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THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. [February 11,1871. 
should answer the same purpose with microscopes 
which these large lenses do with the telescope, the 
principle of construction in the two instruments be¬ 
ing the reverse of each other in this respect. The 
power of the telescope is gained by a long tube and 
an object-glass with a long focus, and to get a very 
powerful telescope you require a very large lens, the 
limit there being the size of the flint glass that can 
be produced of sufficiently good quality. I believe 
at the present time object-glasses for telescopes are 
being made twenty-five inches in diameter, and that 
is the largest yet produced. On the other hand, in 
the microscope the power is gained by diminishing 
the focal length of the lens, and, in diminishing the 
length, the lenses themselves must be made upon a 
more minute scale. I shall not go through the suc¬ 
cessive steps of the process by which these powers 
were obtained, but it is interesting to mark that 
men of science in different countries were at work 
about the same time, in conjunction with mechani¬ 
cians of skill in glass grinding, in producing achro¬ 
matic lenses for the microscope. Dr. Goring, in 
London, within my own recollection, set Mr. Tully, 
who at that time was a celebrated maker of tele¬ 
scopic lenses, to work upon what would now be 
called a very low power for the microscope; and I 
remember very well hearing at the time that Mr. 
Tully, after long and numerous trials, succeeded in 
producing what we should now call a very inferior 
lens; and that, upon Dr. Goring asking what he had 
to pay for it, Mr. Tully said he could not estimate 
his time, at the very least, at less than £50. Lenses 
far superior to that can iioav be bought for about 15s. 
At the same time Professor Secclii, in Italy, and 
M. Chevalier, in Paris, were at work on the subject, 
and they produced achromatic lenses of higher 
power than those of Mr. Tully. Not long after that, 
a most estimable man, not long since deceased, Mr. 
Joseph Jackson Lister, applied very considerable 
mathematical powers to the investigation of the means 
of combining these lenses. It was soon found that 
the telescope plan did not answer at all for the mi¬ 
croscope ; that is, the plan of having the flint glass 
concave between two convex lenses of crown, was 
not effective for the microscope, but a much better 
effect was produced by combining two or three pairs 
of lenses,—a flint concave and a crown convex,— 
and that is the mode of construction which since 
that time has been adopted. Dr. Bentley has kindly 
lent me a diagram, showing the plan of construction 
adopted in object-glasses at the present time. Here 
are three pairs, concave of flint and convex of 
crown. The different curves are ground with very 
great care and accuracy, in such a manner as to 
correct both the spherical and the achromatic aberra¬ 
tion, that is, to bring the rays of the central and 
peripheral parts of the lens to The same focus, and 
thus correct the spherical aberration and prevent 
any foggy appearance, and, at the same time, pre¬ 
vent any achromatic aberration, so that there shall 
be no false colours in any part of the object, and 
especially at its edges. There are different plans of 
combination. Some makers put a single lens in 
front, and a combination of three behind. There are 
different combinations for different powers, and dif¬ 
ferent makers have their different methods, but they 
all go on the same principle; and at the present 
time the most extraordinary accuracy in workman- 
slnp has succeeded in producing lenses of a degree of 
minuteness that can scarcely be conceived. Messrs. 
Powell and Leyland have produced a combination 
lens of only a fiftieth part of an inch focus. I sup¬ 
pose the largest of the three does not exceed in size 
an ordinary pin’s head, whilst the smallest must be 
scarcely larger than a pin’s point, certainly not 
larger than a pin hole in a piece of paper. The 
practical skill which our mechanics have acquired 
has enabled them gradually to cheapen the construc¬ 
tion, and this is a matter of great importance to 
those who desire to use the instrument as one of 
recreation and education. I may take some little 
credit to myself for having promoted this object, for 
in the year 1851, after the Great Exhibition, I in¬ 
duced the Society of Arts to offer a prize for a three- 
guinea achromatic microscope. I was laughed at ex¬ 
cessive^ for thinking that an achromatic microscope 
could be made for three guineas, all the London 
makers saying it was entirely out of the question. 
The few first-rate makers then in the trade would 
have considered it lowering their reputation to con¬ 
descend to anything of the kind. There were, how¬ 
ever, second-rate, or perhaps, 3 r ou may say, third- 
rate makers, who thought it worth their while to see 
what could be done. A jury was appointed by the 
Society of Arts, of which I was a member, and we 
selected a microscope made by Mr. Field, of Bir¬ 
mingham, a well-known Manufacturing optician, and 
he informed me five or six years ago that over 
two thousand of these microscopes had been sold; 
and I dare say since that time more than double the 
number have been disposed of. That was the be¬ 
ginning of an improvement which has gone on con¬ 
tinuously since; and makers both in London and the 
country are continually bringing under my notice 
microscopes, and especially achromatic microscopes, 
of cheaper construction and offered at lower prices. 
They are uoav beginning to feel AAdiat I have been 
preaching for a long time, that the best lenses for 
ordinary use are not those Avliicli serve best to dis¬ 
play Avliat are called test objects. I have some¬ 
times likened the difference betAveen these instru¬ 
ments to that between an ordinary roadster and a 
racehorse. You do not use a thoroughbred race¬ 
horse for ordinary every-day Avork, and, on the other 
hand, a roadster has qualities AA’liicli, though they do 
not enable him to run races, make him a A r ery useful 
animal. It is just the same Avitli regard to these 
different qualities of lenses. There are certain 
achromatic combinations Avliich are very costly, as 
they require a marvellous precision and finish, and 
Avliich, therefore, it is impossible to make cheap. 
On the other hand, there are some combinations of 
Avhat is called a much lower angle of aperture which 
are really best for ordinary use, which give the best 
vieAV of an object, and Avliich can be made at much 
less expense than the others. Very recently there 
have been imported into this country from Berlin a 
series of achromatic lenses of different poAvers at 
most extraordinarily Ioav prices. They are knoAyn 
as Giinlacli’s lenses; and, having examined se¬ 
veral of them, I liaA r e been really astonished at 
the excellence of the work that is produced in com¬ 
parison Avitli the price. For example, a lens of 
about one-tliird of an inch focus, costing 18^., is 
quite equal to a lens AAdiich I gave one of our best 
makers four guineas for a feAv years since. You 
may iioav have a microscope for £5 Avliich I should 
have been most thankful in my earlier days to have 
given £20 for,—I mean Avitli regard to its applica¬ 
bility to working purposes. 
