been proposed by different opticians, and one 
or other will be preferred, according to the 
oe for which it be required. It may be 
id down as a general principle, however, that 
to give the highest effect to the microscope, in 
regard to clearness of view and penetrating 
power, no more than two lenses should be 
employed; and that when a certain amount 
of these may be sacrificed to gain a large flat 
field, three is the largest number which can 
be introduced with any benefit. This prin- 
ciple is founded on the fact that, whenever 
light impinges on the surface of even the most 
transparent body, a part of it only is trans- 
mitted, the remainder being reflected. In the 
e of light through ordinary lenses, there- 
_fore, a certain quantity is lost by reflection 
at each surface; and every multiplication in 
the number of lenses entails, therefore, a 
itive evil, which may or may not be 
_ counterbalanced by the good it effects. In 
the doublet or triplet already described, 
the correction of the aberrations is an advan- 
tage much greater than the injury resulting 
from the substitution of four or six surfaces for 
two; but this is by no means the case in the 
eye-piece, in which (from their low power) the 
aberrations are much less. Hence, when too 
many lenses are employed in it, although the 
field of view (that is, the circle within which 
the image is comprehended) may be very much 
enlarged and rendered flatter, the brilliancy 
and sharpness of the image are so much im- 
paired, and it is invested with so much false 
colour, that, for all scientific purposes, the 
instrument is rather deteriorated than im- 
proved. 
_ The eye-piece which may be most advan- 
_ tageously employed with achromatic object- 
glasses, to the performance of which it is 
“ desired to give the greatest possible effect in 
out the necessity of a large field, is that termed 
the Huyghenian, having been employed by 
Huyghens for his telescopes, although without 
the knowledge of all the advantages which its 
best construction rendered it capable of afford- 
ing. It consists of two plano-convex lenses 
with their plane sides towards the eye. These 
are placed at a distance equal to half the sum 
of cer focal lengths ; or, to speak with more 
precision, at half the sum of the focal length 
of the eye-glass, and of the distance from the 
field-glass at which an image of the object- 
glass would be formed by it. A stop or dia- 
______ phragm must be placed half-way between the 
two lenses. By Huyghens this arrangement 
Was intended merely to diminish the spherical 
aberration ; but it was subsequently shown by 
Boscovich that the chromatic dispersion was 
also in great part corrected by it. Since the 
___- introduction of achromatic object-glasses for 
compound microscopes, it has been further 
_____ shown that all error may be avoided by a slight 
-_—sover-correction of these, so that the blue and 
___ red rays may be caused to enter the eye in a 
: een direction, and thus to produce a colour- 
____lessimage, though not actually coincident (fig. 
MICROSCOPE. 
regard to defining and penetrating power, with- . 
—— 
ee arenes 
ase 
-— 
—ceme OS ee 
L mY a Nn 
Section of Huyghenian eye-piece, adapted to over- 
corrected achromatics. 
LMN, the two extreme rays of three pencils, 
which, without the field-glass, would form a blue 
image convex to the eye-glass at B B, and a red. 
one at RR. By the field-glass, however, a 
blue image, concave to the eye-glass, is formed 
at B’ B’, and ared one at R’R’. As the focus 
of the eye-glass is shorter for blue rays than for 
red rays, by just the difference in the place of 
these images, their rays, after refraction by it, 
enter the eye in a parallel direction, and produce 
a picture tree from false colour. If the object- 
glass had been rendered perfectly achromatic, 
the blue rays, after passing the field-glass, would 
have been brought to a focus at 6, and the red at 
r, so that an error would be produced, which 
would have been increased instead of antago- 
nised by the eye-glass. 
164). Further, the image produced: by the 
meeting of the rays after passing through the 
field-glass is by it rendered convex towards the 
eye-glass, instead of concave, so that every part 
of it may be in focus at the same time, and the 
field of view thereby rendered flat. Those who 
desire to gain more information upon this sub- 
ject than they can from the accompanying dia- 
gram and the explanation of it, may be re-. 
ferred to Mr. Varley’s investigation of the pro- 
perties of the Huyghenian eye-piece in the 
51st volume of the Transactions of the Society 
of Arts, and to the article “ Microscope” in 
the Penny Cyclopedia. 
By an achromatic object-glass for a com- 
pound microscope, therefore, is not meant one 
which simply contains within itself a perfect 
correction for its own errors, but one in which 
the usual order of dispersion is so far reversed 
that the light, after undergoing the series of 
changes effected by the eye-piece, shall come 
uncoloured to the eye. ‘ We can give no spe- 
cific rules,” says the writer of the article just 
