Nee ee 
TRANSACTIONS OF THE SECTIONS. 11 
ization of the part of the sky to which it is directed; the plane of polarization for 
the time being is the only thing we need concern ourselves about, and a very simple 
expression, stated first I believe by M. Babinet,’ defines the position of this plane 
for any given point of the sky; it is this: ‘Fora given point of the atmosphere 
the plane of polarization of the portion of polarized light which it sends to the eye 
coincides with the plane which passes through this point, the eye of the observer 
and the sun.’ The truth of this law may be easily demonstrated without any 
refined apparatus in the following manner :—Let the observer be provided with a 
Nicol’s prism and a plate of Iceland spar cut perpendicularly to the axis, aud stand 
with his back towards the sun; keeping the diagonal of the prism always in the 
same vertical plane, let him direct it successively to every point of the sky within 
that plane; the intensity of the polarization indicated by the brightness of the 
coloured image will vary very considerably at these different points, but the plane 
of polarization indicated by the upright position of the black or white cross, as the 
case may be, will remain unchanged. I leave out of consideration for the present 
the inversion of the plane of polarization observed occasionally near the horizon 
below the neutral point. 
“Tf we direct our analysing apparatus to the zenith during the whole day, the 
change in the plane of polarization of that point of the sky will correspond with the 
azimuths of the sun. Let us now turn our attention to the north pole of the sky : 
as the sun in its apparent daily course moves equably in a circle round this pole, it 
is obvious that the planes of polarization at the point in question change exactly as 
the position of the hour-circles do. The position of the plane of polarization of the 
north pole of the sky will at any peried of the day therefore indicate the apparent 
or true solar time. The point of intersection of the hour-circles, or the north pole 
of the sky, corresponds on only two days of the year with the maximum intensity 
of polarization; these days are the equinoxes; on all other days the points of max- 
imum polarization of the respective hour-circles describe a circle round the point of 
intersection ; but the angular distance thereof, which is greatest at the solstices, 
never exceeding 23° 28’, the polarization has always sufficient intensity to exhibit 
brilliant colours in films of selenite, &c. 
“‘ These points being premised, I proceed to describe the new instrument, which I 
have called the Polar Clock or Dial. It is thus constructed. At the extremity of a 
vertical pillar is fixed, within a brass ring, a glass disc, so inclined that its plane is 
perpendicular to the polar axis of the earth. On the lower half of this disc is a 
graduated semicircle divided into twelve parts (each of which is again subdivided 
into five or ten parts), and against the divisions the hours of the day are marked, 
commencing and terminating with v1. Within the fixed brass ring containing the 
glass dial-plate, the broad end of a conical tube is so fitted that it freely moves round 
its own axis; this broad end is closed by another glass disc, in the centre of which 
is a small star or other figure, formed of thin films of selenite, exhibiting when 
examined with polarized light strongly contrasting colours; and a hand is painted 
in such a position as to be a prolongation of one of the principal sections of the 
crystalline films. At the smaller end of the conical tube a Nicol’s prism is fixed so 
that either of its diagonals shall be 45° from the principal section of the selenite 
films. The instrument being so fixed that the axis of the conical tube shall coincide 
with the polar axis of the earth, and the eye of the observer being placed to the 
Nicol’s prism, it will be remarked that the selenite star will in general be richly 
coloured, but as the tube is turned on its axis the colours will vary in intensity, and 
in two positions will entirely disappear. In one of these positions a small circular 
disc in the centre of the star will be a certain colour (red for instance), while in the 
other position it will exhibit the complementary colour. This effect is obtained by 
placing the principal section of the small central disc 223° from that of the other 
films of selenite which form the star. The rule to ascertain the time by this instru- 
ment is as follows: the tube must be turned round by the hand of the observer 
until the coloured star entirely disappears while the disc in the centre remains red ; 
the hand will then point accurately to the hour. The accuracy with which the solar 
time may be indicated by this means will depend on the exactness with which the 
plane of polarization can be determined; one degree of change in the plane corre- 
sponds with four minutes of solar time. 
