permit.
I feel myself called upon, however, to state, that, since the
completion of Mr. L.’s microscope, Messrs. C. _have enlarged the_
[p252] _apertures of their object-glasses to the requisite angle_,
and have moreover _arrived at the true method of adjusting them_, so
that they are _now_ free from those objections which applied to Mr.
Lister’s, and are in all respects unexceptionably finished.
I know not if any dispute will ever arise hereafter, as to who is
to be considered as _the original maker of effective aplanatic
object-glasses for microscopes_[43]. It is of very little consequence
in the present instance, for it so happens that Mr. Tulley and
Messrs. Chevalier have been so totally unconnected with each other,
and have worked upon such totally different principles, that it must
be evident, on the most superficial consideration, that both are
entitled to the honour; nevertheless I apprehend it can be proved
that Mr. Tulley made an effective one before the Chevaliers, having
completed his in March 1824. The date affixed by Chevalier to his
first instrument is 1825[44].
[Illustration]
It requires moreover a stretch of complaisance, not to be expected on
this side the Channel, to be enabled to admit that the best double
object-glass is (_taken singly_) effective; or that, in consequence,
Chevalier made an _effective one until he had enlarged his apertures,
and combined two together_[45], which combination is not to be met
with in his primitive instruments. Mr. Lister, (to whom the public
is mainly indebted for the present eclaircissement concerning
Chevalier’s instrument,) has, by a peculiar method of his own
discovery, measured the [p253] curves, thicknesses, and diameter,
&c. of that marked 14, which I here give (unfortunately one of the
least effective of the set,) however, in all probability, the rest
are constructed on the same principle; the annexed drawing by Mr. L.
will sufficiently explain itself. Nothing can surpass the beautiful
simplicity of Chevalier’s, or rather Euler’s, curves, which, it will
be observed, are all alike[46]. The production of _deep_ achromatics
must ever be a task of some difficulty, even to those who thoroughly
understand their humours and punctilios; and unscientific artists
will, I think, be much more likely to succeed on the French plan,
than the English one. Two double object-glasses, by themselves,
are very poor things; but, when combined, perform admirably, and
will, I believe, (if the three curves are of equal radii,) be far
more easily executed than one of the triple Tulleian construction.
The dense flint-glass of Guenard, or Frauenhofer, however, will be
an indispensable requisite, from the nature of the curvatures. A
triple Tulleian object-glass, and a thin double one of Chevalier,
(a composition first conceived and adopted by Mr. Lister,) form an
excellent combination, and give a very vivid light, without softness,
dulness, or nebulosity. This, I think, is the extreme number of
glasses which ought to be tolerated. Let it never be forgotten, that
_a really good triple glass will bear an aperture quite sufficient
for ninety-nine objects out of a hundred_. I myself denounce the
practice of combining glasses together, _in all those cases where
they are capable of doing their work alone_. I shall always consider
it as a clumsy, bungling, and unworkmanlike method of obtaining a
short focus, combined [p254] with a large angle of aperture. If a
man aims at perfection, and wishes to distinguish himself in this
branch of optics, let it be done by working perfect triple glasses of
0.2 and 0.3 inch focus, with 0.1 and 0.15 of perfect aperture, like
those of Mr. W. Tulley and Mr. Dollond, or deeper still, if he is
able; and it is with the most cordial satisfaction that I am enabled
to inform my readers, that Messrs. Chevalier (duly appreciating the
regular triple construction as the true form for the microscope) have
applied themselves diligently to the manufacture of this species
of objective, from which they have already had excellent results.
It is a fundamental principle, that all superfluous refractions
and reflections are to be avoided in the construction of optical
instruments. As a radical reformer of microscopes, I can tolerate
no abuses in them, or show any quarter to their abettors. Messrs.
Chevalier have also undertaken the manufacture of achromatic and
catadioptric microscopes, after the fashion of those made by
Professor Amici, of Modena, which were so much and so justly admired
by the cognoscenti of this country.
It only remains for me to observe, that though two double-cemented
object-glasses form the most perfect _combination_ from the fewness
of their surfaces, and consequent brightness of their image, yet a
fusion between the Tulleian and Eulerian constructions seems to be
the _most convenient_ for general use; by this, of course, I mean
a triple glass with a double one, to apply before it occasionally,
à la Lister. Mr. Dollond and Messrs. C. have demonstrated that two
object-glasses _may be combined with the best effect, which are both
good, and work well separately_; but Mr. Tulley has constructed a
double one, which, useless by itself, when applied over a triple one,
(made to act singly,) corrects that excess of spherical aberration
in the concave lens, (by its own excess in the convex,) which, when
the aperture is large, is the eternal vice of all the best single and
compound object-glasses for diverging rays which I ever saw. This is
perhaps the ultimatum of improvement, though a quadruple one, on the
same plan, might have the advantage in greater light and clearness,
from its simplicity, and the paucity of its surfaces.
The quadruple or quintuple object-glasses are those which [p255] are
best adapted for the solar microscope, for they give a full-sized
field of view to this instrument, good to the edges, which no single
object-glass will do, as I have had occasion to remark in my paper on
Mr. Tulley’s aplanatics, unless converted into a compound, by means
of eye-glasses, &c. This popular and highly amusing instrument will
now receive the utmost reformation and improvement of which it is
capable, and become truly scientific in its construction: hitherto it
has been a mere toy, but one degree removed from a magic lantern.
I shall now allow Messrs. C. to say what they can for themselves, and
to detail the various modifications which they have introduced into
their instruments, since they executed Mr. Lister’s order, by giving
a translation of a letter I have received from them on the subject,
and shall conclude by expressing a hope that no national or illiberal
feeling has entered into the composition of this critique, and that I
have used my oil, vinegar, and pepper in correct proportions.
“_Paris, Oct. 15_.
“Sir,
“Accept our thanks for your extreme complaisance in offering
to publish the results obtained by us in the construction of
microscopes. Since the order executed for Mr. Lister, we have
improved those instruments last completed, by greatly enlarging
the diameter of the illuminating mirror, in order to obtain still
greater light. The prism for opaque bodies is diminished about one
half, and by a small modification in its mounting is rendered more
serviceable: the diminution of the length of the body has enabled us
to augment the magnifying powers by different eye-glasses, and the
four double object-glasses placed in a better mounting can be used
separately or in superposition, according to the pleasure of the
observer, either to form quadruple objectives, or even to combine in
a mass together. This last arrangement produces a great accession
of magnifying power, without injuring the clearness of the image or
arresting much light.
“You see, Sir, that we have the pleasure of coinciding perfectly
in your opinions about improvements, for we adopt the quadruple
object-glass as the best, and we give three changes [p256] of
eye-pieces. The double motion given to the body of a microscope is,
in our idea, the defect of all those hitherto constructed; for, as
the optical part should remain perfectly centred with the mirror
and the diaphragms, it is evident that the least derangement of it
from this position must destroy the perfection of the image: the
stage then only should move[47] without affecting the diaphragms
or the mirror, and in this we [p257] have well succeeded in
the construction of the microscope of Sig. Amici. But all these
arrangements much augment the price, and an observer ever so little
practised will always find the object easily enough by means of his
hands.
“The prices of our achromatic microscopes are as follows:—
“_Achromatic microscope_, like Mr. Lister’s—300 francs.
“The same, with the latest improvements, three eye-pieces, and
camera lucida for drawing the magnified objects—400 francs.
“_Amician Microscope_, one horizontal achromatic, the stage
giving all the motions to the object, with a micrometer screw,
five eye-pieces, two camera lucidas, hand magnifier, frog trough,
accessary apparatus, &c. _One catadioptric microscope_, mounted on
the same stand, and adapting itself to the same apparatus; the two
instruments inclosed in a mahogany case—1000 francs.
“We trust that the very moderate prices of these instruments,
together with the care which we bestow on their construction,
will procure us orders for them. Their superiority has been duly
recognized by the jury of the Exhibition of the Products of
Industry, which has been pleased to decree to us a silver medal.
“The Amician achromatic microscope is composed of a tube seven
inches long, at the extremity of which is placed a prism, which
reflects at a right angle the rays which come from the object-glass,
composed (_as in our microscopes executed since 1824_) of four
double object-glasses, which may be used separately, or two, three,
or four at a time. The stand is a square bar, which has a rackwork,
carrying a moveable stage, which, by means of adjusting screws
ingeniously disposed, permits an object to traverse the field
of view in every direction. This disposition gives the power of
determining the real dimensions of objects submitted to observation
by means of the micrometer screw, which is placed at the side, while
the camera lucida affords the means of drawing their outline, and
consequently of measuring the magnifying power.
“The rays proceeding from the object which have passed the
object-glass, and have been rendered horizontal by the prism, are
received by different eye-pieces disposed after the manner of [p258]
Ramsden. Their power can be varied. Each instrument carries six,
five of which can be attached at pleasure either to the catadioptric
or the achromatic. The deepest belonging to the reflector is a
single lens of half a French line focus, and the most powerful of
the achromatic is a line and a half.
“Such, we think, are the details which you required: we wish that
they may prove agreeable to you.
“We beg you to accept the assurance of the high consideration with
which we are, Sir,
“Your very devoted Servants,
“VINCENT CHEVALIER, ainé et fils.”
“69, _Quai de l’Horloge_.”
FOOTNOTES:
[41] The objects employed by me in looking into the defects and
excellencies of these glasses, were an artificial star, and a piece
of enamelled dial-plate, the phenomena presented by which, _when put
out of focus_, incontestably warrant the judgment I have pronounced
upon them, as any, true optician will admit.
[42] When an object-glass is out of adjustment, its maximum of
distinctness is not in the centre of the field of view, but
somewhere else, according to circumstances.
[43] The question must naturally resolve itself into this point,
for achromatics for microscopes, (as they are called,) were made by
Dollond, Martin, and Pollard, many years ago; the only objection to
them was, that they were _not effective_, consequently, _nominal
only_, and useless. I defy any man to produce an _effective_
object-glass, which can satisfactorily be proved to have been made
before 1824.
[44] See the _first_ instruction published by Messrs. C. along with
their microscopes: “Microscope Achromatique selon Euler,” &c. I
apprehend there is a misdate in Messrs. C.’s letter at the end of
this paper, where they state they have made the achromatics _since_
1824, _with four objectives_, which implies that they also made them
before that period, but in another way.
[45] Mr. C. Tulley recommended the combination of two achromatics
together from nearly the beginning of his son W.’s labours upon
them, who rejected the idea, together with myself, as giving rise
to too great a complication, _always supposing that triple glasses
must be used_. I myself combined two triple achromatics together,
for experiment sake, in a very early stage of our proceedings, but
liked not the result, though effects were certainly produced by the
composition, which could not be obtained from the best individual
triple one.
[46] The theory on which these object-glasses are constructed is
contained in a paper of Euler’s, published at St. Petersburgh, in
1774. Messrs. Chevalier have caused it to be inserted entire in the
“Bulletin de la Société d’Encouragement de Paris,” No. CCLIV., for
Aug. 1825.
[47] [Illustration]
This is the theoretical view of the case, the practical one is
different, as frequently happens. _Opaque objects_ are not affected
at all by the eccentricity of the axis of the body: nor can I
recollect that I ever felt any particular inconvenience from the
motion of the optical part, even with transparent subjects, unless
it was thrown _very much indeed_ out of the axis of the illuminating
mirror. On the other hand, it is notorious that _living_ aquatic
insects and animalcules are the most _popular_ and _entertaining_
objects which microscopes can show. These are, for the most part,
abundantly restless; and if the stage on which they are placed has
any motion, their natural unquietness is so much exasperated, that
it becomes almost impossible to get a good observation of them
at all. I once set to work at making some drawings of a variety
of new and original objects of this class (which, I trust, will
one day be published) with a microscope having all the requisite
motions applied to its stage, and am confident that I had thrice
the labour fairly appropriate to the execution of my task from this
oversight. The very tremor produced by the transition of a carriage
in the street, is frequently sufficient to unsettle live objects
when disposed to be still and quiet, and put them in a fidget for
a quarter of an hour. It is very unfortunate that the mountings
of optical instruments are made in general by mere mechanics, who
seldom or never observe with them, and consequently know not the
exigencies which occur in practice. It is still more unfortunate,
that in the science of fitting up microscopes, an ounce of a man’s
own wit is worth about a ton of his neighbour’s. Was it not that
I dislike to verify this adage myself, I should recommend the
following motion to be applied _to the body_:—let the socket of
the arm which carries it have a smooth rotatory motion on the head
of the bar in the usual way, conjoined with another horizontal one
produced by rackwork attached to the said socket. Let the pinion
which belongs to the latter movement be made very strong, so that a
lever about three inches long may project from its centre: this is
to be held in the hand, the thumb and index finger operating on the
rackwork, while the two little fingers give a rotatory motion by
working the lever end. _This rapid double motion is here completely
under the command of_ one hand, _while the other is at liberty to
adjust the focus_. I know of nothing better for general purposes, or
in particular for following the motions of live insects, &c.; but
when _only inanimate ones_ are to be the subject of microscopical
study, I prefer the motion of the stage, for the reasons stated by
Mons. C.
_On the Existence of Chlorine in the Native Black Oxide of
Manganese._ By John M’Mullen, Esq.
In the paper relating to this subject, which the editors of the
Quarterly Journal of Science obligingly inserted in the forty-fourth
number of that work, I described some experiments which I had made,
to show that chlorine is uniformly evolved from the Native Black
Oxide of Manganese by the action of sulphuric acid, under certain
circumstances, which I endeavoured to detail with strict accuracy, so
as to prevent any mistake or failure in the event of the experiment
being repeated.