Page:The wonders of optics (1869).djvu/154

 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