16 On Light. 
relates chiefly to some curious properties of agate. The plate of 
agate which he employed, was bounded by parallel faces, was 
about the fifteenth of an inch thick, and was cut intoa plane, per- 
pendicular to the laminz of which it was composed. When the 
image of a taper reflected from water at an angle of 52° 45’, so as 
to acquire the property discovered by Malus, was viewed through 
the plate of agate, so as to have its lamine parallel to the plane 
of reflection, the flame appeared perfeetly distinct; but when the 
agate was turned round, so that its laminz became perpendicular 
to the plane of reflection, the light which formed the image of 
the taper suffered total reflection, and not one ray of it penetrated 
the agate. Ifa ray of light incident upon one plate of agate is 
received, after transmission, upon another plate of the same sub- 
stance, having its lamin parallel to those of the former, the light 
will find an easy passage through the second plate; but if the 
second plate has its lamine perpendicular to those of the first, 
the light will be wholly reflected, and the luminous object will 
cease to be visible. 
In a second important communication in 1814, on the affec- 
tions of light transmitted through crystallized bodies, after sug- 
gesting that the cultivation of this department of physics may en- 
able us to explain the forms and structure of crystallized bodies, 
a prediction which he himself has since happily fulfilled, the 
Doctor states, that if the light polarized by agate, is incident at 
a particular angle upon any transparent body, so that the plane 
of reflection is perpendicular to the laminz of the agate, it will 
experience a total refraction; if it is transmitted through another 
plate of agate, having its lamine at right angles to those of the 
plate by which the light is polarized, it will suffer total reflection ; 
and if it is examined by a prism of Iceland crystal, turned round 
in the hand of the observer, it will vanish and reappear in every 
quadrant of its circular motion. The pencil of rays to which this 
remarkable property is communicated, is surrounded by a large 
mass of nebulous light, which extends about 7° 30’ in length, and 
1° 7’ in breadth, on each side of the brightimage. This nebulous 
light never vanished with the bright image which is inclosed, 
but was obviously affected with its different changes, increasing 
in magnitude as the bright image diminished, add diminishing 
as the bright image regained its lustre. Light polarized by the 
agate, or by any other means, is depolarized, or partly restored 
to its original state, by being transmitted in a particular direction 
through a plate of mica, or any other crystallized body. 
IV. Of the Production of Light. 
al . . . ’ . 
Some philosophers refer the origin of all luminous phenomena 
to 
