TRANSACTIONS OP THE SECTIONS. 
5 
bare traversed the slit are received in the apparatus. It is advisable, in this case, to 
^lace the diaphragm at the entrance by a system of two lenses forming a colhneator; 
die polarization of the parullel rays is then complete. 
The light which reaches the eye is sometimes very bright; it then becomes neces- 
ury to deaden it by placing a piece of ground glass before the collineator. 
1., studying reflexion or diffusion, or comparing the intensities of two luminous 
Muni-*, the entire system of prisma placed in the anterior part of the apparatus is 
•rnewed; die arrangement of the two separato bundles, which are afterwards united 
In inwuis of the other system of prisms, may be easily understood. 
' When the absorption of light is to be determined, Use apparatus may receive some 
modifications by which it is simplified. The luminous bundle, before its divisions, 
Hit upon a Nicol’s prism which polarize* it, and afterwards upon a bi-rclractive 
pn»m which divides it; by moans of an arrangement of reflecting prisms similar o 
thp preceding, a circular image, composed of two parts polarized in two rectunguia 
pUtie*, is obtained. . . .. , 
The Nicol's polarising prism also serves as an analyser with regard to the igi 
•Inch has traversed the birefractive prism ; the rotation of this prism is also measure 
by moons of an alidade through which it is put in motion. One of the images in- 
erenses in intensity in the proportion of the square of the sinus of the (ingle orme 
b> the principal sections of the analyser and of the bi-refractivc prism ; the o 
ditrciiMu in the proportion of the square of the cosinus, mid their relation varies 
proportion to the square of the tangent of the same angle. . . . , . 
Ah indispensable precaution to be taken in these experiments is to place in iro 
of "‘v analyser a thick plate of rock crystal parallel to the axis, in such a maflnortnai 
principal section m;.y be parallel to that of the analyser. Isiool a prism* cut i 
tin - ordinary manner, and inclined in the tubes enclosing them so that they may nm 
produce a sensible deviation of the luminous rays, form excellent analysers w 
ni’licd with u similar plate, rendering unnecessary any correction with leg-u 
1 gallon in the qinmlity of polarized light reflected ut the first surface of t iese P ’ 
I* tll «y are turned upon their axes. Nicol's prisms cut in the form » g P 
‘d'pipvds do not answer the same purpose hero, us they cause too great a tia -1 
lb# incident ray, , , 
1 he apparatus which forms the subject of this Note, which wi» constrac^ 
i '' cr n !' tl8t > M. Duboseq, may be readily adapted to all the preceding j.Jdin" 
taking it to pieces. Suppressing some of its component parts, and adding 
'. "-refractive prism, the last-mentioned arrangement is produced. oure d 
'"«?"? of ®* u PP a ratus it is easy to prove that the absorbent 
r „. '' l uu |>'“dlnoiiccs the intensity of the light, the peculiar arrange™; J . 
^ protectmgthc eye from the errors which are almost inavitabl,^prndjeea^y 
.r ^UofiutensUy of colour, wlien the spectrum is examined ll 'roug . -fo of 
ihc ,, C0 ? 8e quently nothing to invalidate Newton's opinion as to the » P 7 
p amatic colours which is generally adopted by physicist*. , , 0 f the 
w*** 1 to Sviog . f r r i\ v iil 
r-lative , Whlch 1 «" cr,gaged, principally in a chemical point of7 W, bere _ 
afi cr to , ra, *sparen ey 0 f bodies to the principal rays of the spectrum, P t soine 
uf the n * aVe l ^ e * lonour of bringing forward the results of these oxpe 
“ le meetings of the Association* 
Note. 
'%&£*«», » v,™;.', ^ fSKSaK jjay.s 
R#ri«riil of a ? ( i 0 5 ( * ance with very various causes, is subject to a , ^ 8carc ely 
a ".vthin» a» We arc acquainted, but from which we have as J they are 
'hied. TUp 1 leir relations with the meteorological phsenomei pruve d the 
Pvi 'tonce in »K eSea , rches of MM. Arago, Babiuet, and Brewster ha ‘ lizing 
P " Uri ^tion s ^ e ? ! ® stial *reh of some remarkable points in which t ° (0S jti O n 
008 neutralize each other; these physicists have made known 1 
