156 TRANSACTIONS OF THE [aPR. 1, 
substances emit light of widely different percentages of polar- 
ization, it appears that a study of the relations of different 
bodies in this respect ought either to add something to our 
knowledge of the optical properties of the substances considered, . 
or else, if this particular property is deducable from the already 
known properties,as Arago assumed it to be, its relation to 
these properties ought to be definitely proved. This investiga- 
tion has therefore been undertaken for the purpose, first, of 
making a somewhat wide range of qualitative experiments upon 
the nature and generality of the phenomenon; and, secondly, of 
subjecting Arago’s explanation of the cause to the test of com- 
parison with carefully determined experimental quantities. 
1G 
‘HistortcaL REvIew. 
The simple facts of polarization of light by emission can best 
be observed, and in fact were first noticed, upon platinum. If 
a sheet of that metal be heated to incandescence in the flame of 
a Bunsen burner, and the emitted light examined by means of a 
Nicol prism, or any other instrument adapted to the detection of 
partially polarized light, it will “be observed that when the 
experimenter is viewing the surface normally the emitted light 
exhibits no trace whatever of polarization, but as the instrument 
is inclined so as to receive rays emerging obliquely from the sur- 
face, the light begins to show evidences of polarization in a 
plane perpendicular to the plane defined by the normal and the 
emerging ray, If this plane be called the plane of emission, and 
the angle included between these two directions the angle of 
emission, the complete phenomenon may be roughly described 
by saying that the polarization increases as the angle of emis- 
sion increases, and becomes, in the case of platinum, exceedingly 
strong as the emission angle approaches ninety degrees. 
The announcement of this fact, and the consequent overthrow 
of the common belief that light coming immediately from self 
luminous bodies is always natural, was first made in 1824 by 
Arago. Ina report made in that year to the Royal Academy 
of Sciences (see Annales de Chemie et de Physique (1) 27, p. 
89.) he announced that he had some time before made a series 
of experiments upon the light which emanates from incandescent 
bodies. “ He found that if the bodies are solid or liquid this 
light is partially polarized by refraction when the rays observed 
form with the emitting surface an angle of a small number of 
degrees. As for the light of an ignited gas it presented under 
no inclination traces of sensible polarization.” From these ex- 
