The point of union of the rays of light, formed by means of a glass lens, &c. is called the FOCUS.
Now, as each ray carries with it the image of the object from whence it proceeded, it follows, that if those rays, after intersecting each other, and having formed an image at their intersection, are again united by a refraction or reflection, they will form a new image, and that repeatedly, as long as their order is not confounded or disturbed.
It follows also, that when the progress of the luminous ray is under consideration, we may look on the image as the object, and the object as the image; and consider the second image as if it had been produced by the first as an object, and so on.
In order to gain a clear idea of the wonderful effects produced by glasses, we must proceed to say something of the principles of refraction.
Any body, which is so constituted as to yield a passage to the rays of light, is called a MEDIUM. Air, water, glass, &c. are mediums of light. If any medium afford an easy passage to the rays of light, it is called a RARE MEDIUM; but if it do not afford an easy passage to these rays, it is called a DENSE MEDIUM.
Let Z, Fig. 2. [Plate I.] be a rare medium, and Y a dense one; and let them be separated by the plane surface G H. Let I K be a perpendicular to it, and cutting it in C.
With the center C, and any distance, let a circle be described. Then let A C be a ray of light, falling upon the dense medium. This ray, if nothing prevented, would go forward to L; but because the medium Y is supposed to be denser than Z, it will be bent downward toward the perpendicular I K, and describe the line C B.
The ray A C is called the INCIDENT RAY; and the ray C B, the REFRACTED RAY.
The angle A C I is called the ANGLE OF INCIDENCE, and the angle B C K is called the ANGLE OF REFRACTION.
If from the point A upon the right line C I, there be let fall the perpendicular A D, that line is called the sine of the angle of incidence.