February 11,1871.] THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
641 
THE MICROSCOPE AND ITS 
REVELATIONS.* 
BY W. B. CARPENTER, M.D., F.R.S., F.G.S., F.L.S. 
The modern microscope may, I think, he regarded 
under three aspects—as an instrument of scientific 
discovery, as an educational instrument, and as an 
instrument for affording an almost endless amount 
of rational recreation. I do not intend this evening 
to enlarge upon the first of these uses, because, as I 
understand, the object with which I was asked to 
bring the subject before you was that I might spe¬ 
cially dilate upon the educational advantages, and 
the use of the instrument as a means of rational 
recreation. I hold it extremely important that every 
young man should learn, not only how to work but 
how to pla} r . I think that to find a means of con¬ 
stant and attractive recreation, and especially one 
which combines the double character of quiet work 
at home and occupation for any amount of time, 
and, on the other hand, which will occasionally give 
a zest and interest to a walk abroad, is to find that 
which is one of the very best appliances that any 
home can have. I have found it so myself from my 
own boyhood, for I may say that the modern micro¬ 
scope and I have grown up together. It was just 
about as I was entering on my own educational 
course that that remarkable improvement was effected 
which I shall presently describe to you as the acliro- 
matization of the microscope. It was the introduc¬ 
tion of the achromatic principle, which had already 
been applied to the microscope, which converted it 
from a mere scientific toy, for really it was very little 
better, into an instrument quite on a par with the 
most costly and elaborate telescope, as a means of 
scientific research. As I have said, the microscope 
and I have, in some sense, grown up together; and 
it is an instrument, from my own experiences of its 
uses, in all its aspects most precious in my eyes. It 
is known to most of you that I have made certain 
branches of microscopic research the main study of 
my own life, and I have also felt its advantages in 
the education of my own children so highly, that I 
do not think there is any single means of education 
that, on the whole, I estimate so much. It gives 
that special training which none of our ordinary 
studies do give, that is, the development of habits 
of careful and accurate observation, and in addition 
to that, the habit of reasoning upon observation. 
Those two directions of the mind, the training and 
discipline of the observing powers, and with that 
the teaming and discipline of the power of reason¬ 
ing upon observation. I think distinguish scientific 
study, rightly pursued, from all our ordinary means 
of educational discipline ; as, for instance, the study 
of classics or mathematics, which latter reasons en¬ 
tirely on abstractions, and is confined to reasoning 
of a particular kind, limited in a narrow groove, as 
it were. Here, however, we have a training of the 
faculties of discerning the probable, the improbable, 
the certain and the uncertain,—in fact, all that kind 
of discipline which, in the ordinary walks of life, is so 
extremely valuable. Thus, as I have said, to those 
young people who have been trained in tins habit, 
who have learnt the value of it, and who desire a 
rational occupation of their spare time, and something 
which shall give an attraction to the ordinary work 
of life, nothing can be superior to the microscope. 
* Delivered at the Evening Meeting of the Pharmaceutical 
Society of Great Britain, February 1, 1871. 
Third Series, No. 33. 
These are the reasons, therefore, for which I ad¬ 
vocate, and have often advocated, the use of the 
microscope, especially here, where I know there are 
a large number of young men who are closely con¬ 
fined during the day in business avocations, and who 
yet have more or less time to spare in the evening, 
one great advantage of microscopic study being, that 
it may be carried on as well by lamplight as b} r 
daylight, with the exception of certain advantages 
which daylight has; but, on the other hand, lamp¬ 
light has its advantages, and for much of my own 
study I prefer it, because it is more manageable, and 
can be easily adjusted to exactly what is required. 
A few words, in the first place, upon the micro¬ 
scope itself. I shall not dwell upon its general 
philosophical principles, presuming that you are ac¬ 
quainted with the general construction of the ordi¬ 
nary microscope. All that I shall endeavour to 
explain to you is the great improvements to which I 
have referred under the term acliromatization. 
Achromatic means destroying colour. It does not 
mean that the microscope does not show the colour 
of the objects submitted to it, but that it does not 
show any false colours, or any which do not be¬ 
long to the object. The old microscope was con¬ 
structed in this manner:—The object-glass screwed 
at the lower ends of the tube, and then an eye-piece 
at the upper end, that eye-piece consisting of two or 
more lenses. Now, the object-glass of the old micro¬ 
scope was a simple lens, an ordinary double concave 
lens ; but the simple lens was employed as an ob¬ 
ject-glass, and a compound microscope gives ex¬ 
tremely false effects. In the first place, the spheri¬ 
cal curvature of its'two surfaces does not bring the 
rays of the central part and the spherical, or out¬ 
side part of the lens to the same focus; and there¬ 
fore, if the lens have a large aperture, you have a 
great amount of what is called spherical aberration ; 
that is, the rays that come through the central part 
and the rays that come through the spherical por¬ 
tion do not meet in the same focus. Therefore, if 
you focus it for one, all the other rays produce a sort 
of foggy effect. Then, besides that spherical aberra¬ 
tion, every such lens actsas a prism in dispersing the 
colour, producing that kind of effect which you see 
with any ordinary prism, or with the cut-glass 
prisms on a chandelier, which are so cut for the ex¬ 
press purpose of producing prismatic refractions. 
Therefore, every object seen through such a micro¬ 
scope had fringes of colour produced by the disper¬ 
sion of the coloured raj^s. In order to reduce this to 
the minimum amount, it was necessaryffo contract 
the aperture, and accordingly the object-glass of the 
old microscope had an extremely narrow aperture ; 
it was brought down to merely the central portion 
of the lens ; and even with that, although the spheri¬ 
cal aberration could be considerably reduced, yet 
there was always achromatic aberration, and the 
greater in proportion to the power. Tins defect had 
long previously been corrected in telescopic lenses. 
The large object-glass of telescopes was constructed 
by the union of hint and coarse glass, and the 
corners were ground in such a manner as to cor¬ 
rect almost entirely both the spherical aberration 
and the achromatic. But it was considered quite 
chimerical to attempt to apply the same principle to 
lenses so small as the object-glass of a microscope. 
It was considered that the manipulative skill of 
those who had to grind these glasses was never 
likely to succeed in producing combinations \vInch 
