s8 6 Dr. Brewster on the multiplication of images, and the 
image A b, Fig. 2, across which the fringes are visible, is 
formed by two equal and oppositely polarised pencils, it is 
necessary to the production of the fringes, that one of these 
pencils be either extinguished, or greatly diminished in its 
intensity. Now, if we cover BF, and examine with a prism 
of calcareous spar the pencil VX formed by the rays RS, we 
shall actually find that one of the images, b for example, 
Fig. 2, is very much fainter than the other, and therefore the 
fringes must be formed of the polarised light of A, being in 
this case very faint, owing to the admixture of the remaining 
light of b . If, on the contrary we cover EB, and examine the 
pencil XV formed by the rays rs, we shall find that the 
image b, Fig. 2, is almost wholly extinguished, and conse- 
quently the fringes formed by the polarised light of A are 
remarkably distinct, suffering no diminution of lustre from 
the admixture of oppositely polarised rays. 
If the rays RS, rs, (Fig. 3.) are now polarised before their in- 
cidence upon the rhomboid, the fringes formed by rs, do not 
experience any change, in consequence of their being produced 
by unmixed polarised light; but the fringes formed by RS 
suffer a particular modification. When the plane of incidence 
EBFA forms an angle of 45 0 with the plane of polarisation, 
the fringes are extremely distinct and beautiful, and the same 
thing happens when the rhomboid is turned round 180°. In 
positions at right angles to these, no fringes whatever are 
visible. In the first of these positions, the pencil is not divided 
into two oppositely polarised pencils, whereas in the other 
position, it is divided into two oppositely polarised pencils of 
equal intensity. 
If instead of polarising the incident rays RS, rs, we examine 
