 =De. 19, 1872] NATURE 127 
If, then, a ray of light passes through a piece of glass, | when the beam passes through that the refractive effect is 
such, for instance, as a window glass, the surfaces of | imperceptible. The reason of this is, that when we get 
the light falling on the glass from the air, then travelling 
which are parallel, and inclined to the beam, you see 
nn 
Fic. 8,—Images of objects seen through prisms. 
Unequalfrefrangibility{of the colours of the spectrum 
Fig. 9.—Decomposition of light by the prism. 
pretty much the same effect as at first. But now, if 
instead of having the glass bounded by parallel surfaces, 
through the glass‘and coming into the air again, under 
exactly the same conditions, what is done at the first 
we use a wedge-shaped piece, or a fresm, the sides of 
surface is exactly undone at the second, so that we get 
