one another, so that in neither case an image will be formed behind the lens.

James. To get an image, must the object be beyond the focus F?

Tutor. It must: and the picture will be bigger or less than the object, as its distance from the glass is greater or less than the distance of the object; if a B C (Fig. 12.) be the object, c b a will be the picture ; and if cba be the object, A B C will be the picture. Charles. Is there any rule to find the distance of the picture from the glass?

Tutor. If you know the focal distance of the glass, and the distance of the object from the glass, the rule is this:

"Multiply the distance of the focus, by the distance of the object, and divide the product by their difference, the quotient is the distance of the picture.'

James. If the focal distance of the glass be seven inches, and the object be nine inches from the lens, I say,

7x9 63

2

2

31 inches of course the pic

ture will be very much larger than the ob

ject. For, as you have said, the picture is as much bigger or, less than the object, as its distance from the glass is greater or less than the distance of the object.

Tutor. If the focus be seven inches, and the object at the distance of seventeen inches, then the distance of the picture will be found

thus

7 x 17 119

10

10

12 inches nearly.

CONVERSATION VII.

Images of Objects inverted-Of the Scioptric Ball-Of Lenses and their Foci.

JAMES. Will the image of a candle, when received through a convex lens, be inverted?

Tutor. It will, as you shall see: Here is no light in this room but from the candle, the rays of which pass through a convex lens, and by holding a sheet of paper in a proper place, you will see a complete inverted image of the candle on it.

An object seen through a very small aperture appears also inverted, but it is very imperfect compared to an image formed with the lens; it is faint for want of light,

and it is confused because the rays interfere with one another.

Charles. What is the reason of its being inverted?

Tutor. Because the rays from the extreme parts of the object must cross at the hole. If you look through a very small hole at any object, the object appears magnified. Make a pin-hole in a sheet of brown paper, and look through it at the small print of this book.

James. It is, indeed, very much magnífied. Tutor. As an object approaches a convex lens, its image departs from it; and as the object recedes, its image advances. Make the experiment with a candle and a lens, properly mounted in a long room: when you stand at one end of the room, and throw the image on the opposite wall, the image is large, but as you come nearer to the wall, the image is small, and the distance between the candle and glass is very much increased.

I will now show you an instrument, called a Scioptric Ball, which is fastened into a

.

window shutter of a room from which all light is excluded except what comes in through this glass.

Charles. Of what does this instrument consist?

Tutor. Of a frame A B (Plate 11. Fig. 13.) and a ball of wood c, in which is a glass lens ; and the ball moves easily in the frame in all directions, so that the view of any surrounding objects may be received through it.

James. Do you screw this frame into the shutter?

Tutor. Yes, a hole is cut in it for that and there are little brass screws bepurpose; longing to it, such as those markeds. When it is fixed in its place, a screen must be set at a proper distance from the lens to receive on it images of the objects out of doors. This instrument is sometimes called an artificial eye.

Charles. In what respects is it like the eye?

Tutor. The frame has been compared to the socket in which the eye moves, and

1