6 
REPORT — 1854. 
of these points and the law of their displacement; the points corresponding with the 
maximum of polarization have been studied, and the lines of equal polarization have 
been determined in a particular case, but it cannot be denied that much still remains 
to be done. 
When allowance is made for the accidental circumstances which often introdn: 
such great perturbations into the celestial polarization, there should exist, for va h 
place of observation, a normal state of polarization, depending exclusively spit the 
position of the sun ami local circumstances, which ought to bp characterinblc by 
means of polarimetric observations. The following method, founded on the com¬ 
parison of the intensities uf two images which nre brought to equality under the mo-t 
favourable circumstances (i. c. when they are of the same colour and in contact), nrny 
be employed in this case with advantage. 
Let a and b be the intensities of two polarized bundles of rays, which together at- 
equivalent to the partially polarized bundle, the vibrations of the two component 
bundles being respectively parallel and perpendicular to that of the polarized I j»orto-n 
of this bundle, the quantity of polarized light therein contained will have the vs "■ 
or sim P , y Inking 1, which is the quantity to be estimated. 
Polarimeler ,—The luminous bundle, after traversing some narrow dini'hnvgrr 1 ^ 
arranged in a tube of a length equal to 2$ diameters, falls upon a doubly ram 
prism, which produces, by refraction, two images of the opening «l t« e . 
diaphragm. A Nicol's analysing prism, cut into the form of a right pa™ie ip P l 
placed in the centre of a fried circle, the axis of which coincides with , 
tube; nn alidade furnished with n vernier allows the rotation of tins to be m ■ _, 
Immediately against the terminal diaphragm, there is a cap with an °P e ”J* |irI 
millimetres in diameter ; it is this that furnishes the two polarized images, 
bears a Soleil's quartz-plate, with two rotations, which serves as a polans^P • , 
position of these two opening* may be changed at pleasure m the 
axis of the apparatus. The whole system is enclosed ill a concentric, cR ^ . 
nished with a hinge; by this arrangement it may be turned upon 
m any position by menu of n screw. The apparatus thus arranged is l 
meter-. ** 
Atmopolarimdcr .-To adapt this instrument for observations on 
polarization, it is placed upon a vertical columnar support, standing o * . t „j ,, : j 
screw feet. At the upper part of the support there is a horizontal ,' l rert jr»i 
with levels and n compass, to furnish the azimuth of the point obse j ne the & 
semicircle placed towards the upper part of the column serves to de ^ 
VHtion of this point, and a parallactic circle enables the instruroen 
ranged according to the great circles of the sphere. . , j„r obsff** - 
To ascertain exactly the angular distance from the sun to the 
.a small finder is arranged parallel to the telescope bearing t u .,on 
one of its extremities, and at the other a small disc of transparent > ‘ , 
centre of which the shadow of the point of intersection of the two cro».ea 
>e rown when the apparatus is directed to the sun*. . , , - rC ] e isd'-’f 
minp lT klU 5 observation, the horizontality of the aamtlUiJ 1 ier 0 f 
mined by mean* of the levels nnd supporting screws of the tripod; the 
verticalcnde bring at zero, the parallactic circle and the *‘ J 
is dirp h rj f° ntn, J - Wl '«* t * ,c orientation of the apparatus ha* J*. uar u-pl»“ 
lower d d i!' d8 ,he P°’ nt of Ihe sky under examination, the dotAM J. n „,. 
ho!„vT UP ° n lh r UX ,' R of l, "‘ 1'olarimeter, and the latter is turned jndif* 
t o m™l| r T" y ° f C0lorati " n of ,1|L> ,w,) discs corresponding with •JJf 0 tbc P 1 * 
of 1 ,n'" 1S,n ^Ffpendieularity of the principal section ot the F 1 ■ J )lcp0 |»rF‘ ■ 
when Z “St T1,C W* n " ve cup F. then substituted for that bu! to 
difference «r lffen " C1J ,lie int£ *nsity of the two images become ct , r tain cV 
« with d,? f P ^ a ' 3 I 1 ' 9 " tlu “ P ri n««Pa» section of the analyser form** <■ f ,- 
Wlth tI,at of lk Polarizer; from this it follows that if the pr.nc.pal sec 
prcwnte'lE?" f lU9 a P» rflt « 5 devoted to the orientation and ^ 
h-U adlflS h l,n ^ r - 1, "? ?PmC,,tS as 11 6'eat number of instruments, and 
that adopted by Peltier m his modification of Arago's polarimetcr. 
