Let us try a simple experiment first of all. For the purpose we require two double convex lenses, one capable of magnifying more than the other, a sheet of paper and a candle. We must darken the room in which we make the experiment and, having lighted the candle, we may proceed to make a compound microscope, for that is really what we are about to do. Taking the lens which gives the greatest magnification, we look through it till we can see a clearly defined image of the lighted candle, then we fix the lens at that spot, so that, during the rest of our experiment, the candle and lens remain at the same distance from one another. Now we put the piece of paper as nearly as we can in the position of our eye, moving it nearer or further from the lens till we have a perfectly clear image of the candle thrown upon it. The first thing to strike us is that the image is upside down; it is known as a real, inverted image. Real because it can be thrown upon a screen and inverted—well because it is upside down. There are some images, as we shall learn in a moment, which can be seen but which cannot be thrown upon a screen: they are called virtual images.
Having fixed our sheet of paper in position, we take our second lens, focus it sharply upon the back of the sheet of paper, being careful to keep the centres of the two lenses as far as possible in a straight line with one another. Having obtained a sharp image we remove the paper and gradually advance our second lens towards the first. We soon reach a point where we have a very much larger image of the candle than the first lens gave us; we must fix our second lens at this point for we now have a compound microscope, a very crude one certainly and without the trimmings which make the microscope so useful. Before we proceed to explain what has happened to the light rays we must take our paper screen once more and place it as near as possible to the spot where our eye was situated when we saw the second image. We shall find that, however much we may move our screen to or from the second lens we can never manage to obtain an image upon it for the reason that this second image is virtual, but unlike the first image it is not inverted.
One or two diagrams will help to explain our experiment and, instead of the lighted candle, we will suppose that our object is an arrow—it is easier to draw and serves just as well. The magnification of the object by our first lens may be represented by the diagram below, where AA is the lens, CD the object and D′C′ its image.
The arrow C′D′ shows the point at which we placed our screen, and as our diagram shows, the image is magnified and inverted.
Our second lens, we remember, was focussed on the back of the paper, placed at C′D′; for practical purposes we may ignore the thickness of the paper and say that it was focussed on the image C′D′. Had we left it at that, the further course of the rays through the second lens would be represented by a replica of the diagram we have just given. But, in our experiment, we moved the second lens nearer and nearer to C′D′ till we obtained a clear much magnified erect image of C′D′, let us call this second image C″D″, and represent the course of the light rays by a diagram.
We may well ask, why did the lens AA, our first lens, form a real image whilst the second lens BB, which is precisely similar to AA, except that its magnifying power is not so great, form a virtual image? The formation of a real or a virtual image is nothing to do with magnification, so we repeat—why do two similar lenses form different kinds of images? Let us refresh our memories with the remarks concerning the principal focus of lenses in the [last chapter], then we may try another experiment. The principle focus of a double convex lens, we remember, is the point to which parallel rays of light converge, after passing through the lens. If now our object is further away from the lens than its principal focus, a state of affairs that existed in the case of our lens AA and the object CD, we obtain a magnified, real but inverted image; if, on the other hand, using the same lens if we wish, the object is nearer to the lens than its principal focus, we obtain a magnified virtual and erect image. The form of image then depends on the relative positions of lens and object and not on the magnifying powers of the former.
After this digression, we will see what happens when we combine the diagram showing the real, inverted image, formed by the lens AA with the virtual erect image, formed by the lens BB. In reality we will draw a diagram showing the path of the light rays through our compound microscope.
