4 Improvements in Physical Science (Jan. 
the instrument difficult of execution. Nor is it impossible that the 
polarization or non-polarization of the luminous rays according to 
the nature of the surface from which they proceed may have consi- 
derable influence upon the quantity of light capable of reaching the 
eye through the horn, upon which the value of the instrument as a 
photometer totally depends. Lampadius has given various tables of 
the light emitted by different bodies as measured by his instrument. 
I shall transcribe one, by way of specimen. It exhibits the light 
emitted by the sky in a'clear morning on the 16th of February at 
Freyberg. The photometer was directed to the south-east at the 
height of 45° abeve the horizon. 
Photometer, Photometer, 
48 Mt sulin OVE. 8° GP AO?) cerelnre stad chip BIO 
£90 Ss ol t hee lay 2S G6. 50) Lach. « sete etna tre 
5630 Jiged.. uh 2a d+ Dit * eiscel stu sire alas 
540 cosieltwcW, inBO The sun rose, 
51s BOs J a(stete tO 2G Ti AG coersin'e eb ne VER. 
6.620 Houle. 2G. dou 30 Frio GO: ls yatta o2ibGete tay Oe 
Go WO awiadt te. elisa T5330 pide ess aes OD 
6. Wahine VE dawozs £0 7540 siA tte Wels wie's'etn ap OS 
BO a sratetors = oa. (46 Tesi. (3a. 6 eke [nj 0h oe oaee® 
From this table we see that the twilight began an hour and a half 
before sun-rise. ; 
But the greatest additions that have been made to the science of 
optics during the course of the year consist in the investigations 
respecting the properties of different bodies as far as the polarization 
of light is concerned. The principal experimenters on this subject 
have been Dr. Brewster and M. Biot. Dr. Brewster has been most 
indefatigable, and has published, during the course of 1815, no less 
than six different memoirs on the subject ; five in the Philosophical 
Transactions of London, and one in the Philosophical Transactions 
of Edinburgh. I shall first notice such of the memoirs of Biot as 
have come to my knowledge, and then J shall give an aceount of 
Dr. Brewster’s discoveries. 
1. M. Biot discovered that the tourmaline, when very thin, re- 
fracts doubly, like calcareous spar; but when in thick plates, it 
refracts only singly. From this it is evident that in this mineral 
there exist two distinct causes of polarization ; one belonging to the 
crystalline molecules of the tourmaline, the other depending on the 
plates of which the crystal is composed. The first acts sensibly only 
when the mineral is very thin; the second, when it has a certain 
degree of thickness. 
M. Biot ascertained likewise that, when the agate is very thin, it 
transmits light in every direction, arid possesses the properties of a 
doubly refracting body. The laws observed by Dr. Brewster re- 
specting the agate hold only when it possesses a certain degree of 
thickness, 
2, After Malus had discovered the polarization of light, when 
