Part 2 (1/2)
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 gla.s.s 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 pa.s.sage 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 pa.s.ses 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 are--because the angle formed by the reflected ray is rendered more acute by a convex than by a plane surface, and it is the diminis.h.i.+ng 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 inwards--they 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 gla.s.s is an ill.u.s.tration of this fact; although the rays of light are refracted, or pa.s.sed through the gla.s.s 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 pa.s.s to the consideration of the pa.s.sage of light through bodies.
A ray of light failing perpendicularly through the air upon a surface of gla.s.s or water pa.s.ses on in a straight line through the body; but if it, in pa.s.sing 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 pa.s.ses from a rarer into a denser medium it is refracted towards a perpendicular in that medium; but if it pa.s.ses 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 density--but the chemical const.i.tution 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 pa.s.sing from the centre to the circ.u.mference.
Since the brightness of a reflected image depends upon the quant.i.ty 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 pa.s.sing 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 primitives--and 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, the brilliant diamond, and all the rich and varied hues of nature, both animate and inanimate.
[Ill.u.s.tration: Fig. 3 (hipho_3.gif)]
Now, if we allow this prismatic spectrum (b. Fig. 3.) to fall upon any surface (as at c.) prepared with a sensitive photographic compound, we shall find that the chemical effect produced bears no relation to the intensity of the light of any particular colored ray, but that, on the contrary, it is dispersed over the largest portion of the spectrum, being most energetic in the least luminous rays, and ever active over an extensive s.p.a.ce, where no traces of light can be detected. Fig. 4, will give the student a better idea of this principle. It is a copy of the kind of impression which the spectrum, spoken of, would make on a piece of paper covered with a very sensitive photographic preparation.
The white s.p.a.ce a. corresponds with the most luminous, or yellow ray, (5, Fig. 3) over limits of which all chemical change is prevented. A similar action is also produced by the lower end of the red ray c; but in the upper portion, however we find a decided change (as at d). The most active chemical change, you will perceive, is produced by the rays above the yellow a; viz. 4, 3, 2 and 1 (as at b) the green (4) being the least active, and the blue (3) and violet (1) rays the most so, the action still continuing far beyond the point b which is the end of the luminous image.
[Ill.u.s.tration: Fig. 4 (hipho_4.gif)]
Suppose we wish to copy by the Daguerreotype, or Calotype process, any objects highly colored--blue, red and yellow, for instance predominating--the last of course reflects the most light, the blue the least; but the rays from the blue surface will make the most intense impression, whilst the red radiations are working very slowly, and the yellow remains entirely inactive. This accounts for the difficulty experienced in copying bright green foliage, or warmly colored portraits; a large portion of the yellow and red rays entering into the composition of both--and the imperfections of a Daguerreotype portrait of a person with a freckled face depends upon the same cause.
A yellow, hazy atmosphere, even when the light is very bright, will effectually prevent any good photographic result--and in the height of summer, with the most sensative process, it not unfrequently happens that the most annoying failures arise from this agency of a yellow medium. A building painted of a yellow color, which may reflect the sun's rays directly into the operator's room will have the same effect.