SCIENTIFIC SUMMARY. 
321 
gow, reflecting on this fact and others of a kindred nature, was led to insti- 
tute experiments with the view of determining whether a beam of plane- 
polarized light, when reflected from an iron surface highly magnetized, would 
have its plane of polarization twisted in the act of reflection. A paper 
describing his experiments in detail, and corroborating his assumption, will 
be found in a recent number of the “ Philosophical Magazine ” (May, 1877, 
p. 321). An upright electro-magnet of borse-sboe shape bad its polar 
surfaces polished so as to form reflecting planes. Moreover, a sub-magnet 
was employed to concentrate the magnetic force, by inductive action, upon 
the surface of the iron mirror. A beam of light polarized by transmission 
through a Nicol’s prism was incident at various angles upon this mirror, 
nnd after reflection was analysed by passing through a second Nicol. Let 
t hese prisms be crossed so as to extinguish the light, and then send a current 
through the electro -magnet ; as soon as the iron mirror becomes magnetized 
the obscuration is diminished, showing, of course, that the plane of polari- 
sation has been turned. The direction in which it is turned is contrary to 
that of the magnetizing current. If, for example, the magnetized iron 
mirror become a true south pole, then the plane of polarization is turned to 
the right hand ; if the current be reversed, so that the magnet becomes a 
north pole, the rotation is to the left. It should be remarked that Dr. Kerr, 
following Sir W. Thomson, uses in this respect a different nomenclature 
from that popularly employed. His true south pole would be one that 
points, on the whole, towards the north ; that is to say, what most people 
would call the north pole of the magnet. It seems reasonable enough that 
the great earth-magnet should have its south pole in the southern, and its 
north pole in the northern hemisphere. But if artificial magnets are to 
have their poles similarly named to the like poles of the earth, then (re- 
membering that opposite poles attract) we shall have to call the true south 
pole of the magnet that which points to the magnetic north ; in other words, 
we reverse the common nomenclature. It is in the highest degree desirable 
that uniformity should be attained, and confusion prevented, in the use of 
such common terms. Indeed, it has been suggested to abolish such phrases 
as the north and south ends of the needle, and to substitute others free from 
ambiguity. 
Method of Determining Index of Refraction . — It is by no mer.ns an easy 
matter to determine the index of refraction of a solid substance, say, of a 
gem-stone, or other transparent mineral. And yet the refractive power is 
unquestionably an important physical characteristic, and one which would 
no doubt be often recorded if it could be measured with readiness. Mr. H. 
C. Sorby is therefore to be congratulated on placing in our hands a method 
of extremely simple application (“ Mineral. Magazine,” No. 4, 1877, p. 97). 
He takes a small piece of the transparent substance, having flat parallel 
sides, and placing it on the stage of the microscope, finds the thickness of 
the fragment, and also the extent to which the focal length of the object- 
glass is displaced by looking through this thickness of the medium. These 
measurements are effected by attaching to the body of the microscope a small 
graduated scale with a vernier, enabling the observer to read off to the 
thousandth of an inch. To make the two required measurements is but 
the work of a few moments, and having obtained these data, the index of 
