334 

THE MICROSCOPE 
IMPROVEMENTS IN THE LENSES OF MICROSCOPES 
EF OR some time, people in England have been content to let 
the improvement of the optical powers of the microscope 
remuin entirely in the hands of the makers, believing, apparently, 
that Mr, Lister had effected all in his suggestions and improve- 
ments that could be desired. Dr. Royston Pigott, an able 
mathematician, formerly Fellow of St. Peter’s College, Cambridge, 
and a Doctor of Medicine of that University, was not, however, 
inclined to look at the matter in this way, and for many years 
has been working and experimenting with a view, first, to test 
the accuracy of our best object-glasses, and, secondly, to suggest 
means for their improvement. It should be remembered that 
Oberhauser, Nachet, and especially Hartnack, on the Continent, 
not satisfied with the old system of combinations for object- 
glasses, and not having the benefit of Lister’s researches, have 
made excellent objectives on a totally different system, and 
during the last few years the last-named maker has carried his 
system of ‘immersion lenses” tosuch a point of excellence as 
really to surpass the best glasses on Lister’s system, in definition, 
penetration, working distance, and illumination. Those who do 
not admit the excellence of these objectives, which are now used 
by nearly all German histologists, have probably seen older 
glasses, made at a time when Hartnack had not reached his 
best. It is worth stating, now that the Parisian opticians are 
inaccessible, that Gundlach of Berlin has succeeded in making 
excellent glasses of high power at astonishingly small prices, 
some of his 1-12ths and 1-16ths immersion 1-16ths (so-called), 
being admirable in their performance. They are not, however, 
equal to Hartnack’s glasses, which, though costing far less than 
what similar English glasses cost, yet are more expensive than 
Gundlach’s. It is only fair to all parties concerned to state that 
the terms 1-8th, I-12th, 1-16th, &c., as now applied to an object- 
glass, appear to have no definite meaning, but depend on the 
caprice of themaker, since the magnifying power of glasses, with 
the same fraction assigned to them, differs enormously. 
To return to Dr. Royston Pigott. He found the usual means 
of testing an object-glass by trying if it gave some particular 
appearance with a ‘test object,” such as the Podura-scale, very 
unsatisfactory, since we have no certainty to begin with as to 
what is the true appearance of such an object. He therefore 
examined minute images of objects of which he knew the true 
form, such as a watch-face or thermometer-scale, forming these 
images by aid of mercurial globules and the condenser properly 
adjusted below the microscope-feld. By this means he has 
found that ol\ject-glasses corrected so as to show dark, sharply 
marked spines (like!!!) on the Podura-scale—a favourite test- 
object with our microscope-makers—give false, blurred, and dis- 
torted appearances with his known images, and on making such 
corrections of the objective as to show the known images in 
‘heir true form, he finds that the Podura-scale, examined 
with the corrected objective, is not really marked at all, as 
supposed, but is beset with a series of bead-markings, which 
by intersection, when improperly defined, give the curious 
appearance like notes of exclamation. This important discovery 
of the falsity of our high powers (1-8th to I-16th), has led Dr. 
Royston Pigott to pay more attention to the lower powers, and 
he finds that though you may not get so much actual amplifica- 
tion, you yet get a truer effect, and greater clearness of detail, by 
employing very carefully made low powers (1-2nd to I-5th) and 
increasing the magnifying power at the other end of the micro- 
scope, 7.¢., the eye-piece. We have in this way seen the beaded 
structure of the scales of Podura more satisfactorily than with 
very high objectives even when corrected so far as they would 
~dmit, and we may say the same of some Diatom-valves Cn.Ge5 
LL. Jormosum. It would be most important to know how far 
ee a change of combination would be useful in histological 
work, 
The general upshot of Dr. Royston Pigott’s investigations 
appears to be that. it is desirable to shift the burden hitherto 
cast almost wholly upon the objective, to the other parts of the 
instrument.. We should be content with an objective as high as 
a fifth, or even less. A very deen eye-piece is to be used ; and to 
correct residuary aberrations of the objective, and at the same 
time amplily, Dr. Pigott has introduced an important adjustable 
combination detween the eve-piece and the object-glass. There seems 
to be considerable reason for the step proposed by Dr, Royston 
Pigott. Just as great results were obtained in passing from the 
single lens or combination to the compound microscope of eye- 
NATURE 

| Feb. 23, 1871 

piece and objective, so by adding distinct integral factors to these 
two, such as Dr. Pigott’s ‘*aplanatic searcher,” we may obtain 
excellences quite impossible by any amount of attention bestowed 
on the objective alone, or only with difficulty reached by long 
labour, leading to very high price for high powers. 
Dr. Pigott has, during the past year, published some account 
of his researches in the Quarterly Yuurnal of Microscopical 
Science, and has communicated papers to the Royal Society, one 
of which is about to appear in the Philosophical Transactions. 
Naturally at first the makers in London and the Microscopical 
Society were sorely tried by Dr. Pigott’s exposure of the Podura- 
scale, but we hear, as one good result already obtained, that 
Messrs. Powell and Lealand have constructed a new 1-8th both 
dry and immersion, with great care, which is declared to be the 
best glass yet made. It has been proposed to form a committee 
for the purpose of examining carefully as to penetration, defi- 
nition, and angular aperture, the best glasses of eur English 
makers, the best American glasses, and the best of Hartnack’s, 
Gundlach’s, and others ; the glasses being mounted similarly, 
with private marks only for recognition, so as to prevent all 
possibility of prejudice on the part of the committee. Were 
this done, the result, whichever way it tended, would be eminently 
satisfactory. Of this the writer is sure, that many persons—even 
eminent microscopists—have made up their minds about the 
qualities of foreign objectives, without having seen any, or only 
very poor examples, and then when a really fair specimen of 
such a glass is placed betore them, they exclaim with astonish- 
ment ‘* Why this is the finest glass I have ever seen.” We shall 
be glad to receive suggestions. or assistance, in carrying out the 
proposed comparison of objectives. Dr. Royston Pigott has 
expressed his willingness to aid in such an undertaking. 
ERLE. 

REPORT ON DEEP-SEA RESEARCHES 
Carried on during the months of Fuls, August, and September, 
1870, in H.M. Surveying Ship “ Porcupine” * 
By W. B, Carpenvrrr, M.D., F.R.S., AND J. Gwyn 
Jerrreys, F.R.S, : 
THE equipment of the Porcupine for the previous Ex- 
pedition had been found so complete and satisfactory, 
that nothing more was considered necessary to prepare her for 
the work of the present season than the overhauling of her gear, 
and the manufacture of new dredges, sieves, and other apparatus, 
on the patterns of those which had already proved most service- 
able. We had the advantage of the same excellent commander, 
now promoted to the rank of staff-captain, with his able staff of 
officers ; and we would take this opportunity of again expressing 
our deep sense of obligation to them all for their hearty co- 
operation in our scientific work, and for the unvarying personal 
kindness by which our voyage was rendered a most agreeable 
one. A considerable part of the crew, also, consisted of the 
same steady and experienced men. The Meteorological De- 
partment supplied eight of the protected Miller-Caselia ther- 
mometers, including the two with the performance of which 
we had been so thoroughly satisfied last year ; and we usually 
employed one of these in conjunction with one that had not been 
used in the previous Expedition. 
At the request of the Committee, Mr. Siemens undertook to 
devise an apparatus for testing the depth of sea-water to which 
light, or at least the actinic rays, can penetrate. The foundation 
of the apparatus which he constructed for this purpose is a hori- 
zontal wheel with three radii, each of them carrying a glass 
tube in which a piece of sensitised paper is sealed up. The ro- 
tation of this wheel round a vertical axis brings each of the tubes 
in succession out of a dark chamber in which it ordinarily lies, 
exposes it to light in an uncovered space, and then carries it into 
darkness again. This movement is produced by a spring ; but 
it is regulated by a detent that project from the keeper of an 
electro-magnet, which is made and unmadeby t he completion or 
breaking of a circuit that connects it with a galvanic battery. 
When the magnet is made, it lifts the keeper with its projecting 
detent ; and this allows the wheel to Le carried by the spring 
through one-sixth of its rotation, whereby the first of the tubes is 
brought out into the open space. There it remains until the cir- 
cuit is broken, whereby the magnet is unmade ; the keeper then 
falls, and the wheel is allowed to move through another sixth 
* Ext-acted from the Procecdings of the Roy" Sogisty, 
