336 
without any essential difficulty in practice ; for 
the dispersive power of flint-glass is so much 
aaa than that of crown-glass, that a convex 
ens of the former, the focal length of which 
is 73 inches, will produce the same degree of 
colour with a convex lens of crown-glass whose 
focal length is 4, inches. Hence a concave 
lens of the former material and curvature will 
fully correct the dispersion of a convex lens of 
the latter, and will yet diminish its refractive 
wer only to such an extent as to make its 
us ten inches. The correction for chromatic 
aberration in such a lens would be perfect, if 
it were not that, although the extreme rays, 
violet and red, are thus brought to the same 
focus, the dispersion of the rest is not equally 
compensated ; so that what is termed a secon- 
dary spectrum is produced, the images of ob- 
jects seen through such a lens being bordered 
on one side with a purple fringe, and on the 
other with a green fringe. Moreover such a 
lens is not corrected for spherical aberration ; 
and it must of course be rendered free from 
this, to be of any service, however complete 
may be its freedom from colour. 
eats have long since been able to effect 
the required corrections, with sufficient accu- 
racy for most practical purposes, in the con- 
struction of large object glasses for telescopes ; 
the size of which has been only limited by the 
impossibility of obtaining glasses of large di- 
mensions perfectly free from faults. But it 
has been only of late years, that the construc- 
tion of achromatic and aplanatic object-glasses 
for microscropes has been considered prac- 
ticable,—their extremely minute size appearing 
to forbid the employment of the necessary 
combinations, since a very high amount of 
accuracy is required in the several curvatures, 
in order to obtain any real improvement. About 
the year 1820, however, the attempt was first 
made in France by M. Selligues, who was fol- 
lowed by Frauenhofer at Munich, by Amici at 
Modena, and by Mr. Tulley of London; and 
with these attempts a new era in the history of 
the microscope may be said with truth to have 
commenced. The work has been prosecuted, 
both theoretically and practically, with the 
greatest zeal, and the result has been most 
successful. By combining two or three groups 
of double lenses, each corrected in a particular 
manner, so that the whole is quite free from 
aberration, a perfectly sharp and clearly- 
defined image may now be obtained through 
a lens of many times the aperture of those 
formerly in use; and the differences in the 
representation of the objects under enquiry, 
between such lenses and a good achromatic, 
are such as could not have been, a priori, sus- 
pected. One of the most pleasing results of 
this improvement has been the greatly-increased 
unanimity amongst microscopical observers, 
as to the appearances actually witnessed by 
them; for with the old and imperfect instru- 
ments, great uncertainty could not but exist in 
to many objects, of whose nature ever 
one formed his own opinion, frequently Fheviie! 4 
ing to preconceived ideas; but at present the 
objects are presented to the sight of each ob- 
MICROSCOPE. 
server 
that there is much less scope for the of 
his imagination as to their real character,— 
however much he may exercise it upon their 
history. It would be foreign to the purpose 
of this article to enter into scientific details 
upon the minutiz of the construction of achro- 
matic combinations; but it may not be amiss 
to state that, in the opinion of the author, 
English artists have far su foreigners in 
the construction of lenses of very short focus, 
whilst some foreign combinations which he has 
seen, of low magnifyin wer, possess an 
advantage over those of” ritish make,—the 
constructors of the latter having sometimes 
sacrificed what he deems adequate correctness 
in aiming at a very large aperture.” 
With these preliminary details as to the na- 
ture of the means by which mi ic power 
is obtained, we shall proceed to notice their 
chief applications to practice. Excluding for 
the present the solar and gas microscopes, in 
which an image visible to any number of 
sons at once is formed upon a screen, is 
viewed by them precisely as other surroundi 
objects would be, we shall consider the instru- 
ments (to which the term microscope is more 
commonly applied), whose effects are eke which 
by their influence on the rays of light which 
enter the eye of the observer, and which can 
be used, therefore, by but one at the same 
time. These are distinguished as single or 
simple, and compound microscopes. Each of 
these kinds has its peculiar advantages for the 
anatomist; and we shall, therefore, describe 
the construction and uses of both in some 
detail. Their essential difference consists in 
this,—that in the former the rays of light which 
enter the eye of the observer directly 
from the object itself, after having been subject 
only to a change in their course, whilst in the 
latter an inverted image of the object is formed 
by a lens, which image is viewed by the ob- 
server through a simple microscope, as if it 
were the object itself. The simple mi 
may consist of one lens, but (as will be pre- 
sently shown) it may be formed of ¢wo or even 
three ; but these are so disposed as to produce 
an action upon the rays of light commer 
to that of a single lens. For this kind of mi- 
croscope, therefore, we prefer the term simple 
to single. In the compound microscope, on 
the other hand, not less than two lenses must 
be employed, one to form the inverted image 
of the object, and this being nearest to itis 
called the object-glass, whilst the other . 
nifies that image, being interposed between it 
and the eye of the observer, and is hence 
the poate Both these may be 
of several lenses, as will be hereafter shown ; 
but they are so arranged as to have the func- 
tions of a single lens, and are only combined 
* Those who wish to study the principles which 
hoald” 
now guide opticians in their construction, U 
refer to Mr. J. J. Lister’s paper in the Phil. Trans, 
for 1829, and Mr. Ross’s Tiassa in the Trans. of 
the Society of Arts, vol. ii, ‘ 
of a good instrument, with so” 
much more clear and uniform an pape ws - 
play of 
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