210 
Dr. Brewster on the Affections of Light 
body, so that the plane of reflection is parallel to the plane of 
reflection from the topaz, the fourth set of rings will be visible. 
Hitherto we have attended only to the light reflected from 
ah, the posterior surface of the topaz ; but the light transmitted 
at C exhibits also interesting phenomena. When the observer 
looks through the topaz in the direction FC, so as to see the 
polarising surface GH, the second set of rings is faintly visible. 
They become extremely distinct, however, when viewed 
through a plate of agate having its lamince at right angles to 
the plane AB^6. If the laminae are parallel to the plane ABahy 
the second set is converted into \\\q first set with colours a little 
paler than when it was produced in the former experiments. 
In the preceding experiments the plane of reflection from 
GH has been perpendicular to the plane of reflection from the 
topaz. . We shall now describe the phenomena which take 
place when these planes are parallel to each other, an ar- 
rangement which is represented in Plate V., fig. lo. 
When the observer looks into the topaz in the direction rr\ 
he will perceive the second set of fringes. If the rays rr' are 
transmitted through a plate of agate having its lamiti^ per- 
pendicular to the plane of reflection, xhe fourth set of fringes 
will be seen, but they are very much fainter than they ap- 
peared in the former experiments. When the laminae of 
the agate are parallel to the plane of reflection, the second set 
is faintly visible. The central spots are, however, rather 
larger than before, so that this set has the appearance of 
being the reverse of the first rather than of the third set. 
When the light transmitted in the direction CF is seen by 
the naked eye, it exhibits the first set of rings. If it is examined 
through a plate of agate having its veins perpendicular to the 
