OPTICS. 
(j3(i 
mined by a particular experiment made exprefsly for that 
purpofe with the ftandard-lamp, that value may be written 
inftead of it. When the ftandard-lamp itfelf is made ufe 
of inftead of the lamp C, then the value of A will be i. 
The count’s firft attempts with his photometer were to 
determine how far it might be poftible to afcertain, by di¬ 
rect experiment, the certainty of the affumed law of the 
diminution of the intenfity of the light emitted by lumi¬ 
nous bodies ; namely, that the intenfity of the light is 
every-wliere as the fquares of the diftances from the lu¬ 
minous body inverfely. As it is obvious that this law 
can hold good only when the light is propagated through 
perfeflly-tranfparent fpaces, fo that its intenfity is weak¬ 
ened merely by the divergency of its ray's, he inftituted a 
fet of experiments to afcertain the tranfparency of the air 
and other mediums. 
With his view, two equal wax candles, well-trimmed, 
and which were found, by a previous experiment, to burn 
with exactly the fame degree of brightnefs, were placed 
together, on one fide, before the photometer, and their 
united light w : as counterbalanced by the light of an Ar- 
gnnd’s lamp, well-trimmed, and burning very equally, 
placed on the other fide over againft them. The lamp 
w'as placed at the diftance of 100 inches from the field of 
the photometer, and it was found that the two burning 
candles (which were placed as near together as poftible, 
without their flames affefting each other by the currents 
of air they produced) were juft able to counterbalance the 
light of the lamp at the field of the photometer, w hen 
they were placed at the diftance of 60 8 inches from that 
field. One of the candles being now taken away and ex- 
tinguifhed, the other was brought nearer to the field of 
the inftrument, till its light was found to be juft able, 
fingly, to counterbalance the light of the lamp ; and this 
was found to happen when it had arrived at the diftance 
of 4“5'4 inches. In this experiment, as the candles burnt 
with equal brightnefs, it is evident that the intenfities of 
their united and fingle lights were as 2. to 1, and in that 
proportion oqghr, according to the affumed theory, the 
fquares of the diftances, 6o-8 and 43-4, to be ; and, in fa£l, 
6o'8 2 =r3696'64 is t043'4 2 = i8S3 , 56 as 2 isto 1 very nearly. 
Again, in another experiment, the diftances were. 
With tw’O candies =54inches. Square =2916 
With one candle = 3 3 "6 - - = 1489-96 
Upon another trial, 
With two candles =54-6 inches. Square =2981-16 
With one candle =39-7 - - =1576-09 
And, in the fourth experiment. 
With two candles =58-4 inches. Square =3410-56 
With one candle =42-2 - - =1780 84 
And, taking the mean of the refults of thefe fourexne- 
laments, 
Squares of the Diftances, 
With two Candles. 
With one Candle. 
In the experiment N° 1. 
3696-64 
— 
1883-56 
N° 2. 
2916 
— 
1489-96 
N° 3 - 
2981-16 
— 
1576-09 
N° 4. 
3410-56 
— 
1780-84 
4 ) 
i 3 °° 4 ' 3 6 
4 )^ 73°'45 
Means 
3251-09 
and 
1682-61 
which again are very nearly as 2 to 1. 
With regard to thefe experiments, it may be obferved, 
that were the reiiftance of air to light, or the diminution 
of the light from the imperfedt tranfparency of air, fenfi- 
ble within the limits of the inconfiderable diftances at 
which the candles were placed from the photometer, in 
that cafe the diftance of the two equal lights united ought 
to be, to the diftance of one of them fingle, in a ratio Tefs 
than that of the fquare root of 2 to the fquare root of 1. 
For, if the intenfity of a light emitted by a luminous 
body, in a J'pace void of all rejiftance, be ftiminiftied in the 
proportion of the fquares of the diftances, it muft of ne~ 
ceflity be diminifhed in a (till higher ratio when the light 
paffes through a refilling medium, or one which is not 
perfectly tranfparent ; and, from the differences of thofe 
ratios, namely, that of the fquares of the diftances, and 
that other higher ratio found by the experiment, the re¬ 
finance of the medium might be afcertained. This he 
took much pains to do with refpeft to air, but did not 
fucceed ; the tranfparency of air being fo great, that the 
diminution w-hich light fullers in palling through a few 
inches, or even through feveral feet of it, is not lenlible. 
Having found, upon repeated trials, that the light of a 
lamp, properly trimmed, is incomparably more equal than 
that of a candle, whofe wick, continually growing longer, 
renders its light extremely fluctuating, he fubftituted 
lamps to candles in thefe experiments, and made fuch 
other variations in the manner of conducting them as he 
thought bid fair to lead to a difcovery of the reiiftance of 
the air to light, were it poftible to render that refinance 
fenfible within the confined limits of his machinery. But 
the refults of'them, fo far from affording means for af- 
certaining the reiiftance of the air to light, do not even 
indicate any reiiftance at all ; on the contrary, it might 
almoll be inferred, from fome of them, that the intenfity 
of the light emitted by a luminous body in air, is •dimi¬ 
nifhed in a ratio lefs than that of the fquares of the dif¬ 
tances ; but, as fuch a conclufion would involve an evi¬ 
dent abfurdity, namely, that light moving in air, its 
abfolute quantity, inftead of being diminilhed, actually 
goes on to increafe, that conclufion can by no means be 
admitted. 
The ingenious author’s experiments all confpired to 
fnovv that the refiftance of the air to light is too incon¬ 
fiderable to be perceptible, and that the afiumed law of 
the diminution of the intenfity of light may be depended 
upon with fafety. He admits, however, that means may 
be found for rendering the air’s refiftance to light appa¬ 
rent; and he feems to have thought of the.very, means 
which occurred for this purpofe to M. de Sauffure. Thai 
eminent philofopher, w'i filing to afcertain the tranfparency 
of the atinofphere, by meafuring the diftances at which 
determined'objects ceafe to be vilible, perceived at once 
that his end would be attained, if he Ihoulff find objects 
of which the difappearance might be accurately deter¬ 
mined. Accordingly, after many trials, he found that 
the moment of difappearance can be obferved with much 
greater accuracy when a black objefl is placed on a white 
ground, than when a white objedl is placed on a black 
ground ; that the accuracy was Hill greater when the 
cbfervation was made in the fun than in the fti.ade; and 
that even a ftill greater degree of accuracy was obtained, 
when the white Ipace furrounding a black circle was itfelf 
furrounded by a circle or ground of a dark colour. This 
laft circumftance was particularly remarkable, and an ob- 
fervation quite new. 
If a circle totally black, of about two lines in diameter, 
be faftened on the middle of a large fheet of paper or 
pafteboard, and if this paper or pafteboard be placed in 
fuch a manner as to be expofed fully to the light of the 
fun, if you then approach it at the diftance of three or 
four feet, and afterwards gradually recede from it, keep¬ 
ing your eye cor.ftantly directed towards the black circk, 
it will appear always to decreafe in fize the farther you 
retire from it, and, at the diftance of 33 or 34 feet, will 
have the appearance of a point. If you continue ftill to 
recede, you will fee it again enlarge itfelf; and it will 
feem to form a kind of cloud, the darknefs of which de- 
creafes more and more according as the circumference 
becomes enlarged. The cloud will appear ftill to increafe 
in fize the farther you remove from it; but at length it 
will totally difappear. The moment of the difappear-* 
ance, however, cannot be accurately afcertained ; and 
the more experiments were repeated, the more were the 
refults different. 
M. de Sauflure, having reflected for a long time on the. 
1 means. 
