OPT 
means of remedying this inconveniency, faw clearly that, 
as long as this cloud took place, no accuracy could be 
obtained; and he difcovered that it appeared in confe- 
quence of the contrail formed by the white parts which 
were at the greateft diftance from the black circle. He 
thence concluded, that, if the ground was left white near 
this circle, and the parts of the pafteboard at the greateft 
diftance from it were covered with a dark colour, the 
cloud would no longer be vilible, or atleaft almoft totally 
difappear. This conjecture was confirmed by experi¬ 
ment. M. de Sauflure left a white fpace around the 
black circle, equal in breadth to its diameter, by placing 
a circle of black paper a line in diameter on the middle 
of a. white circle three lines in diameter, fo that the black 
circle was only furrounded by a white ring a line in 
breadth. The whole was palled upon a green ground. 
A green colour was chofen, becaufe it was dark enough 
to make the cloud difappear, and the eafieft to be pro¬ 
cured. The black circle, furrounded in this manner with 
white on a green ground, difappeared at a much lefs dif¬ 
tance than when it was on a white ground of a large fize. 
If a perfeClly-black circle, a line in diameter, be palled on 
the middle of a white ground expofed to the open light, 
it may be obferved at the diftance from 4410 45 feet; 
but, if this circle be furrounded by a white ring a line in 
breadth, while the reft of the ground is green, all fight of 
it is loft at the diftance of only 15J feet. 
According to thefe principles, M. de Sauflure deline¬ 
ated feveVal black circles, the diameters of which increafed 
in a geometrical progreflion, the exponent of which was -§. 
His fmalleft circle was £ or era of a line in diameter; the 
fecond o - 3 ; the third, o‘45 ; and fo on to the iixteenth, 
which was or about 7 inches 3J lines. Each of 
thefe circles was furrounded by a white ring, the breadth 
of which was equal to the diameter of the circle; and the 
whole was palled on a green ground. 
M. de Sauflure, for his experiments, feleCled a ftraight 
road or plain of about 1200 or 1500 feet in circumfer¬ 
ence, which towards the north was bounded by trees or 
an alcent. Thofe who repeat them, however, mull pay 
attention to the following remarks: When a perfon re¬ 
tires backwards, keeping his eye conftantly fixed on the 
pafteboard, the eye becomes fatigued, and foon ceafes to 
perceive the circle; as foon, therefore, as it ceafes to be 
diltinguifliable, you mull futfer your eyes to reft; not, 
however, by Glutting them, for they would when again 
opened be dazzled by the light, but by turning them 
gradually to fome lefs illuminated objeCt in the horizon. 
When you have done this for about half a minute, and 
again directed your eyes to the pafteboard, the circle will 
be again vilible, and you mull continue to recede till it 
difappear once more. You muft then let your eyes reft 
a fecond time, in order to look at the circle again, and 
continue in this manner till the circle becomes actually 
invilible. 
If you wifh to find an accurate expreflion for the want 
of tranfparency, you muft employ a number of circles, 
the diameters of which increafe according to a certain 
progrelfion ; and a comparifon of the diltances at which 
they difappear will give the law according to which the 
tranfparency of the atmofphere decreafes at different dif- 
tances. If you wifn to compare the tranfparency of the 
atmofphere on two days, or in two different places, two 
circles will be fufficient for the experiment. According 
to thefe principles, M. de Sauflure cauled to be prepared 
a piece of white linen cloth eight feet fquare. In the 
middle of this fquare he fewed a perfect circle, two feet 
in diameter, of beautiful black wool; around this circle 
he left a white ring two feet in breadth, and the reft of 
the fquare was covered with pale-green. In the like 
manner, and of the fame materials, he prepared another 
fquare; which was, however, equal to only - f V of the fize 
of the former, fo that each fide of it was 8 inches; the 
black circle in the middle was two inches in diameter, 
’and the white fpace around the circle was 2 inches alfo. 
Vol, XVII. No, 1204, 
I C S. 637 
If two fquares of this kind be fufpended vertically and 
parallel to each other, lb that they may be both illumi¬ 
nated in an equal degree by the fun ; and if the atmo¬ 
fphere, at the moment when the experiment is made, be 
perfectly tranfparent; the circle of the large fquare, which 
is twelve times the fize of the other, muft be feen at 
twelve times the diftance. In M. de Saufiure’s experi¬ 
ments, the fmall circle difappeared at the diftance of 314 
feet, and the large one at the diftance of 3588 feet, 
whereas it Ihould have difappeared at the dillance of 
3768. The atmofphere, therefore, was not perfectly 
tranfparent. This arofe from the thin vapours which at 
that time were floating in it. M. de Sauflure calls his 
inftrument a diaphanomcter; but it ferves one of the pur- 
pofes of a photometer. 
From a number of experiments made with the photo¬ 
meter, count Rumford found, that, by palling through 
a pane of fine clear well-polilhed glafs, inch as is com¬ 
monly made ufe of in the conftruCtion of looking-glafies, 
light loles '1973 of its whole quantity, i. e. of the quan¬ 
tity which impinged on the glafs; that, when light is 
made to pafs through two panes of luch glafs Handing 
parallel, but not touching each other, the lofs is '3184 of 
the whole; and that, in palling through a very thin clear 
colourlefs pane of window-glafs, the lols is only ‘1263. 
Hence he infers, that this apparatus might be very ufe- 
fully employed by the optician, to determine the degree 
of tranfparency of glafs, and direCt his choice in the pro- 
vifion of that important article of his trade. The lofs of 
light, when reflected from the very belt plain glafs mirror, 
the author afcertained, by five experiments, to be one- 
third of the whole which fell upon the mirror. 
For the refults of fome experiments, by the fame inge¬ 
nious philofopher, on the quantities of light emitted from 
bodies in a Hate of combultion, lee the article Light, 
vol. xii. p. 116. 
Profeflor Leilie’s photometer refembles, in the princi¬ 
ples of its conftruCtion, the differential thermometer and 
hygrometer invented by the lame ingenious author. It 
meafures the calorific effect of heat; and is founded upon 
this principle, “ that, if a body be expofed to the fun’s 
rays, it will, in every pollible cafe, be found to indicate 
a meafure of heat exaCtly proportioned to the quantity of 
light which it has abforbed.” This efteCt is produced by 
the following mechanifm: Two tubes, into which ful- 
phuricacid tinged with carmine is introduced, are joined 
together by means of a blow-pipe, and made to Hand pa¬ 
rallel to each other, the interval between them being oc¬ 
cupied by a graduated fcale : the ends of the tubes are 
clofed by two hollow glafs balls, one tranfparent, the other 
blackened ; when the inftrument is expofed to the aCtion 
of light, the blackened ball abforbs light, and, according 
to the author, a proportional quantity of heat; or, by va¬ 
rying the expreflion of the fa 61 , a proportional quantity of 
heat is excited, which produces a dilatation in the aircon- 
tained in the blackened ball, and caufes the coloured li¬ 
quor to rife in the tube terminated by the tranfparent ball, 
which, abforbing no light, receives no acceffion of heat. 
But, though the black ball acquires conftant additions of 
heat, its temperature will not uniformly and perpetually 
increafe; for the accumulated heat will at tail be con¬ 
ducted off’, by the furrounding air, exactly as it is re¬ 
ceived. The depreffion of the liquor, therefore, will mea¬ 
fure the momentary afflux of light. To prevent the irre¬ 
gular effeCts of winds, which might accelerate that difper- 
iion, the inftrument is inclofed within a glafs cafe. But 
this cafe ferves alio an important purpote; for, by con¬ 
fining the circulation of the ambient air, which, alone 
transfers the continual augmentation of heat, it doubles 
the performance of the inftrument. The cylindrical cafe 
fhould be made of clear g'lais, neatly rounded over, and 
hermetically fealed at the end. Its width is not of much 
importance, only it fliould leave a free fpace not lefs than 
j'oth of an inch round the balls, and at leait half an inch 
at the top. Indeed, both the fize and form may be regu- 
7 Z ' lated 
