Student's microscope.
ep, Eye-piece. o, g, Object-glass.
Skeleton of a microscope, showing how an object is magnified.
o, l, Object-lens. e, g, Eye-glass. s, s, Spicule. s´, s´, Magnified image of same in the tube. S, S, Image again enlarged by the lens of the eye-piece.
"But this is only our first step. Those diatoms we spoke of just now will only look like minute specks under even the strongest magnifying-glass. So we pass on to use two extra lenses to assist our eyes, and come to this compound microscope (Fig. 14) through which I have before now shown you the delicate markings on shells which were themselves so minute that you could not see them with the naked eye. Now we have to discover how the microscope performs this feat. Going back again for a minute to our candle and magnifying-glass (Fig. 12), you will find that the nearer you put the lens to the candle the farther away you will have to put the paper to get a clear image. When in a microscope we put a powerful lens o, l close down to a very minute object, say a spicule of a flint sponge s, s, quite invisible to the unaided eye, the rays from this spicule are brought to a focus a long way behind it at s´, s´, making an enlarged image because the lines of light have been diverging ever since they crossed in the lens. If you could put a piece of paper at s´ s´, as you did in the candle experiment, you would see the actual image of the magnified spicule upon it. But as these points of light are only in an empty tube, they pass on, spreading out again from the image, as they did before from the spicule. Then another convex lens or eye-glass e, g is put at the top of the microscope at the proper distance to bend these rays so that they enter our eye in nearly parallel lines, exactly as we saw in the ordinary magnifying-glass (Fig. 13), and our crystalline lens can then bring them to a focus on our retina.
"By this time the spicule has been twice magnified; or, in other words, the rays of light coming from it have been twice bent towards each other, so that when our eye follows them out in straight lines they are widely spread, and we see every point of light so clearly that all the spots and markings on this minute spicule are as clear as if it were really as large as it looks to us.
"This is simply the principle of the microscope. When you come to look at your own instruments, though they are very ordinary ones, you will find that the object-glass o, l is made of three lenses, flat on the side nearest the tube, and each lens is composed of two kinds of glass in order to correct the unequal refraction of the rays, and prevent fringes of colour appearing at the edge of the lens. Then again the eye-piece will be a short tube with a lens at each end, and halfway between them a black ledge will be seen inside the tube which acts like the iris of our eye (i, Fig. 11) and cuts off the rays passing through the edges of the lens. All these are devices to correct faults in the microscope which our eye corrects for itself, and they have enabled opticians to make very powerful lenses.
"Look now at the diagram (Fig. 16) showing a group of diatoms which you can see under the microscope after the lecture. Notice the lovely patterns, the delicate tracery, and the fine lines on the diatoms shown there. Yet each of these minute flint skeletons, if laid on a piece of glass by itself, would be quite invisible to the naked eye, while hundreds of them together only look like a faint mist on the slide on which they lie. Nor are they even here shown as much magnified as they might be; under a stronger power we should see those delicate lines on the diatoms broken up into minute round cups."