OPTICS. 
635 
and of tlie manner of making life of it. They are turned 
likewife without opening the box, by taking hold of the 
ends of their axes, which project below its bottom. 
As it is abfolutely neceffary that the cylinders ftiould 
conftantly remain preciiely perpendicular to the bottom 
of the box, or parallel to each other, it will be befttocon- 
flruft them of brafs; and, inftead of fixing them imme¬ 
diately to the bottom of the box (which, being of wood, 
may warp), to fix them to a ft long thick piece of well- 
hammered plate-brafs ; which plate of brafs may be after- 
vvards faftened to the bottom of the box by means of one 
ft ran ,v fc'rew. In this manner .two of the count’s beft in- 
ftruments are conftruCted ; and, in order to fecure the 
cylinders (till more firmly in their vertical pofitions, they 
are furnifned with broad flat rings, or projections, where 
they reft upon the brafs plate; which rings are T % of an 
inch thick, and equal in diameter to the projection of the 
win 0 * of the cylinder, to the bottom of which they afford 
a firm fupport. Thefe cylinders are likewife forcibly 
puffed, or rather pulled, againft the brafs plate upon 
which they reft, by means of comfrefled fpiral fprings 
placed between the under-fide of that plate and the lower 
ends of the cylinders. Of whatever material the cylin¬ 
ders be conftruCted, apd whatever be their forms or di- 
vnenfions, it is abfolutely neceffary that they, as well as 
every other part of the photometer, except the field, (hould 
be well painted oi a deep black. 
In order to move the lights to and from the photome¬ 
ter with greater eafe and" precifion, the obferver ffould 
provide two long and narrow, but very ftrong and fteady, 
tables ; in the middle of each of which there is a ftraight 
o-roove, in which a Hiding carriage, upon which the light 
fs placed, is drawn along by means of a cord which is 
faftened to it before and behind, and which, paffmg over 
pulleys at each end of the table, goes round a cylinder ; 
which cylinder is furniflied with a winch, and is fo placed, 
n'e-r the end of the table adjoining the photometer, that 
the obferver can turn it about, without taking Ins eye 
from the field of the inftrument. 
Many advantages are derived from this arrangement : 
FirfT the obferver can move the lights as he finds necef- 
ffry’ without the help of an afliftant, and even without 
removing his eve from the fliadows ; fecondly, each light 
is always precifely in the line of direction in which it 
ought to be, in order that the fliadows may be in contaCt 
i-i the middle of the vertical plane of the photometer; and, 
thirdly, the Aiding motion of the lights being .perfectly 
foft and o-entle, that motion produces little or no efteCl 
upon the^lights themfeives, either to increafe or diminith 
their brilliancy. Thefe tables mull be placed at ail angle 
of 60 decrees from each other, and in fuch a fituation, 
with refpeft to the photometer, that lines drawn through 
their middles, in the direction of their lengths, meet in 
a point exactly under the middle of the vertical plane or 
field of the photometer, and from that point the diftances 
ofthe lio-hts are meafured ; thefides of the tables being di¬ 
vided into Englifti inches, and a vernier, fliowing tenths 
of inches, being fixed to each of the Aiding carriages upon 
which the lights are placed, and which are fo contrived 
that they may be raifed or lowered at pleafure ; fo that 
the lights may be always in a horizontal line with the 
tops of the cylinders ofthe photometer. In order that 
the two long and narrow tables or platforms, juft de- 
fcribed may remain immovable in their proper positions, 
they are both firmly fixed to the Hand which fupports the 
photometer; and, in order that the motion of the car¬ 
riages which carry the lights may be as foft and gentle as 
poftible, they are made to Hide upon parallel brafs wires, 
q inches afunder, about of an inch in diameter, and 
well polifhed, which are ftretched out upon the tables from 
one end to the other. 
The ftruCture of the apparatus will be ciearlyffindei- 
ftood by a bare infpeCtion of the Plate. Fig. 7 is a plan of 
the infide of the box, and the adjoining parts of the pho¬ 
tometer Fig. 8. Plan of the two tables belonging to the 
photometer. Fig. 9. The box of the photometer on its 
ftand. Fig. 10. Elevation of the photometer, with one 
of the tables and carriages. 
Having fufticiently explained all the effential parts of 
this photometer, it remains for us to give fome account 
of the precautions neceffary to, be obferved in ufing it. 
And, firft, with refpebt to the diftance at which lights, 
whole intenfities are to be compared, ffould be placed 
from the field of the inftrument, the ingenious and accu¬ 
rate inventor found, that, when the weakeft of the lights 
in queftion is about as ftrong as a common wax candle, 
that light may moll advantageoufly be placed from 30 to 
36 inches from the centre of the field; and, when it is 
weaker or ftronger, proportionally nearer or farther off. 
When the lights are too near, the fliadows will not be 
well defined ; when too far off, they will be too weak. 
It will greatly facilitate the calculations neceffary in 
drawing conclufions from experiments of this kind, if 
fome fteady light, of a proper degree of ftrength for that 
purpofe, be allumed as a ftandard by which all others may 
be compared. Our author found a good Argand’s lamp 
much preferable for this purpofe to any other lamp cr 
candle whatever. As it appears, he fays, from a number 
of experiments, that the quantity of light emitted by a 
lamp, which burns in the fame manner with a clear flame, 
and without fmohe, is in all cafes as the quantity of oil 
confumed, there'is much reafon to fuppofe, that, if the 
Argand’s lamp be fo adjulted as always to confume a given 
quantity of oil in a given time, it may then be depended 
on as a juft ftandard of light. 
In order to abridge the calculations neceffary in thefe 
inquiries, it will always be advantageous to place the 
ftandard-larrip at the diftance of 100 inches from the pho¬ 
tometer, and to affutne the intenfity of its light at its 
fource equal to unity; in this cafe (calling this ftandard- 
light A, the intenfity of the light at its fource —x— 1, and 
the diftance of the lamp from the field of the photometer 
=ni— 100), the intenfity of the illumination at the field 
of the photometer (——) will be expreffed by the frac¬ 
tion Zil2—Tsboo> an d the relative intenfity of any other 
light\vhich is compared with it, may be found by the fol¬ 
lowing proportion : Calling this light B, putting y— its 
intenfity at its fource, and w= its diftance from the field 
of the photometer, expreffed in Englifti inches, as it is — 
= x , or, inftead of 4-, writing its value = T ohooi will 
m? m 
be —-—johoo 5 arR l confequently yis to 1 asw 2 is to 10000; 
n 2 
or the intenfity of its light B at its fource, is to the in¬ 
tenfity of the ftandard-light A at its fource, as the fquare 
of the diftance of the light B from the middle of the field 
of the inftrument, expreffed in inches, is to ioooo; and 
n 2 
hence it is y= —• 
•* 10000 
Or, if the light of the fun, or that of the moon, be com¬ 
pared with the light of a given lamp or candle C, the re- 
fult of fuch comparifon may be b i expreffed in words, 
by faying, that the light ofthe celellial luminary in quef¬ 
tion, at the fin-face of the earth, or, which is the fame thing, 
at the field of the photometer, is equal to the light of the 
given lamp or candle, at the diftance found by the experi- 
r , ien t; or, putting a— the intenfity of the light of this 
lamp C at its lource, and p— its diftance, in inches, from 
the field, when the fliadows correfponding to this light, 
and that correfponding to the celeftial luminary in quef¬ 
tion, are found to be of equal denfities, and putting 
the intenfity of the rays of the luminary at the furface 
of the earth, the refult of the experiment may be ex¬ 
preffed thus, z=— 
; or, the real value of a being deter¬ 
mined 
