280 Dr. Brewster on the multiplication of images , and the 
of the second prism, and that the great angular distance of 
the images, which takes place even when the prisms are con- 
nected by a cement in perfect contact with each, is occasioned 
by the action of the doubly refracting force near the second 
surface of the first prism. 
As all the phenomena of the natural crystal may be imitated 
by an artificial one, there can be no doubt that such changes 
actually take place within the crystal. It is interesting, how- 
ever, to ascertain the principles on which these changes de- 
pend ; and we are fortunately able not only to do this, but 
to apply the principles to the explanation of other phenomena 
exhibited by doubly refracting crystals. 
It may be shown by various experiments, that the division 
of a beam of light into two pencils by double refraction, does 
not take place till the light has penetrated the first surface 
of the crystal, and suffered the ordinary refraction, while at 
the second surface the extraordinary refraction takes place 
before the emergence of the ray. The interposition, there- 
fore, of a film AB of the same refractive power as the crystal, 
though it prevents the ray from suffering any ordinary re- 
fraction, still allows the extraordinary refraction to take 
place just as if the prisms were completely separated. For 
the same reason, the extraordinary refraction again takes 
place at the first surface of the second prism, and the two 
pencils are divided into four, as represented in Fig. 2. 
Since the prisms ABE, ABF, Fig. 4, or the rhomboids which 
they contain, have their homologous sides parallel, the pencils 
Sa, Sb ought not to be divided into two by the second prism 
according to the observations of Huygens and Newton. 
