(132 OPT 
every fifth line longer than the intermediate ones, and 
every tenth longer ttill, for the greater eafe in counting 
the divifions under the microfcope, and is generally ufed 
in meafuringthe magnifying-power of tnicrofcdpes. The' 
ether ivory one is divided into fquares of the joth ar.d 
inoth of an inch, and is commonly employed in mea- 
fui ing opaque objects. Thofe made of glafs are for trans¬ 
parent objedts, which, when laid on them, (how their 
natural fize. That marked on the brafs 100, are fquares 
divided to the 100th 'of an inch: that marked 5000 are 
parallel lines forming nine divifions, each divilion the 
ioooth cf an inch ; the middle divifion is again divided 
into 5, making divifions to the 5000th of an inch. That 
marked 10,000 is divided in the lame manner, with the 
middle divifion divided into 10, making the 10,oooth of 
an inch. The glafs micrometer without any mark is alfo 
divided, the outlidelines into 100th, the next into ioooth, 
and the infide lines into the 4000th, of an inch : thefe 
are again crolled with an equal number of lines in the 
fame manner, making fquares of the 100th, ioooth, and 
4000th, of an inch ; thus demonstrating each other’s fize. 
The middle Square of the ioooth of an inch is divided 
into Sixteen fquares; now, as 1000 fquares in the length 
of an inch, multiplied by 1000, gives one million in an 
inch furface ; by the fame rule, one of thofe fquares divi¬ 
ded into 16 mull be the 16,000,oooth part of an inch fur- 
face ! In viewing the fmalleft lines, Mr. Coventry ufes 
the magnifiers N° 2 or 3. and they are all better Seen, he 
fays, by candle than by day light. 
Photometer, or Measurer of Light. Plate XIV. 
That fome luminous bodies give a Stronger, and others 
a weaker, light, and that fome refledt more light than 
others, are fadts that have been always known, and that 
are Sufficiently obvious. But it is not eafy to eftimate 
with accuracy the comparative intenfity of light afforded 
by any two, or more, luminous objedts. For this pur- 
pofe, it is neceffary to affume as a principle, that the lame 
quantity of light, diverging in all diredtions from a lu¬ 
minous body, remains undiminilhed at all diltances from 
the centre of divergence. Thus we mull fuppofe, 
that the quantity of light falling on every body is the 
fame as would have fallen on the place occupied by its 
lliadow; and, if there were any doubt of the truth of 
the fuppofition,it might be confirmed by fome fimple ex¬ 
periment. It follows that, fine'e the lliadow of a Square 
inch of any furface occupies, at twice the diftance of 
the furface from the luminous point, the fpace of four 
Square inches, the intenfity of the light diminilhes as 
the Square of the diftance increafes. We can judge with 
tolerable accuracy df the equality of two lights by the 
eftimation of the eye ; but we cannot form any idea of the 
proportions of light of different intenfities. If, however, 
we remove two Sources of light to fuch diftances from an 
objeft that they may illuminate it in equal degrees, we 
may conclude that their original intenfities are inverfely 
as the fquares of their diftances. 
To this fubjedt Mr. Bouguer feems to havefirft directed 
his attention. The methods which he ufed for meafuring 
the proportion that two or more lights bear to one ano¬ 
ther, were the following. He took two pieces of wood 
or pafteboard, as EC and CD, fig. 1. and in which he 
made two equal holes P and Q, over ,which he drew 
pieces of oiled or white paper. Upon thefe holes he con¬ 
trived that the light of the different bodies he was com¬ 
paring fhould fall; while he placed a third piece of pafte¬ 
board, FC, fo as to prevent the two lights from mixing 
with one another. Then, placing himfelf Sometimes on 
one fide, and Sometimes on the other, but generally on the 
oppofite fide of this inftrument with refpedl to the light, 
lie altered their pofition till the papers in the two holes 
appeared to be equally enlightened. This being done, 
he computed the proportion of their light by the 
Squares of the diftances at which the luminous bodies 
I c s. 
were placed from the objects. If, for inftance, the dif¬ 
tances were as three and nine, he concluded that the 
lights they gave were as nine and eighty-one. Where 
any light was very faint, he Sometimes made ufeof lenfes, 
in order to condenfe it; and heenclofed them in tubes or 
not, as his particular application of them required. 
To meafure the intenfity of light proceeding from the 
heavenly bodies, or reflefted from any part of the Sky, he 
contrived an inftrument, which refembles a kind of por¬ 
table camera-obfcura. He had two tubes, fig. 2, of which 
the inner was black, fattened at their lower extremities by 
a hinge C. At the bottom of thefe tubes were two holes, 
R and S, three or four lines in diameter, covered with 
two pieces of fine white paper. The two other extremi¬ 
ties had each of them a circular aperture, an inch in dia¬ 
meter ; and one of the tubes confifted of two, one of them 
Sliding into the other, which produced the fame effedt as 
varying the aperture at the end. When this inftrument 
is ufed, the obferver has his head, and the end of the in¬ 
ftrument C, fo covered, that no light can fall upon his 
eye befides that which, comes through the two holes S 
and R, while an affiftant manages the inftrument, and 
draws out or fliortens the tube DE, as the obferver di- 
redts. When the two holes appear equally illuminated* 
the intenfity of the lights is judged to be inverfely as the 
fquares of the tubes. 
In ufing this inftrument, it is neceffary that the objefl 
ftiould Subtend an angle larger than the aperture A or D, 
feen from the other end of the tube; for, otherwise, the 
lengthening of the tube has no effedt. To avoid, in this 
cafe, making the inftrument of an inconvenient length, 
or making the aperture D too narrow, he has recourfe to 
another expedient. He conltrudts an inftrument, repre¬ 
sented at fig.- 3, confifting of two objedt-glailes, AE and 
DF, exadtly equal, fixed in the ends of two tubes fix or 
Seven feet, or, in fome cafes, ten or twelve feet, long, and 
having their foci at the other ends. At the bottoms of 
thefe tubes B, are two holes, three or four lines in diame¬ 
ter, covered with a piece of white paper; and this inftru¬ 
ment is ufed exactly like t!»e former. 
If the two objedts to be obferved by this inftrument be 
not equally luminous, the light that iflues from them 
muft be reduced to an equality, by diminishing the aper¬ 
ture of one of the objedt-glaffes ; and then the remaining 
furface of the two glaffes will give the proportion of 
their lights. But for this purpoSe, the central parts of 
the glafs muft be covered in the fame proportion with 
the parts near the circumference, leaving the aperture 
fuch as is represented at fig. 4. becaufe the middle part of 
the glafs is thicker and lefs transparent than the reft. If 
all the o'ojedts to be obferved lie nearly in the fame direc¬ 
tion, Bouguer remarks, that thefe two long tubes may be 
reduced into one, the two obje'dt-glaffes being placed clofe 
together, and one eye-glafs Sufficing for them both. The 
inftrument will then be the fame with that of which he 
published an account in 1748, and which he called a Jielio- 
vieter, or ajlrometer. 
It is not, however, the abfolute quantity, but only the 
intenfity, of the light, or the number of rays, in propor¬ 
tion to the furface of the luminous body, that is meafured 
by thefe two instruments ; and it is of great importance 
that thefe two things be distinguished. The intenfity of 
light ma)' be very great, when the quantity, and its power 
of illuminating other bodies, may be very Small, on ac¬ 
count of the fmallnefs of its furface ; or the contrary may 
be the cafe, when the furface is large. 
Having explained thefe methods which M. Bouguer took 
to meafure the different proportions of light, wefiiall Sub¬ 
join a few examples of his application of them. 
i.»It is obfervable, that, when a perfon Stands in a place 
where there is a Strong light, he cannot distinguish objedfs 
that are placed in the Shade ; nor can he fee any thing 
upon going immediately into a place where there is very 
little light. It is plain, therefore, that the adtion of a 
Strong 
