The Wonders of Optics

by Fulgence Marion

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.

Fig. 39.—Photo-Electric Microscope.

“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 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 electric 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.