We have used the same lettering as in our previous diagrams and we see that, also as before, a real, inverted image C′D′ of the object CD is formed by the lens AA and a virtual, erect image C″D″ of the image C′D′ is formed by the lens BB, the object CD being further from the lens AA than its principal focus and the image C′D′ being nearer to the lens BB than its principal focus. One very important point we must notice before we leave the diagram. We have mentioned several times that the image formed by BB is erect and so it is, but it is an erect image of an already inverted image, so that the final image of CD, as seen by the eye E is inverted. The fact that objects viewed through the microscope appear upside down is puzzling at first. To all intents our two double convex lenses represent a compound microscope; actually, they should be fixed at either end of a tube, blackened on the inside. The lens AA, nearest to the object, would then be known as the objective and the lens BB nearest to the observer’s eye would be known as the ocular or, more commonly the eyepiece. There are, of course, very many refinements, designed to make the instrument capable of performing the most accurate work, and needless to say these simple lenses would neither give very great magnification nor any clear images. Let us describe a more refined compound microscope than the one we constructed in our darkened room. The optical parts, that is to say the lenses, are the most important parts of every microscope, upon their qualities depend the degree of efficiency of the instrument; the metal portions, known collectively as the stand, contribute to the easier, smoother working of the microscope. [Illustration: _By the courtesy of Messrs. F. Davidson & Co._ THE HEAD OF A DOG FLEA No wonder the flea is an annoying creature. As the plate shows, it is armed with knives, lances and saws, all designed to injure the skin of its victim. ]