opera-glass is readily measured by looking simultaneously at the object with one eye, and at its image with the other, and comparing their relative magnitudes. The best object to be used is a large scale of inches, noting how many divisions as viewed by the eye, equal one or two as seen through the telescope. For high powers the best method is by the dynameter. Focus the telescope on a distant object, and turn it towards the sky, or other bright light. On holding a sheet of paper near the eye-piece, a bright circle of light is seen projected on it, which is really an image of the object-glass formed by the eye-piece. The diameter of the object-glass, divided by that of its image, equals the magnifying power. To measure accurately the diameter of the small circle, a spider line, or eye-piece micrometer, may be used, or a small reading microscope, whose objective is divided in two parts, which may be moved past each other a known amount by a micrometer-screw. Two images of the circle are thus formed, which may be rendered tangent to each other by turning the screw. The parts of the objective have then been moved a distance proportional to the diameter of the circle, which is thus measured with great precision. In cheap telescopes a diaphragm is sometimes inserted near the objective, thus reducing the available aperture, and increasing the sharpness of definition of a poor lens, though diminishing the amount of light without apparently reducing its size. This is detected by turning the eye-piece towards the light, and seeing if it is visible when looking through the very edge of the objective. If not, the diaphragm should be removed or an incorrect value of the magnifying power will be obtained. If focussed for near objects, the magnifying power is much increased; hence for purposes of comparison, objects at a great distance should always be selected when making this measurement.

85. PHOTOGRAPHY. I. GLASS NEGATIVES.

Apparatus. A small darkened chamber or closet is needed for this Experiment. In this, a sink is placed with an abundant supply of water, and over it a shelf for the bottles containing the various reagents described below. A glass or porcelain vessel, shaped somewhat like a card-case, is employed to hold the solution of nitrate of silver, and a flat dish for the hyposulphite of soda. A large number of plates of glass are needed on which the

photographs are to be taken, and racks for holding them. In preparing the solutions, funnels, filter-paper, and other similar chemical apparatus are also required. The closet should be lighted by a gas-burner covered with a yellow shade, to cut off the actinic rays. The camera in which the photographs are taken consists of a blackened box with a convex lens in front, and closed behind by a plate of ground glass, or by the plate-holder which carries the prepared plate of glass.

Great care has been bestowed on various forms of lenses for cameras, and the best forms are somewhat expensive. Three kinds are commonly employed. First, portrait lenses which have a large aperture, and admitting much light work very quickly, but they only take in a cone with an angle of about 60°, and have not much depth of focus. That is, when focussed for a given distance, objects a little nearer or a little farther off will be indistinct. The second kind of lens is adapted for views; a small concave lens is inserted between two which are convex, thus giving a greater depth of focus, but not working so quickly as the preceding. The third class, as the globe and the Zentmayer lenses, take in a large cone of light, as 90°, but work very slowly, requiring one or two minutes even in strong sunlight. They have a great depth of focus, andmay be placed very near the object; but when this is a building, the perspective will be bad if the camera is brought too near. For the same reason, when taking a building, the glass plate should always be vertical, or a distortion will be produced. As no lens is perfectly achromatic, the focus of the actinic rays will not coincide exactly with the visual focus, being less for an under, and more for an over-corrected lens. The latter should always be avoided, and it is best to get one in which the two foci coincide as nearly as possible.

Experiment. Almost all photographic processes depend on first taking a glass negative, that is, a picture in which the bright portions shall be transparent, the dark parts, opaque. For this purpose the plate of glass is first prepared or rendered sensitive, then exposed in a camera obscura so that an image of the object shall fall on it. The plate is then developed, by which the image is rendered visible, and fixed or rendered permanent. These operations are easily performed when all the apparatus is ready, and the baths employed in good condition. The real difficulty in taking photographs, is in preparing the various solutions used, and in renewing them as they deteriorate; accordingly the following receipts are given, which the student should, if possible, try for himself.

The collodion used for coating the plates is made as follows. To 8 oz. of ether add 96 grains of gun-cotton, and then 8 oz. of alcohol. The strength of the latter must be 95 per cent., and it must contain no fusel oil, or the cotton will not be well dissolved. If made in the evening be very careful about lights, as the mixture may take fire from a lamp several feet distant. As the vapor is much heavier than air, there is more danger from lights on the floor than from those above. Dissolve 24 grains of bromide of potassium, in as little water as possible, add 64 grains of iodide of ammonium, and more water if necessary, and pour this into the collodion to iodize it. Pure iodide of ammonium will do, but it is better to have the iodine a little in excess, in which case, the salt will be dark colored instead of white. Too much iodide and too little bromide give a hard picture. In about two days the collodion will be ready for use, and it will keep in good condition about two weeks. Iodide of cadmium, which is commonly used in that which is sold, makes collodion keep better, but renders it less sensitive. If the weather is very hot, there is difficulty from the collodion drying too rapidly. In this case, less ether and more alcohol must be used, as the latter is much less volatile.

The silver-bath by which the plate is rendered sensitive, is made by adding 1 oz. of nitrate of silver to 12 oz. of water, and acidulating it with 30 or 40 drops of pure nitric acid. If used directly it would dissolve the iodides in the plate. Accordingly, a coated plate should be left standing in it over night. Filter, and in twenty-four hours it can be used. More depends on the condition of the silver-bath than on that of any other liquid employed. It should be kept nearly neutral, but always slightly acid. If alkaline the picture is fogged or blurred, and if too acid the action is much retarded. Organic matter is very injurious, and dust should therefore be carefully excluded by a cover. Whenever practicable, it should be exposed in a glass bottle to strong sunlight, which precipitates the organic matter in black flakes, and the latter removed by filtering. After using the bath for some time, it becomes covered with a scum, due to the alcohol and ether from the collodion plate, which is the first indication that the solution is becoming too weak. The iodide of silver is then liable to be precipitated on the plate, forming little spots in

it like pinholes. One half its bulk of water should, in this case, be added, to precipitate the iodide, the solution filtered and boiled down to its original strength.

To develope the picture when taken from the camera, a solution of 1 oz. of proto-sulphate of iron, in about 20 oz. of water is used, and containing 1 or 2 oz. of pure acetic acid, No. 8. This liquid will keep only about three days. The object of the acetic acid is to retard the process, as otherwise the silver would blacken instantly.

The liquid used to fix the picture, is formed by dissolving 1 oz. of hyposulphite of soda in 5 oz. of water.

Be careful that the glass is not rusty or iridescent, as in that case the collodion is liable to cleave off. Double thick glass is preferable on account of its greater strength, unless the plates are small. There must be no "knobs" or glass dust on its surface, nor deep scratches, because these will appear in the positive if the printing is done by sunlight. Almost all plates are slightly curved, and it is essential that the hollow or concave side should be coated. The plate will then conform more nearly to the image in the camera, is easier to coat, is less liable to be scratched if laid down on its face, and is not so likely to be broken in printing.

To clean the glass, mix equal parts of alcohol and ammonia, and add enough rotton-stone to render it viscid. Pour a few drops on the glass and scour with wash leather, letting the plate rest on one corner. Clean also the edges carefully. Let it dry, and then rub off all the rotton-stone with clean flannel. Breathe on the plate, and if clean, the moisture will pass off rapidly and evenly from the surface. Thus cleaned, they will keep for two days. A better method is the following. Soak the plates over night in strong nitric acid, and wash thoroughly under a faucet. Mix up the white of an egg with an egg-beater, taking care that none of the yolk gets in, add 12 ounces of water and 3 or 4 drops of concentrated ammonia, which keeps the egg from souring, and neutralizes any acid that may remain on the plate. Filter through paper or cloth and keep the filtrate in a bottle. Pour it over the plate, and a uniform film is produced, which will last for six months if kept dry and free from dust. Another method is to soak the glass over night in a strong solution of caustic potash

and then wash it, but this is liable to injure the silver bath. If any nitric acid gets in, it merely retards the action, but alkali will cause fogging, or the image will look smoky, as if under a veil.

To coat a plate properly with collodion requires considerable skill and practice, as it is very essential that the coating should be perfectly uniform. First, remove any films of collodion that may have dried on the mouth of the bottle, and take care never to disturb the sediment at the bottom. Regarding the plate as a map, it is held by the S. W. corner with the thumb and forefinger of the left hand, and the collodion poured on just N. E. of the centre. Enough is added to cover about half the plate which is then inclined, so that the liquid shall flow successively to the N. E., the N. W., the S. W. and the S. E. corners, and then tipped so that it will run off into the bottle. Allow it to drain for a few seconds and incline it gently from side to side, to prevent its forming streaks. The ether soon evaporates, and as soon as the film becomes sticky and consistent, the plate is immersed in the silver bath, by laying it on its holder, and lowering it into the liquid. This must be done slowly and steadily, or streaks will appear across the plate. If the plate is too large to be held by one corner while coating it, lay it on the palm of the hand, and interpose a sheet of card-board to prevent the warmth from drying the collodion too rapidly. Still larger plates must be placed on a board and rested on a point attached to the top of a tripod.

After remaining a few seconds in the bath, the plate should be raised gently out of it, when its surface will present a greasy appearance, due to the ether still remaining in the film; soon however, it will appear to be perfectly wet by the solution, and is then ready to be transferred to the camera. The plate-holder consists of a frame closed in front by a slide, and with a hinged back. The latter is opened, the plate put in with the coated side next the slide, and the latter then closed. A black cloth is thrown over it to cut off any stray light that may leak in through cracks in the holder, and it is then carried to the camera. The object to be taken having been placed in the proper position, and in a good light, the camera is turned towards it, when an image will be formed on the ground glass. This is best seen by standing behind the instrument, and cutting off the light by a black cloth thrown