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.
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.
The stand must claim our attention first. The base of the instrument, called the foot, is usually either three-legged or horse-shoe shaped; whatever its form it should be heavy, for only thus can the microscope be steady, and steadiness is essential in all microscopic work. At the top of the foot there is a joint, in order that all the other parts of the stand may be inclined at any angle, from the vertical to the horizontal. Just above the joint is a bent arm of brass, to the forward end of which a brass tube is affixed. This tube is designed to hold the lenses, the objective at its lower end, the eyepiece at its upper end. The tube is always blackened inside; were this not the case, light passing through the objective would be reflected in all directions from the sides of the tube and a clear image of the object could never be obtained. The tubes of microscopes vary in length according to their country of origin; English and American tubes are ten inches long, those of continental make vary from a little more than six inches to rather more than seven inches in length.
Affixed to the lower end of the bent arm of brass, mentioned above, is a flat metal plate, known as the stage; at its centre, there is a circular hole through which rays of light pass to illuminate objects placed upon it. Below the stage, at the edge nearest to the foot, there is a metal peg, over which fits a tube to which a mirror is attached by a moveable joint. The mirror reflects light rays through the opening in the stage. The tube, holding it, can be slipped up and down the peg under the stage, thereby bringing it nearer to or further from the object and so altering the intensity of the reflected light, as we shall explain in a moment. Owing to its moveable joint, it is possible to swing the mirror to the right or left, so that the reflected light rays do not pass directly through the object on the stage, but strike it on one side or the other, thereby giving what is known as oblique illumination.
The cheapest forms of compound microscopes have all the parts we have mentioned, and focussing is carried out by sliding the tube, with its objective and eyepiece, up and down within its holder, in order to bring the objective further from or nearer to the object.
In more expensive instruments there are further refinements, in fact, on some of the very costly present-day instruments, there are so many appendages and appurtenances that it is doubtful whether some of them are not more of a hindrance than a help, at any rate they increase the possibility of trouble by their liability to get out of order. Such microscopes are only of use to very expert workers; there are, however, a good many additional features to be found on quite moderate-priced instruments, features which are a great help to the microscopist.
It is obvious that we cannot attain any degree of accuracy in focussing, especially with high magnifications, when we must perforce raise or lower the tube by hand. To obviate this difficulty, most microscopes are provided with mechanism known as a coarse adjustment; it consists of milled screws at either end of a metal rod; in the centre of the rod there is a little cog-wheel which engages with a row of notches on the tube. By turning the milled screws slightly in either direction, we can impart a considerable upward or downward movement to the tube carrying the objective and focussing at once becomes a more simple matter. The coarse adjustment is only useful for examining objects with a low magnification; if we use it when objects are being highly magnified we run the risk of screwing our objective down upon our object, to the certain destruction of the latter and the probable injury of the former. To obviate such a catastrophe, most of the better class microscopes are also provided with a fine adjustment. By means of this adjustment, which externally takes the form of a single milled screw, a considerable turn of the screw in either direction only imparts a very slight upward or downward movement to the microscope tube. In the best instruments, movements of as little as one hundredth part of a millimetre may be imparted to the tube by the fine adjustment and, seeing that there are about twenty-five and a half millimetres to the inch, it is obvious that a good fine adjustment is very delicate and, being so, must be treated with care. The fine adjustment is used to supplement coarse adjustment in the final focussing, when using high magnifications.