respect that in the transit the telescope can turn completely over, so as to look both backward and forward, while in the theodolite it cannot do so without taking it out of the Ys, or causing the whole of the instrument above the upper plate of the horizontal limb to revolve through an angle of 180°. In the United States and Canada the engineer's transit has almost entirely supplanted the theodolite.

The Spirit Level is an optical instrument used by engineers, surveyors, &c., for the purpose of finding the difference of heights between two or more places. Spirit levels differ more or less in their construction, and

have therefore received different names, such as the Y level, Troughton's, and Gravatt's level, or the "Dumpy." The last is shown in Fig. 103. They all essentially consist of a telescope to which a spirit level is attached, resting on a horizontal bar. The parallel plates and tripod are exactly like those of the theodolite already described.

Optical Toys. A great variety of optical toys owe their effects to the continuance of impressions upon the retina after the objects which produced them have altered their positions. Toys of an amusing character, called thaumatropes, phantaskopes, phenakistoscopes, &c., are constructed upon this principle. A moving object which assumes different positions in performing any action is represented in the successive divisions of the circumference of a circle in the successive positions it assumes. By causing the disc to revolve, these pictures are brought in rapid succession before an aperture through which the eye is directed, so that the pictures representing the successive attitudes are brought one after another before the eye at such intervals that the impression of one shall remain until the impression of the next is produced. In this manner the eye never ceases to see the figure, but sees it in such a succession of attitudes as it really assumes.

The Phenakistoscope, or Magic Circle, is a beautiful and amusing instrument on the same principle. It consists of a circular disc of cardboard, or other material, eight or ten inches in diameter, having twelve slits placed at equal distances in its margin, and in the direction of its radii. This disc can be made to turn rapidly about its axis; and if we look into a mirror through one of the slits when it is revolving, they will appear to stand still in the mirror, owing to the motions of the object and its image being equal and opposite. If a figure were placed beneath each slit, each figure seen in the mirror would be stationary. If the figures were eleven in number, instead of twelve, they would appear to move in one direction; and if they were thirteen, they would appear to move in the opposite direction. Let us now suppose twelve gates to

P

be drawn on a separate disc smaller than the main one, and placed upon it so as not to interfere with its slits; these gates will stand still during the revolution of the disc. If we then place thirteen horses with their riders near the gate, one horse just before he begins to leap, the second horse with its fore-legs raised from the ground, and all the other horses in the different positions of leaping, till the thirteenth horse reaches the ground, the effect will be that each horse and its rider will come up to the slit through which we look faster than the gate; and as each gate arrives, the horse will have advanced of of the circumference of the disc; that is, in one complete revolution it will have moved forward through of the circle. Had there been eleven slits it would have moved backwards. Now during this motion the horse has taken thirteen. different positions in succession, and therefore leaps the gate.

In like manner, there are twelve hedgerows, with several hounds, each of which is represented in thirteen different positions, so that they appear in the act of crossing the hedges, and we have exhibited before us a portion of a fox-hunting scene which produces a very agreeable excitement.

The Anorthoscope is an optical instrument, by means of which two discs, revolving with different velocities, rectify, or make regular, and multiply an extremely shapeless and irregular figure. This instrument, and also the phenakistoscope, were invented by M. Plateau, who might easily have invented simpler names for them.

CHAPTER XVII.

THE PRINCIPLES OF OPTICS APPLIED TO VARIOUS
USEFUL PURPOSES.

Photo-zincography.-One of the most useful applications of the laws of optics is that of producing a photographic facsimile of any subject, such as a manuscript, a map, or a line engraving, and transferring the photograph to zinc, thereby obtaining the power of multiplying copies in the same manner as is done from a drawing on a lithographic stone, or on a zinc plate. This process is called photo-zincography.

of the object of This is obtained When the lens

The first part of the process concerns the production of a negative photograph on glass exactly the same size as the original. by the ordinary wet collodion process. and camera are in adjustment, the plate is covered with the sensitive coating, exposed, developed, and fixed in the ordinary way, and then immersed in a saturated solution of chloride of mercury (corrosive sublimate.) When well whitened by the action of the salt, it is removed, washed with water, and then with a solution of hydrosulphate of ammonia, consisting of ten parts of water to one of hydrosulphate of ammonia of commerce.

In this manner the ground of the negative is rendered extremely dense, without affecting the clearness of the detail. When dried and varnished it is ready for use.

The quality of the paper used is a point of much importance. That which has been found best suited for the purpose is a semi-transparent kind, with a

smooth surface, known by the name of engravers' tracing-paper.

Solutions of gum arabic and bichromate of potassa are prepared by dissolving one part by weight of gum arabic in two parts distilled water, which may be called solution A; and 1 oz. of bichromate of potassa in 10 oz. distilled water, which may be called solution B. When required for use, one part by measure of solution a is mixed with two parts of solution в, and the paper is coated over evenly with the mixture by a flat camelhair brush, and dried. It is then exposed under the negative in the usual way. The time required for printing varies from ten minutes in diffused light to two minutes in sunlight. When all the details appear distinct it should be removed.

The next step is the coating of the whole surface of the print with an even layer of a greasy ink, which is of two kinds; the nature of the object photographed determining what kind is to be used. If it is, for instance, a line engraving of a close nature, a thin ink is applied in the smallest possible quantity to prevent clogging; if, on the other hand, the object is of an open nature, as in manuscript printing, a thick ink with more tenacity is employed. The thin ink is composed of 5 oz. of middle linseed-oil varnish, and 1 oz. of lampblack. The thick ink of 2 oz. of middle linseed-oil varnish, 4 oz. of wax, oz. of tallow, oz. Venice turpentine, oz. of gum mastic, and 13 oz. of lampblack. In order that the ink may form an even coating on the paper, a zinc plate is charged with it by means of a roller, taking care to cover the plate as thinly but as evenly as possible. The paper is laid face downwards on the plate, passed through a lithographic press two or three times, and then removed from the