By the courtesy of Messrs. F. Davidson & Co.
1. A Fly’s Eye
A fly’s eye highly magnified, showing some of the many lenses of which it is composed.
2. Images Seen by a Fly
Photographs of a statue taken through a fly’s eye. Instead of seeing one image of an object, the fly sees as many images as there are lenses in its eyes.
Having cut a section it is hardly likely that, at this our first attempt, it will be good enough to put under the microscope, so we will continue to cut section after section till we have a dozen or more from these we can select the most transparent for examination. The sections of such material as a stem should be kept moist and to do so we will place them in a watch glass containing water. It is often easier, also, to cut our sections if the razor be moistened with water, at anyrate the moisture prevents the sections from adhering to the razor. The sections should be removed from razor to watch glass and from watch glass to slide, by means of a brush, never by means of the fingers. The razor of course, should be well dried with a soft rag before it is put away; rust, besides being unsightly, ruins the cutting edge. If we are really anxious to cut our own sections, and every good microscopist does so, we shall return to the operation again and again, even cutting objects which we have no desire to examine, for the sake of the practice. We shall soon reach a stage where our razor will be dulled and require stropping and the efficient stropping of a razor is, to many people, a more difficult operation than the cutting of sections.
We may point out here, that all sections are not quite so easy to cut as the one we have taken as our example. Some objects are so soft that they need hardening with chemicals before they can be cut, some are so hard that to attempt to cut them would ruin the edge of the razor, though it is wonderful what hard substances may be cut when we have had a little experience; some are so delicate that they must needs be buried in melted wax, then object and wax are cut together and later the wax is separated from the section.
Leaves and very small stems may be sectioned by the beginner, as easily as larger stems. For such objects little sticks of pith are sold as holders. Having cut of our piece of leaf from which we wish to derive a section, we make a slit, with our dissecting knife, down the middle of one end of a piece of pith. The piece of leaf is then placed in the slit and by holding the pith at the sides our piece of leaf is held firmly. Sections are cut through pith and leaf, and the two are floated in water, when the thin slices of pith will float away from the leaf sections.
Having cut a satisfactory section, let us proceed to describe the method of making a slide thereof. We will suppose that we do not wish to make a permanent preparation but one for temporary use. A clean slide must be selected, all through our pages we emphasise the cleanliness of slides, at or near its centre we put a drop of water and, lifting it with a brush, we place our section in the drop of water. If our examination is to be with a low magnification, we need not use a cover slip, nevertheless it is worth while to cultivate the habit of using one. The cover slip is not only a protection for our object but for our objective. Water you may argue cannot harm the lenses of the objective. Perhaps not, we will not argue the point but, when the water dries on the objective, it leaves a certain amount of deposit on the glass and this deposit must be rubbed off. The less often the lenses are rubbed the better for them, glass especially very highly polished optical glass, is far more easily scratched than many people imagine and a scratched lens is an inefficient lens. When using high magnifications a cover glass should always cover our object and the same remark applies to objects examined in Canada Balsam. This substance is likely to cause serious trouble if it finds its way on to an objective. It must be removed, that is obvious, but it sets hard, it must not be scraped away for fear of damaging the optical glass and, as it is used to cement the lenses together, there is the great danger that any solvent used to remove the Balsam from the face of the objectives, may also dissolve their setting. Our digression may seem somewhat unnecessary, but the very great importance of keeping all chemicals and even water, from coming into contact with the lenses of our instrument cannot be insisted upon too strongly.
In whatever substance we examine our object, water, glycerine or Balsam, there is a right and a wrong way of applying the cover slip. It must not be dropped or laid down flat upon the object, if we do this we shall certainly imprison a number of air bubbles and that must be avoided. One edge of the cover slip must be laid against the edge of the mountant, as the liquid used for mounting our object is called, then placing a needle beneath the cover slip it must be gently lowered into position. We shall now find that all air bubbles are driven out as the cover slip is lowered.