306 Royal Society. 



to force the air in a rapid current through the apparatus. Apjohn's 

 dew-point observations are then compared, and the errors are found to 

 be similar to the preceding, and apparently from the same cause. 



To make the formula generally useful, the author gives a table of 

 the depression of dew-point below temperature for every degree of 

 depression of the moist bulb, at every 5° of temperature from 0° to 

 100°, and for every 10° from 100° to 140°, which he protracts on a 

 chart, so as to give the dew-point in evei-y case with little more 

 trouble than is required for reading a common thermometer, and 

 also at the same time the elasticity of vapour in the atmosphere. 



" Experiments on the influence of Magnetism on Polarized Light." 

 By Professor Carlo RIatteucci. Communicated by Sir John F. W. 

 Herschel, Bart., V.P.R.S. &c. 



The object of this notice is to communicate some recent experi- 

 ments on diamagnetism, and particularly on the influence of mag- 

 netism on polarized light. The following extracts are in the words 

 of the author: — - 



" The apparatus I employed in these experiments was an electro- 

 magnetic apparatus invented by RI.Rumkorf, and described by M.Biot 

 at a meeting of the Academy of Sciences of Paris, and consisting of 

 a powerful electro-magnet, of which the soft iron cylinder is traversed 

 by a hole in the direction of the length of the axis, through which 

 hole the ray of polarized light is made to pass ; and the voltaic cur- 

 rent which I employed on this occasion was that of seven pair of 

 Grove's construction. I made my first experiment with a piece of 

 heavy glass, which I received from Faraday himself. In order to 

 assure myself of the exact amount of rotation induced by magnetic 

 action, I caused the ray of light, before it reached the heavy glass, 

 to pass through the system invented by M. Soleil, consisting of two 

 equal plates of perpendicular quartz, placed side by side ; the one 

 turning to the right, the other to the left. I ascertained, first of all, 

 the rotation produced by making the current pass sometimes in one 

 direction, and sometimes in the other ; the two rotations, one to the 

 right, the other to the left, thus produced, were exactly the same. 

 Then I compressed slightly the middle part of the piece of heavy 

 glass, in tlie same manner as one compresses pieces of glass. I was 

 then obliged to turn the eyepiece in a certain direction in order to 

 restore the image to its first condition ; in my experiments I always 

 had to turn it, after compression, towards the right. I next made 

 the current pass, first in one direction, then in the other. The ge- 

 neral facts which I have observed constantly and without exception 

 are the following : — The rotation produced by the magnet on the com- 

 pressed j}iece of heavy glass is not the same to the right as it is to the 

 left : the rotation produced by the magnet is considerably greater in 

 the direction of the rotation produced by compression than it is in the 

 contrary direction : the rotation pi'oduced by the magnet on the com- 

 pressed heavy glass, and in the direction of the rotation produced by the 

 compression, is greater than that produced by the same magnet on r/lass 

 which has not been compressed, and the rotation in the contrary direc- 

 tion is less. The following are the numerical results. 



