551 
OPT 
covery, by reafoning in a manner quite contrary to the na¬ 
ture of things. At length, however, M. Euler was con¬ 
vinced of the reality and importance of Mr. Dollond’s 
difcoveries; and frankly acknowledged, that perhaps he 
thould never have been brought to alfent to it, had he not 
been affured, by his friend M. Clairaut, that the experi¬ 
ments of theEnglifli optician might be depended upon. 
The experiments of M. Zeiher, however, gave him the 
moil complete fatisfaftion with refpecl to this fubjeft. 
This gentleman demor.flrated, that it is the lead in the 
compolition of glafs which produces the variation in its 
difperfive power; and, by increafing the quantity of lead 
in the mixture, he produced a kind of glafs, which occa¬ 
fioned a much greater feparation of the extreme rays than 
the flint-glafs which Mr. Dollond had made ufe of. . 
From tliefe new principles M. Euler deduces theo¬ 
rems concerning the combination of the lenfes; and, in 
a manner fimilar to M. Clairaut and d’Alembert, points 
out methods of conftrufting achromatic telefcopes. 
While he was employed upon this' fubjeft, he informs 
us, that he received a letter from M. Zeiher, dated Peterf- 
burgh, 30th of January, 1764, in which he gives him a 
particular account of the fuccefs of his experiments on the 
compofition of glafs; and that, having mixed minium and 
fand in different proportions, the refult of the mean re- 
fraftion and the difperiion of the rays varied according to 
the following Table. 
Proportion 
of Minium 
to Flint. 
Ratio of the Mean 
RefraClion from 
Air into Glafs. 
Difperfion of Rays 
in Comparifon of 
Crown Glafs. 
I. 
— 3:1 
2028 : 
IOOO 
4800 
IOOO 
II. 
— 2 , : I 
1830 : 
IOOO 
355 ° 
IOOO 
III. 
— 1 : 1 
1787 : 
1000 
3259 
IOOO 
IV. 
— 4 : 1 
1732 : 
IOOO 
2207 
IOOO 
V. 
— k • 1 
1724 : 
IOOO 
1800 
IOOO 
VI. 
— 4 : 1 
1664 : 
IOOO 
1354 
IOOO 
From this Table it is evident, that a greater quantity of 
lead not only produces a greater difperiion of the.rays, 
but alfo increafes the mean refraClion. The firft of thefe 
kinds of glafs, which contains three times as much minium 
as flint, will appear very extraordinary ; fince, hitherto, 
no transparent fubftance has been known, whofe refrac¬ 
tive power exceeded the ratio of two to one, and lince the 
difperiion occafioned by this glafs is almolt five times as 
great as that of crowm-glafs, which could fcarcely be be¬ 
lieved by thofe who entertained any doubt concerning the 
fame property in flint-glafs, the effeft of which is three 
times as great as crown-glafs. 
Here, however, M. Euler announces to us another dif- 
covery of M. Zeiher, no lefs furprifing than the former, 
and which difconcerted all his fchemes for reconciling 
the above-mentioned phenomena. As the fix kinds of 
glafs mentioned in the preceding Table were compofed of 
nothing but minium and flint, M. Zeiher happened to 
think of mixing alkaline falts with them, in order-to give 
the glafs a confidence more proper for dioptric ufes : this 
mixture, however, greatly diminifhed the mean refradtion, 
almolt without making any change in the difperiion. Af¬ 
ter many trials, he is faid to have obtained a kind of glafs, 
which occafioned three times as great a difperfion of the 
rays as the common glafs, at the fame time that the mean 
refradtion was only as r6i to 1. though we have not heard 
that this kind of glafs was ever ufed in the conltruction 
of telefcopes. 
The heft refradting telefcopes, conftrudled on the prin¬ 
ciples of Mr. Dollond, are Hill defedlive, on account of 
that colour which, by the aberration of the rays, they 
give to objefts viewed through them, unlefs the objeft- 
glafs be of fmall diameter. This defedt philofophers have 
endeavoured to remove by various contrivances; and 
Bofcovich has, in his attempts for this purpofe, difplayed 
much ingenuity; but the philofopher whofe exertions 
I c s. 
have been crowned with moll fuccefs, and who has per¬ 
haps made the moll important difcovery in this fcience, 
is-Dr. Robf. Blair, proleffor of pradtical altronomy in the 
college of Edinburgh. By a judicious fet of experiments, 
he has proved, that the quality of dilperfing the rays in a 
greater degree than crown-glafs is not confined to a few 
media, but is poflefled by a great variety of fluids, and 
by fome of thefe in a moll extraordinary degree. He has 
fltown, that, though the greater refrangibility of the violet 
rays than of the red rays, when light pafles from any me¬ 
dium whatever into a vacuum, may be conlidered as a law 
of nature; yet, in the palfage of light from one medium 
into another, it depends entirely on the qualities of the 
media which of thefe rayslhall be the molt refrangible, or 
whether there fhall be any difference in their refrangibi¬ 
lity. In order to correft the aberration arifing from dif¬ 
ference of refrangibility among the rays of light, he in- 
flituted a fet of experiments, by which he detected a very- 
fingularand important quality in the muriatic acid. In all 
the difperfive media hitherto'examined, the green rays, 
which are the mean refrangible in crown-glafs, were 
found among the lefs refrangible; but, in the muriatic 
acid, thefe fame rays were found to make a part of the 
more refrangible. This difcovery led to complete fuccefs 
in removing the great defeft of optical inilruments, viz. 
that difiipation or aberration of the rays which arifesfrom 
their unequal refrangibility, and which has hitherto ren¬ 
dered it impoflible to converge all of them to one point 
either by Angle or oppofite refractions, A fluid, in which 
the particles of marine acid and metalline particles hold 
a due proportion, at the fame time that it feparates the 
extreme rays of the fpeclrum much more than crowji- 
glafs, refrafts all the orders of the rays in the fame pro¬ 
portion that glafs does: and hence rays of all colours 
made to diverge by the refraction of the glafs, may either 
be rendered parallel by a fubfequent refraClion in the 
confine of the glafs and this fluid; or, by weakening 
the refraftive denfity of the fluid, the refraClion which 
takes place in the confine of it, and glafs may be ren¬ 
dered as regular as reflection, without the lealt colour 
whatever. The doCtor has a telefcope, not exceeding 
15 inches in length, with a compound objeft-glafs of 
this kind, which equals in all refpeCts, if it does not fur- 
pafs, the betl of Dollond’s 4a inches long. See Phil. Tranf. 
Edin. vol. iii. 
Refrattion of the Atmofphere. —Tables of refraClion have 
been calculated by Mr. Lambert, with a view to correct 
inaccuracies in determining the altitudes of mountains 
geometrically. The obfervations of Mr. Lambert go upon 
the fuppofition that the refractive power of the atmof¬ 
phere is invariable : but as this is by no means the cafe 
his rules mull be confidered as true only for the mean 
ftate of the air. 
Dr. Nettleton obferved a remarkable variety in the re¬ 
fraftive power of the atmofphere, which demonftrates how 
little we can depend upon the calculated heights of moun¬ 
tains, when the obfervations are made with an intlrumenr 
and when the refraftive power of the air is to be taken 
into the account. Being defirous to learn, by obferva- 
tion, how far the mercury would defcend in the barome¬ 
ter at any given elevation, he propofed to meafure the 
height of fome of their higheft hills; but, when he at¬ 
tempted it ? he found his oblervation fo much dillurbed 
by refraClion, that he could obtain no certain refult. 
Having meafured one hill of a conliderable height in a 
clear day, and obferved the mercury at the bottom and 
at the top, he found, that about 19 feet or more were 
required to make the mercury fall ^ of an inch ; but, af¬ 
terwards repeating the experiment when the air was ra¬ 
ther grofs and hazy, he found the fmall angles fo much 
increafed by refraction as to make the hill much higher 
than before. He afterwards frequently made obfervations 
at his own houfe, by pointing a quadrant to the tops of 
fome neighbouring hills, and obferved that they would 
< a . appear 
