282 Dr. Brewster on the multiplication of images, and the 
a plate of glass GH, Fig. 6. upon the second surface CD, by 
means of a transparent cement EF. The rays S a, S b have 
now emerged completely from the calcareous spar, and being 
reflected from the glass plate GH, they again enter the cry- 
stal, and are subdivided as formerly at the line mn, into the 
four pencils ce, cf, dg, dh* 
In a specimen of calcareous spar examined by Mr. Martin, 
twelve images were seen, arranged in three rows. The 
middle row, consisting of six, was produced by two interrupt- 
ing planes situated in the manner shown in Fig. 7 , while the 
other two rows were formed by reflection from the sides of 
the rhomboid. Mr. Brougham examined a specimen which 
afforded six images in some positions, besides other two, 
which, as this able writer justiy remarks, were reflected from 
the sides of the specimen. 
. Sect. IV. On the cause of the colours with which the images are 
affected. 
As there are some specimens of calcareous spar in which the 
multiplication of the images is not accompanied with the pro- 
duction of colours, the one phenomenon is not necessarily con- 
nected with the other, the multiplication of the images depend- 
ing merely on the interruption in the regular structure of the 
mineral, and the colours upon the thickness and crystalline 
nature of the vein by which that interruption is produced. 
We have already seen that the double image A b (Fig. 2.) 
is in general white, while a, and B, are affected with the same 
prismatic colour, and that when A h is coloured at particular 
angles of incidence, its colour is always complementary to 
* Malus believed that the fact of the subdivision of the reflected pencils was general ; 
but tnere is obviously a particular angle of reflection at which four pencils are not 
formed. 
