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he second and third boards； because the same quantity of light is diffused over a space four and sixteen times greater。 These same rays may be collected and their intensity again increased。
Rays of light are reflected from one surface to another； Refracted； or bent； as they pass from the surface of one transparent medium to another； and Inflected； or turned from their course； by the attraction of opaque bodies。 From the first we derive the principles on which mirrors are constructed； to the second we are indebted for the power of the lenses； and the blessings of sight；for the light acts upon the retina of the eye in the same manner as on the lens of a camera。 The latter has no important bearing upon our subject。
When a ray of light falls perpendicularly upon an opaque body； it is reflected bark in the same line in which it proceeds； in this case the reflected ray returns in the same path the incident ray traversed； but when a ray falls obliquely； it is reflected obliquely； that is； it is thrown off in opposite direction； and as far from the perpendicular as was the incident ray； as shown at Fig。 2； a representing the incident ray and b the reflected。 The point； or angle c made by ＇hipho_2。gif＇ the incident ray； at the surface of the reflector e f； with a line c d； perpendicular to that surface； is called the angle of incidence； while the angle formed by the reflected ray b and the perpendicular line d is called the angle of reflection； and these angles are always equal。
It is by this reflection of light that objects are made visible； but unless light falls directly upon the eye they are invisible； and are not sensibly felt until after a certain series of operations upon the various coverings and humors of the eye。 Smooth and polished surfaces reflect light most powerfully； and send to the eye the images of the objects from which the light proceeded before reflection。 Glass； which is transparent transmitting lightwould be of no use to us as a mirror； were it not first coated on one side with a metalic amalgam； which interrupts the rays in their passage from the glass into the air； and throws them either directly in the incident line； or in an oblique direction。 The reason why trees； rocks and animals are not all mirrors； reflecting other forms instead of their own； is； that their surfaces are uneven； and rays of light reflected from an uneven surface are diffused in all directions。
Parallel rays falling obliquely upon a plane mirror are reflected parallel； converging rays； with the same degree of convergence； and diverging rays equally divergent。
Stand before a mirror and your image is formed therein； and appears to be as far behind the glass as you are before it； making the angle of reflection equal to that of incidence； as before stated。 The incident ray and the reflected ray form； together； what is called the passage of reflection； and this will therefore make the actual distance of an image to appear as far again from the eye as it really is。 Any object which reflects light is called a radiant。 The point behind a reflecting surface； from which they appear to diverge； is called the virtual focus。
Rays of light being reflected at the same angle at which they fall upon a mirror； two persons can stand in such a position that each can see the image of the other without seeing his own。 Again； you may see your whole figure in a mirror half your length； but if you stand before one a few inches shorter the whole cannot be reflected； as the incident ray which passes from your feet into the mirror in the former case； will in the latter fall under it。 Images are always reversed in mirrors。
Convex mirrors reflect light from a rounded surface and disperse the rays in every direction； causing parallel rays to diverge； diverging rays to diverge more； and converging rays to converge less They represent objects smaller than they really arebecause the angle formed by the reflected ray is rendered more acute by a convex than by a plane surface； and it is the diminishing of the visual angle； by causing rays of light to be farther extended before they meet in a point； which produces the image of convex mirrors。 The greater the convexity of a mirror； the more will the images of the objects be diminished； and the nearer will they appear to the surface。 These mirrors furnish science with many curious and pleasing facts。
Concave mirrors are the reverse of convex； the latter being rounded outwards； the former hollowed inwardsthey render rays of light more converging collect rays instead of dispersing them； and magnify objects while the convex diminishes them。
Rays of light may be collected in the focus of a mirror to such intensity as to melt metals。 The ordinary burning glass is an illustration of this fact； although the rays of light are refracted； or passed through the glass and concentrated into a focus beneath。
When incident rays are parallel； the reflected rays converge to a focus； but when the incident rays proceed from a focus； or are divergent； they are reflected parallel。 It is only when an object is nearer to a concave mirror than its centre of concavity； that its image is magnified； for when the object is farther from the mirror； this centre will appear less than the object； and in an inverted position。
The centre of concavity in a concave mirror； is an imaginary point placed in the centre of a circle formed by continuing the boundary of the concavity of the mirror from any one point of the edge to another parallel to and beneath it。
REFRACTION OF LIGHT：I now pass to the consideration of the passage of light through bodies。
A ray of light failing perpendicularly through the air upon a surface of glass or water passes on in a straight line through the body； but if it； in passing from one medium to another of different density； fall obliquely； it is bent from its direct course and recedes from it； either towards the right or left； and this bending is called refraction； （see fig。 3； b。） If a ray of light passes from a rarer into a denser medium it is refracted towards a perpendicular in that medium； but if it passes from a denser into rarer it is bent further from a perpendicular in that medium。 Owing to this bending of 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 densitybut the chemical constitution of bodies as well as their density； is found to effect their refracting power。 Inflamable 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 Steller 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； and some rays being more refracted than others； we procure an elongated image of the luminous beam； exhibiting three distinct colors； red； yellow and blue； which are to be regarded as primitivesand from their interblending； seven； as recorded by Newton； and shown in the accompanying wood cut。 These rays being absorbed； or reflected differently by various bodies； give to nature the charm of color。 Thus to the eve is given the pleasure we derive in looking upon the green fields and forests； the enumerable varieties of flowers； the glowing ruby； jasper； topaz； amethist； and emerald