284 Dr. Brewster on the multiplication of images , and the 
When the position of the sulphate of lime is changed, the 
depolarisation is increased, and the double image Ah is no 
longer white, but always displays the colours complementary 
to those of a and B. In particular positions of the sulphate, 
the middle images become white at an oblique incidence. 
In the specimen of calcareous spar represented in Fig. 7, the 
colours are by no means brilliant, and they appear only when 
the incident ray falls obliquely upon the rhomboid, with an 
inclination towards the base of the first prism. The reason of 
this will appear from Fig. 4. When the ray is incident ob- 
liquely towards the base of the first prism, as rS, it is refracted 
in the direction S m, and passes through the interrupting stra- 
tum AB nearly at its least thickness; whereas when it is inci- 
dent obliquely in the direction ^ S, it is refracted into the line 
Sw, and passes obliquely through the stratum at a thickness too 
great to produce the complementary colours. We are pre- 
sented therefore with a method of determining rudely, the 
comparative thickness of the strata by which the colours are 
produced. In two very fine specimens, the colours are exhi- 
bited at almost every inclination, and they vanish when the 
inclination is near its maximum, and when the ray passes 
obliquely through the stratum. 
Hence it follows, that the colours are produced by the 
transmission of polarised light through a crystallized vein, 
and that the phenomena change their character with the 
thickness of the vein. 
We have already seen that the images may be multiplied 
without being coloured, but they cannot be coloured without 
being multiplied, as the separation of the oppositely polarised 
pencils is necessary to the production of the colours. This is 
