One or two practical remarks on the proper handling of the pocket-lens may be of use. Do not always employ the same eye in looking through the lens, but use the eyes alternately. There is always a temptation to employ the same eye, which receives a kind of training in vision; but it is a temptation always to be resisted. With some persons the right eye is most in favor, and with others the left; and when the favorite eye gets all the work, it too frequently suffers. Whether you look with the right or the left eye, keep both eyes open.
At first the beginner will find a little difficulty in restricting his vision to one eye while the other remains open, just as a beginner on the piano-forte feels himself puzzled when he tries to make his right hand go one way and his left hand another; but in either case a little practice and plenty of perseverance are sure to overcome all obstacles, and in a wonderfully short time the difficulty will not only be overcome, but forgotten.
We speak here with some feeling, because, while engaged on a work on the microscope, we were necessarily obliged to work much at night, and inadvertently employed the left eye more than the right; the consequence of which imprudence was that we have been obliged ever since that time to give the left eye perfect rest, as far as artificial vision goes, and, except when looking through a binocular instrument, we have not ventured to use it either to a microscope or a telescope. The vision accommodates itself to circumstances with wonderful ease, and the observer learns the curious art of cutting off all communication between the unused eye and the brain; so that, although the objects around may imprint themselves upon the retina, the mind is as totally unconscious of them as if they had no existence.
If possible, always examine an object without removing it, as thereby you see it as it is, without altering any of the conditions with which it is surrounded. Should this not be practicable, take the object to be viewed in the left hand and the lens in the right. Place the wrists of the two hands together, and then you will find that one supports the other, and that the lens can be held in the proper focus without the least difficulty. After you have used the lens for some little time, you will learn to hit upon the right focus almost to a hair’s breadth,—so as to lose no time, a matter of some importance when a living creature is to be examined, especially if it be in motion.
We are now about to suggest a very simple piece of mechanism, by which the pocket-lens can be converted into a microscope that will serve for dissection and many other purposes.
Melt three or four pounds of lead in an iron ladle, and make a mold, consisting of a hollow hemisphere of paper or cardboard, through the center of which an iron rod has been passed. The hollow of the paper should resemble an ordinary saucer. Pour the lead into the saucer, and let it cool. The paper mold will be scorched by the heat and rendered useless, but an outer coating of lead will be cool and hard before the paper is quite destroyed. Next take a piece of stout brass wire and a wine-cork; twist the wire round the cork several times; cut off one end close to the cork; sharpen the other, and turn it up.
Bore a hole through the cork, just large enough to allow the upright rod to slip through it, and there is the “stand” of your microscope. Now take your pocket-lens, and get an optician to bore a hole through one end of it, just large enough to receive the upturned end of the wire; slip the lens on the wire, and the microscope is complete.
The cork, though grasping the upright stem with tolerable firmness, can be slid up and down so as to insure the correct focus, and can be pushed aside whenever the object has to be viewed with the naked eye, and must not be removed from its place. This instrument is a capital one for dissecting purposes, and will answer quite as well as those expensive affairs that are to be purchased in the shops.
If the object be transparent, and requires to be seen by transmitted light, the following plan will answer:—Take a thin piece of wood, cut or punch a round hole out of the middle, and support it on four legs. Wires or wooden pegs fixed in corks will answer the purpose well, and if the corks be glued to the corners of the board, the legs can be inserted or removed at pleasure. The wood of which cigar-boxes are made will answer the purpose very well. Its dimensions should be about three inches in length by two in width. Now buy one of the doll’s looking-glasses that are sold for a penny, and put it under the stand. Lay a flat piece of glass over the hole, place the object upon it, and direct the light through it by means of the mirror below. If such a mirror cannot be obtained, it is easy enough to make one, by mounting a piece of looking-glass in a cork frame, and making it swing on pivots, like the glasses of our dressing-rooms.
The young microscopist must remember that when he is examining any object by transmitted light, he must arrange it as flatly as possible on the glass. In many cases, a still neater manipulation is required—as, for example, when the petals of flowers are under examination. Thin glass is to be purchased at any optician’s, and if cut in squares, instead of circles, is very much cheaper, and quite as useful for all practical purposes. Lay the petal on the glass plate, place a piece of the thin glass upon it, and press it gently while examining it. If it still remains thick and dull, put a drop of pure water on the petal, and replace the thin glass, when the structure will almost invariably be detected.