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Tutor. Yes; it will be precisely so much as if the rays had come from a radiant point X, which is the centre of the concavity of the glass.
Charles. Is that point called the focus ?
Tutor. It is called the virtual or imaginary focus. Thus the ray a, after passing , through the glass A B, will go on in the direction g h, as if it had come from the point x, and no glass been in the way: the ray b, would go on in the direction m n, and the the direction r s, and so on. The
ray in the centre suffers no refraction, but proceeds precisely as if no glass had been in
James. Suppose the lens had been con- . cave only on one side, and the other side had been flat, how would the rays have diverged?
Tutor. They would have diverged after passing through it, as if they had come from a radiant point at the distance of a whole diameter of the convexity of the lens. Charles. There is then a great similarits
in the refraction of the convex and concave lens.
Tutor. True : the focus of a double convex is at the distance of the radius of convexity, and so is the imaginary focus of the double concave; and the focus of the planoconvex is at the distance of the diameter of the convexity, and so is the imaginary focus of the plano concave.
You will find that images formed by a concave lens, or those formed by a convex lens, where the object is within its principal focus, are in the same position with the objects they represent: they are also imaginary,
for the refracted rays never meet at the foci whence they seem to diverge.
But the images of objects placed beyond the focus of a convex lens are inverted, and real, for the refracted rays do meet at their
of the Nature and Advantages of Light---...Of
the Separation of the Rays of Light by means of a Prism ------And of compounded Rays, &c.
TUTOR. We cannot contemplate the nature of light without being struck with the great advantages which we enjoy from it. Without that blessing our condition would be truly deplorable.
Charles. I well remember how feelingly Milton describes his situation after he lost his sight :
With the year Seasons return; but not to me returns
Day, or the sweet approach of ev'n or morn,
Tutor. Yet his situation was rendered comfortable by means of friends and relations, who all possessed the advantages of light. But if our world were deprived of light, what pleasure or even comfort could we enjoy. “How," says a good writer, « could we provide ourselves with food, and the other necessaries of life? How could we transact the least business? How could we correspond with each other, or be of the least reciprocal service without light, and those admirable organs of the body, which the Omnipotent Creator has
adapted to the perception of this inestima. ble benefit?”
James. But you have told us that the light would be of comparatively small advantage without an atmosphere.
Tutor. The atmosphere not only refracts the rays of the light, so that we enjoy longer days than we should without it, but occasions that twilight, which is so beneficial to our eyes ; for without it the appearance and disappearance of the sun would have been instantaneous ; and in every twenty-four hours we should have experienced a sudden transition from the brightest sun-shine to the most profound darkness, and from thick darkness to a blaze of light.
Charles. I know how painful that would be, from having slept in a very dark room, and having suddenly opened the shutters when the sun was shining extremely bright.
Tutor. The atmosphere reflects also the light in every direction, and if there were no atmosphere, the sun would benefit