Intelligence and Miscellaneous Articles. 11 



could turn about a vertical axis so as to orient the luminous beam 

 in various directions. 



With this arrangement it is ascertained that, if the system bo 

 made to coincide in direction with the plane of the magnetic meri- 

 dian, the same position of the polarization-plane is not obtained 

 whether we look towards the south or towards the north; a great 

 number of closely concordant measurements gave an angular differ- 

 ence of about 6'-5 between the two positions. On the contrary, 

 if we place ourselves in a position perpendicular to the magnetic 

 meridian, we get the same direction of the plane of polarization 

 whether we look towards the east or towards the west ; and this 

 position is the bisectrix of that which we have in viewing towards 

 magnetic north and south. 



It may hence be concluded that the angular difference observed 

 is a rotation of the plane of polarization of the light, due to the 

 action of the earth ; the number 6'*5 measures the double of the 

 rotation for yellow light and in the special conditions of the experi- 

 ment. The direction of this rotation is the same as that of the 

 rotation of the earth ; it is the direction of an electric current 

 which, on the hypothesis of Ampere, would give rise to the pheno- 

 mena of terrestrial magnetism. 



It must be remarked that the number we have given only refers 

 to observations made in the laboratory of the Museum d'Histoire 

 naturelle, in proximity to more or less considerables masses of iron. 

 To ascertain with more precision the action of the globe, and to 

 utilize this method for estimating the intensity of the earth's mag- 

 netism, it would be necessary to use the same precautions as for 

 ordinary observations of terrestrial magnetism, and to amplify the 

 phenomenon by taking a longer tube. This is what I am at present 

 engaged in. 



The system arranged as above indicated exhibits remarkable sen- 

 sitiveness to the action of magnetism : an ordinary bar magnet, 

 held in the hand, and brought parallel near to the tube, first in one 

 direction, then in the other, is sufficieutto make manifest a rotation 

 of the plaue of polarization that may attain to upwards of 1°. 



It is interesting to compare this direct measurement with an esti- 

 mation, made by Mr. Gordon*, of the magnetic rotation produced 

 by 1 ceutim. of bisulphide of carbon under a magnetic action equal to 

 unity. The result found by calculating from this number the action 

 of the terrestrial horizontal component is, that 2 - 5 metres of bisul- 

 phide of carbon should give, with yellow light, a single rotation of 

 6'- 8 — instead of 3''25, which results from our direct observation. 

 The difference may be due to exterior perturbations. If we adopt 

 the latter number, we see that, under the conditions in which we 

 have placed ourselves, the double rotation of 1 metre of bisulphide 

 of carbon would be 2'*6, and that of 1 metre of water 0'*8. — Comptcs 

 llendas tie VAcademie des Sciences, April 29, 1878, tome lxxxvi. 

 pp. 1075-1077. 



* Philosophical Transactions, 1877, Part I. 



