1^6 Dr Brewster on. a singular Eocperiment 
tude^ there issues forth; some splendid discovery to interest and 
instruct mankind. Yet if we trace the histories of these ne- 
glected individuals, and contemplate the influence which, in 
afteti ages,, they exercise on human . affairs, we shall see new 
reason to admire the splendid privileges of the mind, and that 
unbounded influence which the silent progression of time, 
slowly, but; permanently creates, for the triumphant produc- 
tions of its powers. I remain, Dear Sir, yours, &c. 
Plymouth, ) 
May 4. IsA i Harvey. 
Art. XXIII — Account of a singular Experiment depending 
on the Polarisation of Light by Reflexion. By David 
Brewster, L. L. D., F. R S. Lond., h Sec. R. S. Ed. 
^ N the course of some experiments on the polarisation of light 
by reflexion, I was led to observe, that a film of aqueous va- 
pour might be breathed upon a plate of glass, without altering 
the distinctness of the image reflected from it surface. This 
effect arises from the extreme minuteness of the particles of 
water which compose the aqueous film ; for when they are too 
large, which sometimes happens, the reflected image is more or 
less obliterated. By a little practice, however, it is easy to 
transfer a sufficiently attenuated film to a small plate of glass. 
Let two glass reflectors A, B, Plate II. Fig. 7., be ar- 
ranged in the usual manner, so that a ray of light R is incident 
on- A, at an angle of about 56'", while the reflected ray AB is 
incident at the same angle upon B, so placed that the plane 
of reflexion from B may be at right angles to the plane of re 
flexion from A. When this adjustment is well made, scarcely a ray 
of light will be reflected from B, and the candle from which the 
ray R proceeds will be nearly invisible. Close to A, place ano- 
ther reflector of glass C, and let the ray R from the same 
candle be incident upon C at an angle of about 53°, while the 
reflected ray CD is received upon a second reflector D, (pla- 
ced near B,) at an angle of 56°, and in such a manner that the 
plane of reflection from D is at right angles to the plane 
of reflection from C» As the ray R is incident upon C at an 
