the rays of
light the angles of refraction and incidence are never equal.

Transparent bodies differ in their power of bending light--as a general
rule, the refractive power is proportioned to the density--but the
chemical constitution of bodies as well as their density, is found to
effect their refracting power. Inflammable bodies possess this power
to a great degree.

The sines of the angle of incidence and refraction (that is, the
perpendicular drawn from the extremity of an arc to the diameter of a
circle,) are always in the same ratio; viz: from air into water, the
sine of the angle of refraction is nearly as four to three, whatever be
the position of the ray with respect to the refracting surface. From
air into sulphur, the sine of the angle of refraction is as two to
one--therefore the rays of light cannot be refracted whenever the sine
of the angle of refraction becomes equal to the radius* of a circle,
and light falling very obliquely upon a transparent medium ceases to be
refracted; this is termed total reflection.

* The RADIUS of a circle is a straight line passing from the centre to
the circumference.

Since the brightness of a reflected image depends upon the quantity of
light, it is quite evident that those images which arise from total
reflection are by far the most vivid, as in ordinary cases of
reflection a portion of light is absorbed.

I should be pleased to enter more fully into this branch of the science
of optics, but the bounds to which I am necessarily limited in a work
of this kind will not admit of it. In the next chapter, however, I
shall give a synopsis of Mr. Hunt's treatise on the "Influence of the
Solar Rays on Compound Bodies, with especial reference to their
Photographic application"--a work which should be in the hands of every
Daguerreotypist, and which I hope soon to see republished in this
country. I will conclude this chapter with a brief statement of the
principles upon which the Photographic art is founded.

SOLAR and Stellar light contains three kinds of rays, viz:

1. Colorific, or rays of color.

2. Calorific, or rays of heat.

3. Chemical rays, or those which produce chemical effects.

On the first and third the Photographic principle depends. In
explaining this principle the accompanying wood cuts, (figs. 3 and 4)
will render it more intelligible.

If a pencil of the sun's rays fall upon a prism, it is bent in passing
through the transparent medium; an