It is common for such lights to be seen at a distance of between fifty and sixty miles. The apparatus is mostly arranged in the form of an octagon, and is generally provided with additional reflecting mirrors at those parts above the light which are out of the range of the lenses. The light shining fully in eight directions at one time, can scarcely be missed by any ship within range; but in order to guard against any possibility of accident, the optical apparatus is often made to revolve by clockwork, so that every point of the ocean is illuminated in turn. By using coloured glasses, or by causing the light to disappear at distinct intervals, different lighthouses may be identified by ships that are out of their reckoning. Fig. 36 represents the interior of the lantern of a first-class lighthouse, showing the arrangement of the lenticular rings round the central lens. If ever the student should pass through Havre, he should not miss the opportunity of seeing this noble apparatus, which is one of the finest ever manufactured. [Illustration: FIG. 36.—Lantern of a First-class Lighthouse. ] ------------------------------------------------------------------------ CHAPTER VIII. OPTICAL INSTRUMENTS.—THE SIMPLE AND COMPOUND MICROSCOPE. THE SOLAR AND PHOTO-ELECTRIC MICROSCOPE. 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.