On a mode of Printing enlarged and reduced Positives, etc., from Collodion Negatives.

To explain the manner in which a photograph may be enlarged or reduced in the process of printing, it will be necessary to refer to the remarks made at page 20, on the conjugate foci of lenses.

If a collodion negative be placed at a certain distance in front of a camera, and (by using a tube of black cloth) the light be admitted into the dark chamber only through the negative, a reduced image will be formed upon the ground glass; but if the negative be advanced nearer, the image will increase in size, until it becomes first equal to, and then larger than, the original negative; the focus becoming more and more distant from the lens, or receding, as the negative is brought nearer.

Again, if a negative portrait be placed in the camera slide, and if the instrument be carried into a dark room, a hole be cut in the window-shutter so as to admit light through the negative, the luminous rays, after refraction by the lens, will form an image of the exact size of life upon a white screen placed in the position originally occupied by the sitter. These two planes, in fact, that of the object and of the image, are strictly conjugate foci, and, as regards the result, it is immaterial from which of the two, anterior or posterior, the rays of light proceed.

Therefore in order to obtain a reduced or enlarged copy of a negative, it is necessary only to form an image of the size required, and to project the image upon a sensitive surface either of collodion or paper.

A good arrangement for this purpose may be made by taking an ordinary portrait camera, and prolonging it in front by a deal box blackened inside and with a double body, to admit of being lengthened out as required; or, more simply, by adding a framework of wood covered in with black cloth. A groove in front carries the negative, or receives the slide containing the sensitive layer, as the case may be.

In reducing photographs, the negative is placed in front of the lens, in the position ordinarily occupied by the object; but in making an enlarged copy it must be fixed behind the lens, or, which is equivalent, the lens must be turned round so that the rays of light, transmitted by the negative, enter the back glass of the combination, and pass out at the front. This point should be attended to in order to avoid indistinctness of image from spherical aberration.

A portrait combination of lenses of 2½ or 3¼ inches is the best form to use, and the actinic and luminous foci should accurately correspond, as any difference between them would be increased by enlarging. A stop of an inch or an inch and a half aperture placed between the lenses obviates to some extent the loss of sharp outline usually following enlargement of the image.

The light may be admitted through the negative by pointing the camera towards the sky; or direct sunlight may be used, thrown upon the negative by a plane reflector. A common swing looking-glass, if clear and free from specks, does very well; it should be so placed that the centre on which it turns is on a level with the axis of the lens.

The best negatives for printing enlarged positives are those which are distinct and clear; and it is important to use a small negative, which strains the lens less and gives better results than one of larger size. In printing by 2¼ lens for instance, prepare the negative upon a plate about two inches square and afterwards enlarge it four diameters.

Paper containing chloride of silver is not sufficiently sensitive to receive the image, and the print should be formed upon collodion, or on iodized paper developed by gallic acid.

The exposure required will vary not only with the intensity of the light and the sensibility of the surface used, but also with the degree of reduction or enlargement of the image.

In printing upon collodion the resulting picture is positive by transmitted light; it should be backed up with white varnish, and then becomes positive by reflected light. The tone of the blacks is improved by treating the plate first with bichloride of mercury, and then with ammonia.

Mr. Wenham, who has written a most practical paper on the mode of obtaining positives of the life size, operates in the following way:—he places the camera, with the slide containing the negative in a dark room, and reflects the sunlight in through a hole in the shutter, so as to pass first through the negative and then through the lens; the image is received upon iodized paper, and developed by gallic acid.

On Printing Collodion Transparencies for the Stereoscope.—This may be done by using the camera to form an image of the negative in the mode described at the last page; but more simply by the following process:—Coat the glass, upon which the print is to be formed, with collodio-iodide of silver in the usual way, then lay it upon a piece of black cloth, collodion side uppermost, and place two strips of paper of about the thickness of cardboard and one-fourth of an inch broad, along the two opposite edges, to prevent the negative being soiled by contact with the film. Both glasses must be perfectly flat, and even then it may happen that the negative is unavoidably wetted; if so, wash it immediately with water, and if it be properly varnished no harm will result.

A little ingenuity will suggest a simple framework of wood, on which the negative and sensitive plate are retained, separated only by the thickness of a sheet of paper; and the use of this will be better than holding the combination in the hand.

The printing is conducted by the light of gas or of a camphine or moderator lamp, diffused daylight would be too powerful.

The employment of a concave reflector, which maybe purchased for a few shillings, ensures parallelism of rays, and is a great improvement. The lamp is placed in the focus of the mirror, which may at once be ascertained by moving it backwards and forwards until an evenly illuminated circle is thrown upon a white screen held in front. This in fact is one of the disadvantages of printing by a naked flame—that the light falls most powerfully upon the central part; and less so upon the edges, of the negative.

(From Humphrey's Journal, No. 17, Vol. 8.)

On the Use of Alcohol for Sensitizing Paper.

I have practised for some time the following simple method, which appears to me to be very superior for cleanliness and celerity in working, for depth of tone, and especially for purity of white in its results. By means of it T have produced very satisfactory results upon paper which was otherwise nearly worthless.

To your sensitizing solution (which should be not less than 60 grains to the ounce), whether simple nitrate or ammonio-nitrate, add 50 per cent, of alcohol. Float the paper upon the solution for 40 seconds.

This method answers equally well for albumenized or plain paper. You will find that the solution penetrates the paper which flattens instantaneously upon it. It becomes as transparent as though it were oiled, and every minute air-bubble or defect in the paper is rendered visible. Remove the air-bubbles by pressing upon the paper about an inch from the bubble, and thus driving it out under the paper. In doing this, if the solution flows partially over the back of the paper, shake it until the paper is wholly immersed, which will prevent any unevenness in printing. The paper reassumes its transparency in the toning bath, but it will dry a pure white. The sensitising solution will not become materially discolored even after frequent applications of albumenized paper. Should it become so much discolored as to give a dark hue to the paper, shake it in a bottle with two drachms Of animal charcoal and leave it a night to settle. It will filter clear. A very small portion of your solution may be made available in sensitizing a sheet of paper by pouring it upon a clean glass, the size of the paper or a little larger, which is carefully levelled and nicely laying down the paper upon it. This is useful when your solution is too small to float in your trays. The alcohol causes it to flow and be absorbed with perfect evenness.

To remove the papers from the solution and dry them:—Provide a dozen or more clothes-pins, of the kind that are supplied with a ring of india-rubber for a spring. Into the top drive a pin firmly and bend it to a hook. Lift a corner of the paper by passing under it the point of a quill tooth-pick, and attach to it one of the clothes-pins; lift the edge out by this, and attach another to the other corner. You may thus carry the sheet by the pins and hang it upon a line to dry without touching it with the fingers, a matter of some importance to Amateurs of the art, who must have unstained hands for their day's business.

It may be worth while here to add the following simple and economical method of printing, which I have found to surpass in convenience and afford all the advantages of the most expensive printing frames. Four common clothes-pins, such as work with a wire spring supply pressure enough for a ⁴/₄-plate. Lay your prepared paper upon the negative, and next to it about twenty separate leaves of thin common wrapping paper cut to the size of the negative; next a sheet of tolerably stiff and smooth writing paper, and lastly, a piece of glass as a back to the whole. Let the glass back be pushed from the lower edge of the papers about ¹/₂₀ of an inch, or just so far as to enable you to pinch the negative and papers with the thumb-nail and forefinger. Attach a pin to each corner and your negative is prepared for exposure. Now, to examine your picture without endangering its displacement:—remove the pins from one end, and place it, face downwards, on a table, the other end with pins attached projecting an inch beyond the edge. Hold down the back glass with the left hand, while with the right you remove the pins and pinch the papers and negative together between the forefinger and thumb-nail. Upon the smooth sheet of paper you can easily slide the back glass an inch from the edge. Hold it there, and on the uncovered margin attach three of the pins with as deep a bite as they will take. You may now examine your picture to within an inch of its margin, as you would turn over the leaves of a book. To replace the back, lay it again on the table and slide the back glass up to the pins before you remove them. The rest of the process is obvious.

The minutiÆ of my communication may excite a smile with some, but I shall always act upon the principle, that nothing is more out of place than an apology for minuteness in describing manipulations.

G. B. C.

Recovery of Silver from waste Solutions,—from the black Deposit of Hypo Baths, etc.

The manner for separating metallic silver from waste solutions varies according to the presence or absence of alkaline hyposulphite and cyanides.

a. Separation of metallic Silver from old Nitrate Baths.—The silver contained in solutions of the nitrate, acetate, etc.; may easily be precipitated by suspending a strip of sheet copper in the liquid; the action is completed in two or three days, the whole of the nitric acid and oxygen passing to the copper, and forming a blue solution of the nitrate of copper. The metallic silver however separated in this manner, always contains a portion of copper, and gives a blue solution when dissolved in nitric acid.

A better process is to commence by precipitating the silver entirely in the form of chloride of silver, by adding common salt until no further milkiness can be produced. If the liquid is well stirred, the chloride of silver sinks to the bottom, and may be washed by repeatedly filling the vessel with common water, and pouring off the upper clear portion when the clots have again settled down. The chloride of silver thus formed may afterwards be reduced to metallic silver by a process which will presently be described.

b. Separation of Silver from solutions containing alkaline Hyposulphites, Cyanides or Iodides.—In this case the silver cannot be precipitated by adding chloride of sodium, since the chloride of silver is soluble in such liquids. Therefore it is necessary to use the sulphuretted hydrogen, or the hydrosulphate of ammonia, and to separate the silver in the form of sulphuret.

Sulphuretted hydrogen gas is readily prepared, by fitting a cork and flexible tubing to the neck of a pint bottle, and having introduced sulphuret of iron (sold by operative chemists for the purpose), about as much as will stand in the palm of the hand, pouring upon it 1½ fluid ounces of oil of vitriol diluted with 10 ounces of water. The gas is generated gradually without the application of heat; and must be allowed to bubble up through the liquid from which the silver is to be separated. The smell of sulphuretted hydrogen being offensive, and highly poisonous if inhaled in a concentrated form, the operation must be carried on in the open air, or in a place where the fumes may escape without doing injury.

When the liquid begins to acquire a strong and persistent odor of sulphuretted hydrogen, the precipitation of sulphuret is completed. The black mass must therefore be collected upon a filter, and washed by pouring water over it, until the liquid which runs through gives little or no precipitation with a drop of nitrate of silver.

The silver may also be separated in the form of sulphuret from old hypo baths, by adding oil of vitriol in quantity sufficient to decompose the hyposulphite of soda; and burning off the free sulphur from the brown deposit.

Conversion of Sulphuret of Silver into Metallic Silver.—The black sulphuret of silver may be reduced to the state of metal by roasting and subsequent fusion with carbonate of soda; but it is more convenient, in operating on a small scale, to proceed in the following manner:—first convert the sulphuret into nitrate of silver, by boiling with nitric acid diluted with two parts of water; when all evolution of red fumes has ceased, the liquid may be diluted, allowed to cool, and filtered from the insoluble portion, which consists principally of sulphur, but also contains a mixture of chloride and sulphuret of silver, unless the nitric acid employed was free from chlorine; this precipitate may be heated in order to volatilize the sulphur, and then digested with hyposulphite of soda, or added to the hypo bath.

The solution of nitrate of silver obtained by dissolving sulphuret of silver is always strongly acid with nitric acid, and also contains sulphate of silver. It may be crystallized by evaporation; but unless the quantity of material operated on is large, it will be better to precipitate the silver in the form of chloride, by adding common salt, as already recommended.

On the Use of Test Papers.

The nature of the coloring matter which is employed in the preparation of litmus-paper has already been described at page 98.

In testing for the alkalies and basic oxides generally, the blue litmus-paper which has been reddened by an acid may be used, or, in place of it, the turmeric paper. Turmeric is a yellow vegetable substance which possesses the property of becoming brown when treated with an alkali; it is however decidedly less sensitive than the reddened litmus, and is scarcely affected by the weaker bases, such as oxide of silver.

In using test papers observe the following precautions:—they should be kept in a dark place, and protected from the action of the air, or they soon become purple from carbonic acid, always present in the atmosphere in small quantity. By immersion in water containing about one drop of liquor potassÆ in four ounces, the blue color is restored.

Test-papers prepared with porous paper show the red color better than those upon glazed or strongly sized paper. If the quantity of acid present however is small, it is not sufficient in any case simply to dip the paper in the liquid; a small strip should be thrown in, and allowed to remain for ten minutes or a quarter of an hour.

If the paper, on immersion, assumes a wine-red or purple tint, in place of a decided red, it is probably caused by carbonic aid gas: in that case the blue color returns when the paper is washed and held to the fire.

Blue litmus-papers may be changed to the red papers used for alkalies by soaking in water acidified with sulphuric acid, one drop to half a pint.

The Salting and Albumenizing Paper.

Take of

If distilled water cannot be procured, rain water or even common spring water[L] will answer the purpose. To obtain the albumen, use new-laid eggs, and be careful that in opening the shell the yelk is not broken; each egg will yield about one fluid ounce of albumen.

[L] If the water contained much sulphate of lime, it is likely that the sensitiveness of the paper would be impaired (?).