1813.

BAROMETER.

THERMOMETER.

WIND.

De

grees

Morning. Mid-day. Evening. Morning. Mid-day. Evening. Morn-1 Mid-Even- of

Jan. 127 7.527 7.627 7.4 0.5 8.6

ing. day. ing. Wind

Observations.

227 7.5 27

7.627

A Cloudy.

4.8

4.0

3.2

0.0

1.4

10.9 27

2.8

1.5

0.5 N N N B

0.0 7.2 NNW NW C Snow.

Ditto.

7 27 2.7 26

826

8.4 26

0.0-12-2

-

N

927 6-2 27

7.227

4.9-11.5-10-8-100 S S

3.6 1.1 SW SW

9.8

4.0

12 26 6.226

9.226 11.3

3.227 4.3

5.227 3.6

1527 1.827

5.7 27

7.7

2.6 2.9

1.4 3.2

3.0

18 26

10.426

-1.1 SW SW SW

20 27

2-4 27

E

Ditto.

22 27 11 4 28

D Ditto.

D Ditto.

25 28 2.628 1.828 0.9

26 27 11-427 11.828 0.1

27 27 11-627 10.0 27 9.3

28 27 7-227

6.0

6.7

6.8

D Ditto.

29 27 5.927

C Ditto.

Sun.31 28

6-123

(The Supplementary Register will appear in the next Number.)

ARTICLE IV.

Description of a Method of making Doubly Refracting Prisms perfectly Achromatic. By David Brewster, LL.D. F.R.S. London and Edinburgh.

In the course of my experiments on the polarisation of light, I have long been in the habit of making use of perfectly achromatic prisms of calcareous spar; but until I ascertained from M. Biot that the method of constructing such prisms had neither occurred to himself, nor, as far as he knew, to any other person, I did not think of laying it before the public.

When we form a prism of calcareous spar for the purpose of analyzing light that has been submitted to the action of crystallized bodies, and correct the colour of one of the images by the equal and opposite dispersion of a prism of glass, we shall always find that a considerable portion of the colour of the other image

remains uncorrected. Even when the refracting angle of the prism is small, this uncorrected colour is very perceptible; but when the refracting angle is great, which it is often required to be when we wish to see at one view the systems of rings produced by crystallized bodies, the spectrum of the uncorrected pencil is extremely large, and alters completely the nature of the light which we are investigating. This is the form of the achromatic prism which I at first employed, and which has been used by M. Biot,* and, I believe, also by Malus.

R

In order to construct a prism in which the dispersions of both the images shall be simultaneously corrected, let us suppose A B C to be the prism of spar, and let two prisms of glass, Bed D, Ced, be constructed, so that the first is capable of correcting A the colour of the most refracted image, while the second, C e d, is capable of correcting the colour of the least refracted image. These prisms must be cut in such a manner that their joining surface, de, bisects the angle, mrn, formed by the two pencils when the ray Ris incident in the desired direction. If these two prisms are made

M.

2

n

of the same kind of glass, Ced will generally have a less refracting angle than Bed D; and the surfaces Cd, D d, will form at d an angle less than 180°. But if it is thought desirable to have Cd D a straight line, it may be accomplished by selecting crown and flint glass of the proper refractive and dispersive powers, and making Bed D of flint glass, and Ced of crown glass. The surface Cd D should be covered with an opaque plate, E F G H, as shown in the annexed figure, having an aperture, op, a little larger than the pupil, and having the line of junction, d, of the prisms for one of its dia

meters.

E

H

d

F

If calcareous spar were capable of receiving a perfect polish, the achromatic prism would now be complete; but as it appears to be impossible to give its surface a very fine lustre, we must endeavour to produce, by other means, an equivalent effect. When the surfaces of the calcareous spar are as highly polished as possible, the surfaces B e, Ce, of the two prisms must be cemented to the surface C B by a transparent cement, whose refractive power is a mean between the refractive powers by which the pencils rm, rn, are influenced; and by means of the same cement, plates of well-polished and well-annealed glass

*See Biot's Traité de Physique, tom. iv. p. 268.

must be cemented to the surface A B. By this means we shall have a prism in which the imperfections of the polishing are removed, while the two images are perfectly white, and nearly equally luminous. If we wish one of the two images to be still more perfect, it must be done at the expense of the other image, by using a cement of the same refractive power as that which corresponds to the image which we wish to improve. This compound prism will admit of still farther improvement, by cementing, in every case, the prism of glass Ced to the calcareous spar with a cement of the same refractive power with that which corresponds to the ray r m, and the other prism Bed D, with a cement whose refractive power is equal to that which corresponds to the ray r n.

These practical directions respecting the nature of the cement are of greater importance than we are apt to imagine. They are deduced from a series of new experiments on the action of crystallized surfaces, and on the extension of the doubly refracting force beyond the surfaces of bodies, which will soon be published.

Venlaw, Oct. 15, 1817.

ARTICLE V..

Of Isothermal Lines, and the Distribution of Heat over the Globe. By Alex. de Humboldt.*

THE unequal distribution of heat over the globe is one of those phenomena which has been long known as a general fact, but which cannot be exactly ascertained, with respect to its particular laws, until we have more correct data furnished by observation and experiment. To furnish these data is the immediate object of this paper; they are deduced from a great number of facts which have not been published; and if they are not sufficient to enable us to form a correct theory, they may, at least, lay a foundation for it, and will be useful in pointing out to travellers those objects to which they ought especially to direct their attention.

The distribution of vegetables and of organized beings in general dépends upon the circumstances that are connected with latitude, longitude, and elevation; and of these one of the most important is atmospherical temperature. The means which the author enjoyed of making observations, during his residence in South America, have enabled him to establish some very valuable data, which could not have been obtained in any other situation, more especially those which require to be made at great heights above

* Abridged from the third volume of the Memoirs of the Society of Arcueil. VOL. XI. N° III.

M