Oct. 28, 1886 | 
NATURE 
623 
liquid media, which he proposed to use in his compound 
lenses ; a fluid lens being formed in a cavity between two 
lenses of glass. He also suggested combinations of two 
or more fluid lenses. He found that in the spectrum of 
hydrochloric acid the green rays lay much nearer the 
violet than in the spectra of most metallic solutions ; and 
he proposed to use the chlorides of antimony and of 
mercury in various proportions along with hydrochloric 
acid, or with sal-ammoniac, in order to obtain a fluid 
which, while having a different absolute dispersion from 
crown glass, should have its relative dispersion or relative 
distribution of the rays of different colours proportionately 
identical throughout the spectrum.  Blair’s object-glasses 
for telescopes, though regarded at the time as of singular 
merit, never came into use. The only recent serious 
attempt to revive a triple lens for approximating to a cor- 
rection of the secondary spectrum by achromatising for 
three rays has been that of Prof. C. S. Hastings, of Yale, 
who has used three kinds of glass. 
What Blair proposed to effect with his liquid combina- 
tions Prof. Abbé claims to have now achieved by his dis- 
covery of new kinds of optical glass. To Abbé we owe 
the remark that, in addition to the secondary chromatic 
aberration of our so-called achromatic lenses, there exists 
a second defect, termed by him the chromatic difference 
of the spherical aberrations. ‘This term he uses to denote 
the fact that with the crown and flint glasses used by 
opticians, though the curves of the lenses be calculated 
to correct the spherical aberration, taking in each case 
the mean refractive power, there will be a slight 
residual spherical aberration for all rays not of mean 
refractive index; the lens being spherically under cor- 
rected for red rays and spherically over-corrected for 
blue. 
Having realised so far back as 1880 that these defects 
were inherent in the use of such glass as opticians had at 
their disposal, Abbé determined to make a resolute 
attempt to discover new kinds of glass which should be 
free from these vices. The research involved no less a 
field of work than the examination of the optical pro- 
perties of all known chemical substances which undergo 
vitreous fusion and solidify in non-crystalline transparent 
masses, together with a detailed comparison of their 
physical and chemical properties. The work was begun 
so far back as January of the year 1881 by Prof. Abbé 
and his coadjutor, Dr. Schott, then of Witten in West- 
phalia, now of Jena. Dr. Schott undertook the chemical 
portion and the melting processes ; Prof. Abbé and: his 
assistant, Dr. Riedel, conducted the optical examinations 
of the products. At first only small quantities, from 20 
to 60 grammes in weight, were melted at once ; all kinds 
of chemical elements being tried with the view of ascer- 
taining their influence on the refractive and dispersive 
powers. Not only were glasses of ordinary kinds having 
silicic acid for their chief constituent tried, but also glasses, 
resembling Faraday’s famous “ heayy-glass,” made from 
boric acid and also phosphoric acid. So, by the end of 
1881, a series of fundamental facts in chemical optics 
were gathered together for future use. The next point 
was to use these chemical researches as the basis for the 
production of real glass possessing the necessary quali- 
ties of hardness, colourlessness, and unalterability. To 
carry out this work, Dr. Schott moved to Jena early in 
1882, and set up a special laboratory for scientific glass- 
making with proper blast-furnaces, and smelting-pots in 
which quantities of 10 kilogrammes could be melted 
at once. Until the end of the year 1883 this research- 
laboratory was occupied almost exclusively in working 
toward the solution of two practical problems. The first 
of these was the production of pairs of kinds of flint and 
crown glass, such that the dispersion in the various re- 
gions of the spectrum should be, for each pair, as nearly 
as possible proportional. The object of this was to per- 
mit of a higher degree of achromaticity than hitherto 
possible, by diminishing the secondary colouring effects 
inseparable from the irrationality of the ordinary silicate 
flint and crown glasses. The second problem was the 
production of a greater multiplicity in the gradations of 
optical glass in respect of the two chief optical constants, 
the index of refraction and the mean dispersion. Though 
this has not generally been regarded as an important 
need in optics, it was considered by Prof. Abbé and Dr. 
Schott as of quite equal importance to the first. For the 
silica glasses hitherto used constitute, according to their 
composition, a simple series, from the lightest crown to 
the densest flint, in which, with an increase in refractive 
index, there is practically always an increase in the mean 
dispersion. But there is no doubt that for many pur- 
poses of practical optical instrument-making, particularly 
in the designing of optical combinations for special pur- 
poses, it would be a great benefit for the optician to have 
at his command other kinds of glass in which the refrac- 
tion and dispersion are not related in the way in which 
they are in the silica glasses ; for example, a glass having 
great refraction and small dispersion. Hence the multi- 
plication in the available kinds of optical glass opens 
out new possibilities of great practical moment. Pending 
the publication of these very valuable scientific investiga- 
tions, only a very brief account can be given of the actual 
results. The first problem has been satisfactorily solved, 
with the result that achromatic lenses of a much more 
perfect kind than has ever been attainable are now in the 
market ; and the second has also been successfully car- 
ried out, a whole series of new glasses of graduated proper- 
ties having been introduced into the optical trade. 
Down to the autumn of 1883 the matter was one of syste- 
matic scientific research ; but at that date, encouraged by 
the substantial fruit borne by the investigations, a further 
step was taken. In conjunction with the brothers Dr. 
Carl Zeiss and Dr. Rod. Zeiss, of Jena, whose names are 
household words to every microscopist, a commercial 
enterprise was set on foot for establishing a new glass- 
foundry. This establishment, which, under the style of 
Schott and Company, has been at work since the autumn 
of 1884, produces glass of all kinds, old and new, on a 
large scale. ‘The first application of the new glass to the 
microscope comes naturally from the famous firm ot 
Zeiss. To his new microscope lenses Prof. Abbé gives 
the distinctive name of afochromatic objectives. He 
claims for them great superiority in the. finer qualities of 
definition, the new dry apochromatic lens giving an image 
equal to that of an ordinary achromatic water-immersion 
objective. He also claims that the more perfect correc- 
tions permit equal magnification to be obtained by using 
a longer-focus objective with an eye-piece of higher power 
than hitherto has been usual, thus obviating some of the 
difficulties of very short-focus objectives. Moreover the foci 
for visible and for photographic purposes are identical. 
Special compensating eye-pieces have been also devised 
for use with the new apochromatic objectives. 
Whether these new appliances are found to fulfil under the 
test of experience all that their inventor claims for them re- 
mains yet to be seen ; but it cannot be doubted that a great 
step has been taken. It ought also to be recorded to the 
credit of all concerned that no attempt is being made to 
secure to one firm a monopoly of the new materials, but 
that the new optical glass is offered to the optical trade 
without any restriction or patent to stand in the way of 
further development. Nor less honourable or significant 
is it that the researches of the “ Glass-technical Labora- 
tory” of Jena should have been supported by a very 
liberal and several times repeated subvention from the 
Prussian Government. Such a result arising from the 
endowment of research makes ample answer to the easily- 
uttered assertion that such endowments, if given, would 
be wasted on useless fancies and trifling schemes. We 
sincerely congratulate Prof. Abbé and Dr. Schott on the 
completion of their most meritorious labours. 
