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. [Illustration] 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. [Illustration]