The wonders of optics/Optical instruments
The lenses and mirrors whose properties we have
been considering in the previous chapters, have been
combined in different ways for the purpose of examining
objects too small or too distant to be perceived by
the human eye. To instruments used for the former
purpose the name of microscope has been given, from
two Greek words signifying small and to see. In like
manner the name of telescope is also derived from two
Greek words, meaning distant and to see. Besides
these two classes of optical instruments, others have
been devised to facilitate the depicting of natural
objects, either by means of the pencil or of photography,
or to amuse the eye by optical illusions. Thus
we have the camera obscura, the camera lucida, the
magic lantern, the phantasmagoria, and numberless
other instruments of the same sort, most of which will
be described in the latter part of this book.
There are two sorts of microscopes, the simple and the compound; the one consisting of a single convex lens, and the other of several combinations of both convex and concave lenses.
When speaking of convex lenses, we described the properties of the ordinary magnifying glass, or simple microscope. The uses of this instrument are almost too well known to need description. It is used by old people, the lenses of whose eyes have become flattened by old age, by watchmakers for examining the minute portions of their work, by jewellers for the same purpose, and by most people for examining maps, engravings, and photographs. Simple microscopes are generally mounted in horn, ivory, or metal handles for convenience' sake. Some simple microscopes consist of two or more lenses mounted together in order to increase the magnifying power. The student must distinguish between several lenses mounted together in this way, and the true compound microscope, which is a comparatively complicated optical arrangement, as we shall see presently. When two single lenses are thus mounted together, the power of the combination is equal to the powers of each added together.
There is good reason for supposing that the simple microscope is a comparatively ancient invention. Seneca, who lived in the first century, declares that in his time it was well known that, when writing was looked at through a globe full of water, it appeared larger and blacker. In the eighth century we find the use of magnifying spectacles for old people common in most countries, and yet it was only at the beginning of the seventeenth century that a true optical instrument, in the form of a telescope, was invented. It only needed the placing of two magnifying glasses in a line to discover the principle of the telescope, but nearly a thousand years elapsed after the first introduction of these glasses before an accident rendered the principle evident.
In fig. 37 we see the commonest form of microscope in the hands of an observer; and by examining the following figure and tracing out the path of the rays, we shall easily discover the principles on which its action depends.
The object to be looked at is placed at a (fig. 38), on a piece of thin glass usually called a slide. A small
Fig. 37.—The Compound Microscope.
converging lens placed at b collects the rays proceeding from the object, and transmits them as far as c d, where they come under the influence of a second converging lens B, which causes them to spread out still more before they reach the eye. Consequently we not only
Fig. 38.—The Theory of the Compound Microscope.
see the image of the object magnified by the lens b, but still more enlarged by the action of the lens B, and appearing considerably enlarged at C D. The lens placed in front of the object is called the objective or object-glass; that placed nearest the eye, the eye-piece. These names apply equally to the similar lenses used in telescopes and other optical instruments. The instrument shown in fig. 38 is the simplest possible compound microscope, and is very rarely used. The eye-piece is generally constructed of two lenses, and the object-glass of as many as eight; the object in multiplying the lenses being, not only to increase the magnifying power, but to decrease certain defects inherent in all lenses whose surfaces are parts of spheres.
The amplification depends mainly upon the power of the objective, but different eye-pieces are also used to increase the apparent size of the objects to be examined. Thanks to the investigations of modern philosophers, we are enabled to magnify objects to 2,000 times their diameter with perfect distinctness; that is to say, the surface of the object appears to occupy 4,000,000 times its natural extent. Under such a power a hair would appear about six inches thick, a fine needle would look like a street post, and a grain of sand like a mass of rock. Although it is possible to employ compound microscopes of such a high magnifying power in the investigation of certain classes of objects, all ordinary preparations are best seen under a power of 500 or 600 diameters. It would be utterly impossible to give our readers the slightest idea of the benefits conferred on the human race by this marvellous instrument. Suffice it to say, that no naturalist or surgeon ever attempts the most simple investigation into the structure of any body without the aid of the microscope. It has already shown us that a world of creatures exists which, although invisible to the eye of man, are possessed of wonderful forms, colour, and beauty of structure, and is daily adding to our knowledge in this direction. We can hardly submit any substances to this marvellous instrument without discovering animal or vegetable life of the most vivid character. A drop of scum from the surface of a stagnant pool is instantly seen to be peopled with animal and vegetable life, when submitted to microscopic examination. At one moment a rolling ball glistening like glass slowly revolves past our view; then a little fellow like a piece of spiral spring screws his way along, backing when he meets with an obstacle; or a shuttle-shaped vegetable, apparently made of glass, with green balls inside him, slowly works his way from side to side, or, possibly, a mad battledore-shaped being dashes past at an inconceivable rate.
As it is indispensable that the object should be well lighted, a concave mirror is placed below it to reflect the rays of light from a lamp or white cloud, through the object when it is transparent. When it is opaque, it is illuminated by the rays of light being concentrated upon it by means of a convex lens. The name microscope appears by common consent to be applied more particularly to the compound instrument, the epithet of magnifier or magnifying-glass being kept for simple microscopes, although they are all, strictly speaking, microscopes.
In the ordinary compound microscope, it is only possible for one person to see the object to be examined at once; for popular exhibitions of microscopic objects the reflecting microscope has been devised, by means of which the images of the objects to be looked at are thrown upon a screen. The principle of this instrument is the same as that of the magic lantern and phantasmagoria, of which we shall speak presently. Fig. 39 (see next page) represents the photo-electric microscope, so called from the objects being reflected by the electric light.
The jars seen on the ground are the cells of a voltaic battery, by which the electricity is generated. The luminous rays starting from the incandescent charcoal points are reflected through the tube and its lenses by the reflector placed at the back of the instrument, and are concentrated upon the object to be magnified. The image thus produced passes through a second system of converging lenses, and is projected upon the screen magnified some millions of times according to the power of the object-glass employed.
"The experiments made with the photo-electric microscopes,"
says M. Ganot, "are amongst the most curious
and pleasing to be found in the whole range of physical
science. With this instrument it is possible to show the
smallest objects magnified almost indefinitely to an unlimited
number of spectators. A human hair will appear
as large as a broomstick, an ordinary flea will look the
size of a sheep, and the tiny cheese mite, as well as the
smallest animalcules, will be visible in all their beauty of
form and colour as clearly as if they were seen with the
naked eye. One of the most remarkable experiments to
be made with this instrument is that which shows the
circulation of the blood. The tail of a live tadpole is inserted between two plates of glass, or on an instrument specially made for the purpose, and placed in the microscope armed with a somewhat low power. The
Fig. 39—Photo Electric Microscope.
spectator immediately perceives upon the screen a mass of rivers and rivulets, all flowing with the red corpuscles forming the blood of the animal, and rushing through its veins and arteries with inconceivable rapidity. Another interesting experiment consists in dissolving a small quantity of sal-ammoniac in warm water, and passing a small portion of the solution across a warm glass slide. When placed in the microscope the water gradually evaporates, leaving behind a mass of feathery crystals, whose growth may be watched atom by atom, each crystalline molecule grouping itself around the others in forms resembling a mass of fern-leaves."
The apparatus we have been describing is sometimes illuminated with the rays of the sun, as in the following figure.
Fig. 40.—Solar Microscope.
It is then called the solar microscope, and exhibits objects with great beauty and clearness. The use of the sun's rays, however, has, in our own country at least, been entirely superseded by the electric and lime light. The latter method of illumination, which consists in projecting a stream of oxygen and hydrogen upon a ball of lime, is cheaper and more certain than the elec-tric light, although the latter is possibly the more brilliant of the two. The construction of the solar microscope differs but little from the instrument already described, and may be readily understood from the foregoing figure. The large mirror is placed outside the window of the room in which the microscope stands, so that the solar rays are reflected upon the surface of a series of convergent lenses, and from thence on to another mirror, from which it is again reflected through the microscope. As the position of the sun is constantly changing, it is necessary to connect the outside mirror with a train of clockwork. It may be mentioned that an instrument of this kind, for reflecting the sun's rays, is called a heliostat.
The student will, no doubt, at once perceive that if we concentrate the light of the sun upon an object, we shall also concentrate the heat, and either melt or consume it. A screen is therefore used in such cases, which will allow the light to pass while holding back the rays of heat. A solution of alum is found to answer the purpose admirably.