Part 15

As for the kind of light that is employed, there is nothing which equals that of a white cloud; but such clouds are rare, and are at the best extremely transient, and can only be seen by day, various artificial methods of illumination have been invented. Novices generally think that when the sky is bright and blue they will be very successful in their illumination, and feel grievously disappointed at finding that they obtained much more light from the clouds, whose disappearance they had anxiously been watching. Finding that the blue sky gives scarcely any light at all, they rush to the other extreme, turn the mirror towards the sun, and pour such a blaze of light upon the object, that the eye is blinded by the scintillating refulgence, and the object is often injured because the mirror is capable of reflecting heat as well as light.

In the daytime there is nothing better than the “white-cloud illuminator,” which is made easily enough by means of plaster of Paris. A sheet of thin white paper fastened against a window-pane is also useful; and the simple plan of dabbing the glass with putty will have a beneficial effect in softening the light, when the window has a southern aspect. In default of these conveniences, it will be often sufficient to fix a piece of white letter paper over the mirror, or even to dull its surface with wax. At all events, he who aspires to be a true microscopist must be ready with expedients, and if he finds himself in a difficulty, he must summarily invent a method of obviating it.

At night a lamp is necessary; candles are useless, because they have two faults--they flicker, and they become lower as they burn. The latter defect can be cured by using a candle-lamp, but no arrangement will cure the flame of flickering; it is peculiarly trying to the eyes, and destructive of accurate definition. An ordinary moderator lamp answers pretty well, and a small one is even better for the microscopist than one of large dimensions. The chief drawback to the moderator lamp is that the flame cannot be elevated or lowered, so that the only way to procure a light at a higher elevation is to stand the lamp on a block of wood or a book. Small lamps are, however, made expressly for the microscope, and, if possible, should be procured, and used for no other purpose, and intrusted to no other hands.

If you want a really brilliant, clear, white light, you must trim the lamp yourself. A small piece of pale blue or neutral-tint glass interposed between the lamp and the microscope has a wonderful effect in diminishing the yellow hue which belongs more or less to all artificial lights which are produced by the combustion of oil or fat. We have no doubt but that in a few years we shall be rid of the clumsy and dirty machines that we call lamps, and have substituted for them the pure brilliancy of the electric light.

Whatever lamp you use, a shade is absolutely necessary, in order to defend the eyes. Let me here warn my young readers that they cannot be too careful of their eyes. In the exuberance of youthful strength and health we are too apt to treat our eyes as unceremoniously as our digestion, and in later years we awake to unavailing repentance.

Another point which calls for extreme attention is the perfect cleanliness of the glasses. It is astonishing how a tiny dust-mote, or the least condensation of damp, will diminish the powers of the microscope, and how often the instrument is blamed for indistinctness when the real fault lies in the carelessness of the operator. Even when the greatest care is taken, dust is sure to settle on the glasses, especially on the eye-piece, and before using the microscope the glasses ought to be carefully examined. Never wipe them with an ordinary handkerchief, but get a piece of new wash-leather; beat it well until no dust issues from it, and then put it into a box with a tightly-fitting cover. Use this, and nothing else, for cleaning the glasses, and you will avoid those horrid scratches with which the eye-glass and object-glass of careless operators are always disfigured.

Moisture is very apt to condense on the glasses and to ruin their clearness. If the microscope be brought from a cold into a warm room, the glasses will be instantly covered with moisture, just as the outside of a tumbler of cold water is always covered with fine dew when brought into a warm room. The microscope should therefore be kept at least an hour in the room wherein it is to be used, so that the instrument and the atmosphere may be of the same temperature. You should make the microscope a trifle warmer than the surrounding atmosphere, and so avoid all danger of condensation. When changing the object glass or eye-piece, always keep the hand as far away from the glass as possible, and manipulate with the tip of the forefinger and thumb. The human skin always gives out so much exhalation, that even when the hand is cold the glasses will be dimmed; and it is a peculiarity of such moisture, that it adheres to the glasses with great pertinacity, and does not evaporate like the dew which is condensed from the atmosphere.

In order to insure perfect success in this important particular, the young microscopist will do well to get the optician from whom he purchased his instrument to explain its construction, and to give him a lesson or two in the art of taking it to pieces and putting it together again; for unless each glass can be separately cleaned, no one can be quite sure that the instrument will perform as it ought to do. The best method of ascertaining whether it is quite clean is to throw the light upwards by means of the mirror, and then to turn the eye-piece slowly round. If any dust or moisture has collected either upon the eye-glass or the “field-glass,” which forms the second lens of the eye-piece, it will be immediately detected. Turning the object-glass will in a similar manner detect impurities upon its surface.

THE END.

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