TRANSACTIONS OF SECTION 4A. 931 
he says that there appears to be no electro-optic double refraction in the case of a 
uniformly charged Franklin’s plate. The results obtained were shown by the 
author to prove that electrostatically strained glass acts in the polariscope as if 
compressed along the lines of electric force, and this always, whether the electric 
field is uniform or not. 
13. On Magnetic Double Circular Refraction. 
By De Wirt B. Brace, Ph.D. 
The main object of the investigation has been to determine whether the re- 
fractive index of a medium under magnetic stress experiences a change for cir- 
cularly polarised light when the direction of propagation is that of the lines of 
force. 
In connection with the investigation, several important equations have been 
deduced for the case of reflection and refraction near the critical angle. A very 
small change in the refractive index produces a very great change in the angle of 
deviation and in the amount of reflected light. A slight change in the angle of 
incidence may produce a very great change in the angle of deviation of the re- 
flected or refracted ray. 
In the first experiment a piece of Faraday glass was placed in a strong mag- 
netic field, and one of the two interfering rays from a Jamin’s interference 
refractor allowed to pass through it in the direction of the lines of force. When 
the ray was circularly polarised, a displacement of the bands was observed, 
the direction of which depended on whether the ray was right or left-handed 
circularly polarised. This displacement became less and less distinct as the ray 
was more and more elliptically polarised. Every ray is then broken up into its 
opposite circular components, and either the velocity of propagation, or the phase 
(period), or both, changes. The observed displacement was ‘1355, while the 
value calculated from a double rotation of the plane of polarisation of 49° 20’ 
was ‘137. 
Several experiments for direct observation were made, which seemed to indicate 
no change in the velocity of propagation, but a change in the phase. In one of 
these experiments, a prism of glass was placed between the poles of a magnet, so 
that the rays were parallel to the lines of force and perpendicular to the first face 
of the prism, and were refracted out at the second face at a very large angle. 
Refraction then took place at a surface near the critical angle, and aslight change 
in the refractive index due to the induced magnetic stress would produce a very 
large deviation, in accordance with the equations found. The two halves of the 
narrow image were oppositely circularly polarised, so that each should have been 
displaced in the opposite direction. There should also have been a slight change 
in the intensity of the two halves. Nothing of the sort could be observed. 
Direct measurements of the rotation of the plane of polarisation, and comparison 
with a Fresnel’s double quartz prism, showed that the effect was within the limits 
of observation if such a change in the refractive index had occurred. 
14. Determination of the Heliographic Latitude and Longitude of Sun- 
spots. By Professor A. W. Tuomson. 
In the ‘ Observatory,’ published monthly, we have for any day—The Position 
Angle of the Sun’s Axis, and the Heliographic Latitude and Longitude of the 
‘Centre of Sun’s Disc. 
The method devised by me consists in throwing the image of the sun from an 
equatorial telescope on to adise representing the sun, with lines of latitude and 
longitude drawn thereon, and reading off the latitude and longitude of any spot. 
The latitude of the centre of sun’s disc varies from 0° to about 7°; a cardboard 
‘ Inaugural Dissertation,‘ Ueber die magnetische Drehung der Polarisationsebene 
und einige besondere Falle der Refraction.’ Berlin, Aug. 12, 1885. 
s 302 
