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8 _ Improvements in Physical Science (JAN. 
4. Dr. Brewster ascertained by a great number of observations 
that the index of refraction is the tangent of polarization. In the 
memoir in which he establishes this law he gives a full detail of the 
laws of the depolarization of light by reflection from the first sur- 
faces of transparent bodies ; but I am unable to give an account of 
this part of his paper without entering into details inconsistent with 
the nature of this sketch. 
5. Some specimens of calcareous spar have the property of multi- 
plying images, and of exhibiting a beautiful set of complementary 
. colours. This was first observed by the late Professor Robison, of 
Edinburgh, who communicated the fact to Mr. Benjamin Martin. 
These specimens were examined in succession by Martin, Brougham, 
and Malus, and they were generally ascribed to fissures in the 
crystals. Dr. Brewster has shown that fissures are inadequate to 
produce the effect, that it is owing to a fissure filled up with erys- 
tallized calcareous matter, and he has succeeded in imitating these 
specimens by cementing a thin film of sulphate of lime between 
two prisms of calcareous spar. The colours are produced by the 
transmission of polarized light through the crystallized film. 
6. When a luminous body is viewed through two parallel plates 
of equal thickness, placed at the distance’of about the tenth of an 
inch from each other, if one of the glasses be inclined a little, till 
the reflected image of the luminous body is distinctly separated from 
the bright image formed by transmitted light, and is received upon 
the eye placed behind the plates: under these circumstances the 
reflected image is crossed with about fifteen beautiful parallel 
fringes, The three central fringes consist of blackish and whitish 
siripes; and the exterior ones, of brilliant red and green light. 
These fringes are formed by the joint action of the four reflecting 
surfaces of the glass, for they are destroyed when the action of any 
of these surfaces is prevented by coating it with Canada balsam. 
The direction of these fringes is always parallel to the common 
section of the four reflecting surfaces which exercise an action 
upon the incident light. Their breadth is inversely as the inclina- 
tion of the plates.. Their magnitudes are inversely as the thick- 
nesses of the plates which produce them at a given inclination ; and 
in general the magnitude of the fringes is in the compound inverse 
ratio-of the thickness of the plates, and of their angle of inclina- 
tion. Dr. Brewsicr conceives that these fringes may be explained 
by Newton’s Theory of Fits of easy Reflection and Transmission of 
Light. 
A curious set of observations on the colours exhibited by thin 
plates of glass placed upon each other, or by a convex lens placed 
upon a plane glass surface, by Mr. Knox, has been published in the 
last half volume of the Philosophical Transactions. He merely 
describes the phenomena which he observed, without attempting to 
explain them. They consist of certain sets of coloured fringes. 
They were tangents to the primary coloured rings of Sir Isaac 
Newton; or, when two sets of primary colours were produced, 
