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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 ex treme 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. Itis, 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 VOL. III.
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.
Fames. Do you screw this frame into the shutter?
Tutor. Yes, a hole is cut in it for that purpose; and there are little brass screws belonging to it, such as those marked s. 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 ar.
Charles. In what respects is it like the
Tutor. The frame has been compared to the socket in which the eye moves, and
the wooden ball to the whole globe of the eye; the whole in the ball represents the pupil, the convex lens corresponds to the crystaline humour,* and the screen to the retina.
James. The ball by turning in all directions is very like the eye, for without moy. ing the head I can look on all sides, and upwards and downwards.
Tutor. Well, we will now place the screen properly, and turn the ball to the garden :-Here you see all the objects perfectly expressed.
James. But they are all inverted.
Tutor. That is the great defect belonging to this instrument; but I will tell you how it may be remedied: take a lookingglass and hold it before you with its face towards the picture on the screen, and inclining a little downwards, and the images will appear erect in the glass, and even brighter than they were on the screen.
* These terms will be explained hereafter.
Gharles. You have shown us in what manner the rays of light are refracted by convex lenses, when those rays are parallel. Will there not be a difference if the rays converge, or diverge before they enter the lens ?
Tutor. Certainly: if rays converge before they enter a convex lens, they will be collected at a point nearer to the lens than the focus of parallel rays. But if they diverge before they enter the lens, they will . then be collected in a point beyond the focus of parallel rays.
There are concave lenses as well as convex, and the refraction which takes place by means of these differs from hat which I have already explained.
Charles. What will the effect of refraction be, when parallel rays fall upon a dou• ble concave lens ?
Tutor. Suppose the parallel rays a, b, c, d, &c. (Plate 11. Fig. 14.) pass through the lens a B, they will diverge after they have passed through the glass.
Fames. Is there any rule for ascertaining the degree of divergency?