« ForrigeFortsett »
ANGLES FOR CONSTKUCTING A HORIzONTAL DIAL FoR LATITUDE FRoM 500 TO 59£3.
Whether for amusement or instruction, there is no instrument so deservedly popular as the Microscope. Other amusements are soon exhausted, but the httle world which the microscope reveals is inexhaustible: there is always something wonderful or something new to be seen, and the instruction it affords is unparalleled. Many beautiful objects may be observed by a single lens, which can be folded and carried in the waistcoat-pocket; but there is a limit to the use of such instruments, and the only satisfactory microscope is the compound microscope, which a good optician will supply at a cost of from three guineas upwards. It may be said that cheaper instruments can be had, which appear to do their work well. Perhaps so; but as we are about to recommend only what we know to be worthy of recommendation, we should not name a lower priced instrument than such a one as can be procured of Mr. C. Baker, of No. 244 High Holborn, for three guineas. It is not with any invidious spirit that this name is given. Other opticians may supply microscopes as good at the price, but this instrument will serve to illustrate all we have to say about the microscope, and we shall adopt it as the standard of all our observations.
The microscope we have named is packed in a neat polished mahogany box, with lock and key. The size of this box is 10 in. high, and 6 in. deep, by 7 in. wide. At the top is a brass handle by which it may be carried, and when fully replenished its weight is about 7 lbs. So much for the case and the microscope within it. But we must open the case and take out the instrument. On opening the door we observe at the bottom of the case a neat little mahogany drawer divided in two parts: one part is " racked " for holding glass slides and mounted objects, the other portion will contain small articles of apparatus, which we shall describe hereafter.
We draw out the stand of the microscope, which is clamped to a square of mahogany, so as to ensure greater steadiness, an object of importance in a microscope; from the left side of the case we take the brass tube or lx>dy, and screw this to the stand, so that it presents nearly the appearance indicated in our woodeut (rig. 1). A little cylindrical brass box slides into a hole at the top right-hand corner of the case. This we take down, unscrew the top carefully, take out the combination of glasses set in a neat kind of brass nozzle, which piece of apparatus is usually called ikvtobjective or objectglass. There is a screw at one end of this objective and a lens at the other. Let us screw this nozzle or objective into its proper place at the end of the tube or body of the microscope, and then, behold! it is the complete original of which our woodeut is a copy. Having put it together, the next step must be a careful examination of all its parts, and an appreciation of how these parts arc to be employed in the examination of objects.
It is not our purpose to enter into a dissertation on the science of opties, for which we have neither room nor inclination; what we most desire is to instruct our reader how to use the mysterious little piece of machinery which has just been unpacked. The "why and wherefore" will be sought by-and-bye, and there are plenty of means of acquiring the theory when it is wanted, Big boys would be more likely to try and use such an instrument at once than to sit down and ponder over " the reason why," and little boys arc not a whit less curious or impatient than their ciders.
bome one will perhaps read these pages before he has obtained his microscope, and would like to know how high it stands, so that he may imagine what its appearance would be under a glass shade. For the especial benefit of such a one we have measured the instrument, and declare its full elevation to be 13 inches.
Place the left hand firmly upon the mahogany slab which supports the instrument, then with the right hand hold the top of the tube or body; draw the tube backwards, and it will be found to move easily to any angle, so that a tall boy or a short one, a man standing or a man sitting, can either of them look comfortably down the tube without any danger of dislocating his neck, which might be the case if the tube were fixed bolt upright
As the body moves freely on the pivots the lower portion will be seen to carry with it a circular mirror, which is attached near the bottom; this mirror, by an admirable arrangement of joints, can be turned in any direction. The use of such facility of motion will be seen by-and-bye. Above the mirror is a square brass plate with a round hole in the centre; this is called the stage, and upon it the objects to be viewed by the microscope must be placed. A movable bar passes up and down on the upper surface of this plate, which is useful to retain the slide containing the object in position. Above this stage is the tube with the object-glass screwed in at the lower end, and the eye-piece at the upper end. At the side of the tube or body, near the bottom, is a screw with a milled head, which may be moved by the thumb and finger: this is called the fine adjustment. It will be time enough by-and-bye to speak of its uses. Below the tube are two other milled heads, one on each side. Turn one of them towards you with the thumb and finger: they move easily, freely, and smoothly, and with their motion, behold! the tube of the microscope, with the eye-piece at the top and its object-glass at the bottom, glides up and down just as the operator wishes! A firm, steady, gradual motion here is a necessity in all good instruments. These milled heads and the screws which they move we call the coarse adjustment. Now, having learnt the names of all the parts of the microscope which at present we desire to know, let us put it to work.
Take a slide containing a mounted object, which is transparent, and one may be purchased of an optician for a shilling, which will serve as a model for
imitation. (Fig. 2.) It consists of a strip of glass, 3 in. long and I in. wide. In the middle, a circular disc of very thin glass is fastened by means of Canada balsam, and in the middle of the disc, between the thin glass and the thick glass, lies the little object to be examined—perhaps it is the tongue of the house-fly. Place the slide upon the stage of the microscope, with the label and the thin glass upwards. If the object-glass is in the way, turn it up by means of the coarse adjustment, so that the slide containing the object may lie flat on the stage, with its bottom edge resting against the movable bar which crosses the stage. This bar, you now see, will prevent the slide falling
off the stage, even when the body is bent down to a convenient angle for looking through the tube. Then with the coarse adjustment bring the end of the object-glass to within about half an inch of your object. The tube is now supposed to lean just at the angle you wish, so that you may look down it in an easy position without bending the neck uncomfortably. If it is daylight, your microscope will be standing on a steady table by the window; if it is too dark for daylight, the microscope will be on a tabic equally steady, with a lamp, the flame of which should be only 6 or 8 in. away from the mirror of your microscope. Take the object off the stage for a few moments. Replace it by a piece of writing-paper large enough to cover the hole in the stage. Now turn the mirror with your finger and thumb, in such a manner that the light which falls upon it from the lamp shall be reflected up, and form a bright spot on the piece of paper just in the centre of the hole of the stage. When the light is thus properly adjusted, take off the piece of paper, and place the glass slide containing the object upon the stage in its place, so that the light reflected from the mirror falls just in the centre of the object, which may be kept in place by sliding up the movable bar. Now look down the tube. You see nothing but a bright disc of light like a full moon, perhaps. That is fortunate, because it proves that your light is properly managed. Now for the object. By means of the coarse adjustment you move the "body" of the microscope, and consequently the object-glass, a little nearer to the object. Proceed slowly, cautiously, and by no means near enough for the object-glass to touch the slide. Gradually, as you look down the tube, you will sec the shadow of the object, and at length the indistinct image of the object will appear upon the bright disc of light at the bottom of the tube. Quit your hold of the milled head of the coarse adjustment, and apply your thumb and finger to the milled head of the fine adjustment. Turn this also slowly and gradually till the object appears clear and distinct upon the bright field or disc of light at the bottom of the tube. Now, if you a have followed the instructions we have given, you will have overcome the first difficulty of seeing an object through your microscope. But why quit the use of the coarse adjustment, and apply the fine adjustment just as the object became distinct? For this reason: the . fine adjustment has a much finer screw than the coarse adjustment, and by turning its head you will more easily hit upon that desirable point where the object is exactly in focus, and can be seen most clearly. A halfturn of the milled head, up or down, will throw the object out of focus, make it less clear and distinct, and convince you that you have hit upon the exact focus of the lens. c
Whilst you are admiring the tongue of a fly, seen for the first time in your own microscope, we have a few words to offer about the objective, or object-glass, which you have taken out of the box, and placed upon the instrument, and used in looking at your first object.
The object-glass is called a half-inch objective, because its magnifying power is about equal to that of a lens with a focus of halffan inch. But this object-glass is itself a compound
of three glasses or lenses. Unscrew it from the tube and take it in your
One lens — equal to i.l inch power.
I f, however, when purchasing your instrument, you desire to possess glasses with a higher magnifying power, you may have, instead of the above combination, and at the same price, a triplet, or combination of three powers, the highest of which will be equal to a J in., and which will divide into two other powers, one of which will be r| in., and the other $ in., which may be represented thus:
One lens — equal to ?1 inch power.
Either combination, therefore, may be had; but if our advice were sought, we should recommend the first or lowest powers to any youth commencing with the microscope.
In order that some idea may be formed of the magnifying power of these glasses, we have ascertained that they magnify objects, each power as many diameters as arc placed opposite to them in the following table:
1 i inch power . . (one lens) . . 35 diameters.
I „ . . (two lenses) . . 75 .,
1 „ . . (three lenses) . . 150 „
S „ . . (one lens) 90 „
ft » . . (two lenses) . . 175 „
< „ . . (three lenses) . . 300 „
By 150 diameters it is meant that the tongue of a fly, for instance, viewed with the -J-in. power, is magnified so that it appears 150 times as wide or as long as it really is.
Having explained how this microscope is to be used, and what it is capable of performing, we will enumerate the remainder of the appliances which are supplied with the three-guinea instrument.
There is a pair of brass hand-tweezers for picking up little objects that you may desire to examine, which it would be difficult to raise with the thumb and finger. (Fig. 4.)
Then there is a pair of stage forceps for holding a minute insect for examination under the microscope, which is held fast at b, whilst the arm, a, is placed in a hole in the brass stage. The forceps can be turned round freely in almost any direction, so that the point, b, at which the insect is held, may be brought under the object-glass, and so turned that all parts of the insect may be examined readily. (Fig. 5.)
Finally, there is a " live box," the top of which can be raised (not unscrewed) and a drop of water containing any very small water-flea, or other minute aquatic insect, can be placed therein, the cap replaced, and when the "live