220 Dr. Brewster on the Pohirisation of Light. 
obliqirty of its incidence. I then substituted a plate of glass 
instead (.f the mica, and a similar result was obtained, though 
the quantity of polarised light was considerably less than in the 
first experiment. By adding one plate of glass after another, 
the number of polarised rays was increased by the addition of 
each plate, and when the plates amounted tojifteeny the trans- 
mitted pencil was wholly polarised at an angle of about 
7c* 17', and possessed all the properties of that species of 
light. 
When a beam of light, polarised in this manner, is viewed 
through a piece of agate having its laminae parallel to the plane 
of refraction, the bright image of the object from which the 
ray proceeds will vanish, and the nebulous light will be a 
maximum ; but if either the plates of glass or the agate is 
turned round, so as to bring the lamime perpendicular to the 
plane of refraction, the nebulous light will vanish, and the 
bright image will recover its full lustre. 
If we now substitute in place of the agate another series of 
plates, having their refracting surfaces parallel to those of the 
first series, and having the same inclination to the emergent 
ray as the first series had to the incident ray, the ray emerg- 
ing from the first series will be transmitted through the second 
series, like ordinary light ; but if the refracting surfaces are 
perpendicular to each other, the ray emerging from the first 
series will refuse to penetrate the second series, and the object 
from which it proceeds will be invisible. 
Having thus ascertained that the pencil of light polarised 
by transmission comported itself, in every respect, like one 
of the pencils formed by double refraction, my next object 
was to ascertain the law of the phenomena in relation to the 
