The History and Practice of the Art of Photography [8]
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 density--but 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 primitives--and from their interblending, seven, as recorded by Newton, and shown in the accompanying wood cut. These rays being absorbed, or
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 density--but 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 primitives--and from their interblending, seven, as recorded by Newton, and shown in the accompanying wood cut. These rays being absorbed, or