We have already mentioned that the optical parts of the compound microscope are of greater importance than what may be termed the mechanical portions and the objectives are more important than the eyepieces. Better results can always be obtained with a good, high-power objective and a low-power eyepiece, than with an inferior objective and a good quality eyepiece. The merits of the eyepiece, however great, will not be adequate compensation for the failings of the objective. Modern objectives are composed of several lenses and of a combination of flint and crown glass, as we explained in our [last chapter]. They are so designed that they can be screwed into the lower part of the microscope tube. The focal length of each objective is, or should be, marked upon it; as a general rule, however, it may be taken that the smaller the lower lens, the shorter its focal length and therefore the greater its magnifying power.
The form of eyepiece most usually met with is known as Huyghen’s. It consists of two plano-convex lenses, with their flat or plane surfaces directed away from the objective. The smaller of the two lenses is situated nearer to the eye of the observer and is known as the eyeglass; its function is to magnify the image formed by the objective. The larger, lower lens is known as the field or collecting glass; it renders the image clearer though, in so doing, it reduces the magnification of the eyeglass. In instruments provided with more than one eyepiece we shall wish to know which gives the greater magnification; this is or should be marked upon the metal rim surrounding the eyeglass but, in general, it may be stated that the shorter the eyepiece the greater its magnification. We repeat again, increase your magnification always, when possible, by using higher power objectives rather than eyepieces with greater magnifying powers. Sometimes it is necessary to use a greater magnification than our most powerful objective will give us; then we must fit our most powerful eyepiece and draw out the upper part of the microscope tube—in the best instruments they are made to pull out, after the manner of the telescope. The effect of so doing will be to increase the magnification considerably but, at the same time, the definition or clearness is seriously impaired.
For the examination of practically all our microscopic objects we require a number of slides, little glass slips of good, thin, clear glass. They may be used over and over again unless we make permanent preparations, but we are hardly likely to do so in our early days. The slides are held in place on the microscope stage, either by a pair of clips attached thereto or by resting against a bar running across the stage. We may here remark that it is essential always to keep one’s microscope slides absolutely clean. Dirty slides denote the careless worker; moreover, dirt when magnified is misleading. Objects which are being examined in water or any other liquid should be covered with a cover-slip, an exceedingly thin circle or square of glass. The cover-slip is as much a protection for the objective as for the object and its cleanliness also, is all important.
We have not mentioned any refinements such as the mechanical stage, by means of which slides on the stage may be rotated, moved to the front and to the back of the stage or from side to side. We have omitted these because they are not essential even for the very best work; they lend additional comfort to the use of the microscope but, again, they are not essential. The microscopist who requires such luxuries may learn about them in the larger text-books on the microscope.
CHAPTER V
ANIMAL LIFE IN THE PONDS AND STREAMS
The enthusiastic microscopist will probably never lack material for his instrument, whatever branch of microscopical work he may decide to make his own. To the student of Pond Life, either animal or vegetable, there is granted a never-ending store of beautiful and interesting objects. Because one pond has been thoroughly searched and all that it can offer has been carefully examined, we must not conclude that no other pond will be worth our attention. Though indeed many little animals occur over and over again in practically every pond, there are other equally interesting animals which only occur in certain localities and for these we must keep a sharp look-out.
The apparatus needed for the collection of the denizens of ponds and streams, need consist of no more than a net with a very fine mesh and a jar in which to bring our captures home; for, of course, animals which dwell in water need not be dried on the journey home. Various useful accessories for the student of pond life are sold at very reasonable rates by most scientific instrument makers.
We shall find many representatives of the animal world in our pond and exceedingly interesting most of them will prove. From the mud we may obtain the “protean animalcule,” known to scientists as Amœba Proteus, the most lowly of all animals. Though this creature is plentiful and just visible to the naked eye, he is not easy to separate from his surroundings. He is almost colourless and therefore paler than the mud. Having secured him on the end of a glass rod, let us examine him in a drop of water on a slide. At first he will remain motionless, as a protest against being disturbed; we shall not have to wait long, however, for soon one part of his body will be seen to protrude and then grow larger and larger till it forms a false foot; other parts may follow suit, till he is more elongate than oval, and he moves in the direction of his false foot with a curious gliding motion. His pace is not great and has been calculated at a twenty-fifth of an inch in an hour. Really the “protean animalcule” is little more than an animated drop of jelly, a fact we can substantiate by watching him feed. His food consists of minute water plants such as diatoms, and when one of these plants comes within his line of march he simply surrounds it with his false feet and, as it were, flows around it. When he has digested all he can he flows away from the undigested portions; he has no mouth or any of the organs usually associated with animal anatomy.