608 SIR DAVID BREWSTER ON CIRCULAR CRYSTALS. 
and perpendicular to the plane of primitive polarisation, and the disc of bright 
light disappeared at their point of intersection. In the opposite position of the 
analyser, the luminous disc appeared at the point of intersection, and the two 
luminous sectors that were horizontal were brighter than the two vertical ones. 
These phenomena are shewn in Figs. 1 and 2, Plate XVI. 
In some cases, the circular space enclosing the sectors was very small, and in 
others large, and frequently when two halos were produced, there were two sets 
of luminous sectors, separated by an interval equal to that between the halos. 
It is very obvious that the halos were produced by the crystals of the Oil of 
Mace, the smaller halo by the larger crystals, and the larger halo by the smaller 
crystals existing among the larger ones. In order to explain the luminous sec- 
tors, I inferred that each halo was composed of two, the one lying above the other, 
and having every alternate sector polarised in opposite planes; or, in other words, 
that these two halos were the two images formed by the double refraction of the 
elementary crystals, and were oppositely polarised, as all such images are. But 
though this inference was correct, as I afterwards proved, yet I could not see with 
the microscope the actual form of the circular crystals by which the double re- 
fraction and polarisation were produced. 
After the publication of Mr Tatzot’s paper, I repeated the experiment with 
oil of mace, and having adopted different methods of cooling it under pressure, I 
soon discovered with the microscope, and by the aid of polarising films when the 
microscope could not alone detect the structure, that the phenomena which I have 
described were produced by circular crystals varying from invisibility to the 200th 
or 300th of an inch in diameter, and exhibiting, when of this size, distinct and 
beautiful sectors in polarised light. 
Having thus discovered a method* of distinguishing true quaquaversus po- 
larisation, or that which is produced by invisible crystalline particles with their 
axes lying in all directions, from that apparent quaquaversus polarisation which 
is produced by the same class of particles combined in circular crystals, I was 
anxious to prosecute the subject of circular crystallisation, by examining a great 
number of doubly-refracting substances. 
With this view I received from Mr Tarzor the preparation of Borax and Phos- 
phoric acid which he had found to give the best circular crystals, and from Dr 
Dow er of Richmond a quantity of the Lithovanthate of Ammonia, which yields 
circular crystals with more certainty and less trouble than the preparation of 
* This method consists in placing a film of selenite (sulphate of lime) between the polariser 
and the substance to be examined. If the polarising structure is produced by cireular crystals, it 
will appear covered with spots, or minute sectors, of two different colours, the one being a tint a 
little lower, and the other a tint a little higher, than that of the selenite. The higher tint is the sum 
of the tints of the two substances, and the lower their difference, the tint of the selenite being in- 
creased by that of two of the sectors, and diminished by that of the other two. 
