127 
Dr. Wollaston on the methods of, &c. 
of his contrivance,* that he fully succeeded in accomplishing 
this end. But although he informs us that the means em- 
ployed, as best suited to his views, had exactly the effect of 
doubling the amount of deviation produced by ordinary means, 
he has not chosen to explain the mode of construction he 
adopted, and has merely referred to a certain artist living at 
that time in Paris, who was in possession of his secret, and 
skilful in applying it to the construction of micrometers. 
As I have reason to think that the method to which he 
alludes in his memoir has never yet been described, I design, 
in the present communication, to explain a combination which 
I have found advantageous, and which I think must be the 
same as that of M. Rochon. 
I shall hope to render the principles of this construction 
intelligible to every one acquainted with the original obser- 
vation of Huygens on the properties of polarised light, and 
to enable any competent artist to cut wedges from hexagonal 
prisms of rock crystal, in the positions requisite to produce, 
by their combination, the double effect to which I allude. 
There are three principal directions in which a crystal may 
be cut specifically different from each other, which require to 
be distinctly understood. 
In the first place, let us suppose a prismatic crystal to be 
placed with its axis in a vertical position, and a portion to be 
cut off from the base by a plane surface at right angles to the 
axis, and sufficient to form a wedge of 20 degrees, by giving 
it a second surface duly inclined to the former. For distinc- 
tion, this may be called the horizontal wedge. 
Next, let the crystal be bisected vertically by a plane 
* Journal de Physique, An. 9. 
