base of which is the pupil of the eye, and its height is the distance from us to the sun.

James. But the breadth of the eye is nothing when compared to a line ninety-five millions of miles long.

Tutor. And for that reason, the various rays that proceed from a single point in the sun are considered as parallel, because their inclination to each other is insensible. The same may be said of any other point as c. Now all the rays that we can admit by means of a small aperture or hole, must proceed from an indefinitely small point of the sun, and therefore they are justly con-/ sidered as parallel.

If now we take a ray from the point a, and another from c, on opposite points of the sun's disk, they will form a sensible angle at the eye; and it is from this angle a Ec that we judge of the apparent size of the sun, which is about half a degree in di

C

ameter.

Charles. Will the size of the pupil of the eye make any difference with regard to the appearance of the object?

Tutor. The larger the pupil, the brighter will the object appear, because the larger the pupil is, the greater number of rays it will receive from any single point of the object. And I wish to remember what I have told you before, that whenever the appearance of a given object is rendered larger and brighter, we always imagine that the object is nearer to us than it really is, or than it appears at other times.

James. If there be nothing to receive the rays (Fig. 8.) at f, would they cross one another and diverge?

Tutor. Certainly, in the same manner as they converged in coming to it; and if another glass F G, of the same convexity as D E, be placed in the rays at the same distance from the focus, it will so refract them, that, after going out of it, they will be parallel, and so proceed on in the same manner as they came to the first glass.

Charles. There is, however, this difference; all the rays, except the middle one, have changed sides.

Tutor. You are right, the ray B, which entered at bottom, goes out at the top b; and a, which entered at the top, goes out at the bottom c, and so of the rest.

If a candle be placed at f, the focus of the convex glass, the diverging rays in the space Ff G, will be so refracted by the glass, that after going out of it, they will become parallel again.

James. What will be the effect if the candle be nearer to the glass then the point ƒ?

Tutor. In that case, as if the candle be at g, (Plate 11. Fig. 10.) the rays will di verge after they have passed through the glass, and the divergency will be greater or less in proportion as the candle is more or less distant from the focus.

Charles. If the candle be placed farther from the lens than the focus f, will the rays meet in a point after they have passed through it?

Tutor. They will: thus if the candle be placed at g, (Plate 11. Fig. 11.) the rays, after passing the lens, will meet at x; and this point x will be more or less distant from

the glass, as the candle is nearer to, or farther from its focus. Where the rays meet, they form an inverted image of the flame of the candle.

James. Why so?

Tutor. Because that is the point where the rays, if they are not stopped, cross each other to satisfy you on this head, I will hold in that point a sheet of paper, and you now see that the flame of the candle is inverted.

Fames. How is this explained?

Tutor. Let A B C (Plate 11. Fig. 12.) represent an arrow placed beyond the focus , of a double convex lens d e f, some rays will flow from every part of the arrow, and fall on the lens; but we shall consider only those which flow from the points A, B, and C. The rays which come from A, as a d, A e, and af, will be refracted by the lens, and meet in A. Those which come from B, as в d, B e, and в f, will unite in b, and those which come from c, will unite in c.

Charles. I see clearly how the rays from Bare refracted, and unite in 6; but it is not

so evident with regard to those from the extremities A and c.

Tutor. I admit it; but you must remember the difficulty consists in this, the rays fall more obliquely on the glass from those points than from the middle, and therefore the refraction is very different. The ray B F in the centre suffers no refraction, в d'is refracted into b; and if another ray went from N, as N d, it would be refracted to n, somewhere between b and a, and the rays from A must, for the same reason, be refract ed to a.

James. If the subject A B C is brought nearer to the glass, will the picture be removed to a greater distance?

Tutor. It will: for then the rays will fall more diverging upon the glass, and cannot be so soon collected into the corresponding points behind it.

Charles. From what you have said, I see that if the object A B C be placed in F, the rays, after refraction, will go out paral lel to one another; and if brought nearer to the glass than F, then they will diverge from