Chromatic Aberration is another difficulty that opticians have to contend with in the manufacturing of lenses. It will be remembered, that in a former page (133) a beam of light is decomposed by pa.s.sing through a gla.s.s prism giving seven distinct colors--red, orange, yellow, green, blue, indigo and violet.
Now, as has been said before, the dissimilar rays having an unequal degree of refrangibility, it will be impossible to obtain a focus by the light pa.s.sing through a double-convex lens without its being fringed with color. Its effect will be readily understood by reference to the accompanying cut.
[Ill.u.s.tration: Fig. 8b (amdg_8b.gif)]
If L L be a double convex-lens, and R R R parallel rays of white light, composed of the seven colored rays, each having a different index of refraction, they cannot be refracted to one and the same point; the red rays, being the least refrangible, will be bent to r, and the violet rays, being the most refrangible, to v: the distance v r const.i.tutes the chromatic aberration, and the circle, of which the diameter is a l, the place or point of mean refraction, and is called the circle of least aberration. If the rays of the sun are refracted by means of a lens, and the image received on a screen placed between C and o, so as to cut the cone L a l L, a luminous circle will be formed on the paper, only surrounded by a red border, because it is produced by a section of the cone L a l L, of which the external rays L a L l, are red; if the screen be moved to the other side of o, the luminous circle will be bordered with violet, because it will be a section of the cone M a M l, of which the exterior rays are violet. To avoid the influence of spherical aberration, and to render the phenomena of coloration more evident, let an opaque disc be placed over the central portion of the lens, so as to allow the rays only to pa.s.s which are at the edge of the gla.s.s; a violet image of the sun will then be seen at v, red at r, and, finally, images of all the colors of the spectrum in the intermediate s.p.a.ce; consequently, the general image will not only be confused, but clothed with prismatic colors."
To overcome the difficulty arising from the chromatic aberration, the optician has only to employ a combination of lenses of opposite focal length, and cut from gla.s.s possessing different refrangible powers, so that the rays of light pa.s.sing through the one are strongly refracted, and in the other are bent asunder again, reproducing white light.
To the photographer one of the most important features, requiring his particular attention, is, that he be provided with a good lens. By the remarks given in the preceding pages, he will be enabled, in a measure, to judge of some of the difficulties to which he is occasionally subjected. We have in this country but two or three individuals who are giving their attention to the manufacture of lenses, and their construction is such, that they are quite free from the spherical or chromatic aberration.
CHAPTER V.
To make Plates for the Daguerreotype--Determining the Time of Exposure in the Camera--Instantaneous Process for Producing Daguerreotype--Galvanizing the Daguerreotype Plate--Silvering Solution--Daguerreotype without Mercury--Management of Chemicals--Hints and Cautions--Electrotyping--Crayon Daguerreotypes--Illuminated Daguerreotypes--Natural Colors in Heliography--Multiplying Daguerreotypes on one Plate--Deposit in Gilding--Practical Hints on the Daguerreotype.
TO MAKE PLATES FOR THE DAGUERREOTYPE.
I do not give the method employed by our regular plate manufacturers; this is not important, as the operator could not possibly profit by it from the fact of the great expense of manufacturing. The following will be found practical:
Procure a well planished copper plate of the required size, and well polish it, first with pumice stone and water, then with snake stone, jewelers" rouge. Plates can be purchased in a high state of preparation from the engravers. Having prepared the copper-plate, well rub it with salt and water, and then with the silvering powder. No kind answers better than that used by clock-makers to silver their dial-plates. It is composed of one part of well washed chloride of silver, five parts of cream of tartar, and four parts of table salt.
This powder must be kept in a dark vessel, and in a dry place. For a plate six inches by five, as much of this composition as can be taken up on a shilling is sufficient. It is to be laid in the centre of the copper, and the figures being wetted, to be quickly rubbed over every part of the plate, adding occasionally a little damp salt. The copper being covered with the silvering is to be speedily well washed in water, in which a little soda is dissolved, and as soon as the surface is of a fine silvery whiteness, it is to be dried with a very clean warm cloth. In this state the plates may be kept for use. The first process is to expose the plate to the heat of a spirit flame, until the silvered surface becomes of a well-defined golden-yellow color; then, when the plate is cold, take a piece of cotton, dipped in very dilute nitric acid, and rub lightly over it until the white hue is restored, and dry it with very soft clean cloths. A weak solution of the hydriodate of potash, in which a small portion of iodine is dissolved, is now pa.s.sed over the plate with a wide camel"s hair brush. The silver is thus converted, over its surface, into an ioduret of silver; and in this state it is exposed to light, which blackens it. When dry, it is to be again polished, either with dilute acid or a solution of carbonate of soda, and afterwards with dry cotton, and the smallest possible portion of prepared chalk: by this means a surface of the highest polish is produced. The rationale of this process is, in the first place, the heat applied dries off any adhering acid, and effects more perfect union between the copper and silver, so as to enable it to bear the subsequent processes. The first yellow surface appears to be an oxide of silver with, possibly, a minute quant.i.ty of copper in combination, which being removed leaves a surface chemically pure.
Another Method.--The best and simplest mode with which we are acquainted is to divide an earthenware vessel with a diaphragm: one side should be filled with a very dilute solution of sulphuric acid, and the other with either a solution of ferroprussiate of potash, or muriate of soda, saturated with chloride of silver. The copper plate, varnished on one side, is united, by means of a copper wire, with a plate of zinc. The zinc plate being immersed in the acid, and the copper in the salt, a weak electric current is generated, which precipitates the silver in a very uniform manner over the entire surface.
Another Method.--A piece of bra.s.s or of polished copper, bra.s.s is preferred, is perfectly planished and its surface made perfectly clean.
A solution of nitrate of silver, so weak that the silver is precipitated slowly, and a brownish color, on the bra.s.s, is laid uniformly over it, "at least three times," with a camel"s hair pencil.
After each application of the nitrate, the plate should be rubbed gently in one direction, with moistened bitartrate of pota.s.sa, applied with buff. This coat of silver receives a fine polish from peroxide of iron and buff. Proofs are said to have been taken on it, comparable with those obtained on French plates.
M. SOLIEL"S PROCESS FOR DETERMINING THE TIME OF EXPOSURE IN THE CAMERA.
M. Soliel has proposed the use of the chloride of silver to determine the time required to produce a good impression on the iodated plate in the camera. His method is to fix at the bottom of a tube, blackened within, a piece of card, on which chloride of silver, mixed with gum or dextrine, is spread. The tube thus disposed is turned from the side of the object of which we wish to take the image, and the time that the chloride of silver takes to become of a greyish slate color will be the time required for the light of the camera to produce a good effect on the iodated silver.
INSTANTANEOUS PROCESS FOR PROCURING DAGUERREOTYPES.
The following method of producing Daguerreotypes has by some been named as above. Most experienced operators have been long acquainted with the effect of the vapor of ammonia upon the chemically coated plate. I will here insert Mr. W. H. Hewett"s plan of proceeding. This gentleman, in referring to it (published in 1845), says:
"This improvement consists in using the vapor of ammonia, as an object to accelerate the action of light upon the plate. The effect is produced upon a simple iodized plate, but still more upon a plate prepared in the ordinary way, with both iodine and bromine. By this means, the author obtained impressions instantaneously in the sunshine, and in five to ten seconds in a moderate light; and he hopes to be able to take moving objects. It can be applied by exposing the prepared plate over a surface of water, to which a few drops of ammonia have been added (sufficient to make it smell of ammonia); or the vapor can be introduced into the camera during the action. In fact, the presence of ammonia, in the operating-room, appears to have a good effect, as it also neutralizes the vapors of iodine and bromine that may be floating about, and which are so detrimental to the influences of light upon the plate."
GALVANIZING THE DAGUERREOTYPE PLATE.
In consideration of the importance of galvanized plates, I shall endeavor to give as plain and concise a manner of manipulation as possible. For some time it was a question among the operators generally, as to the beneficial result of electrotyping, the Daguerreotype plate, but for a few years past our first operators have found it a fact, that a well electro-silvered surface is the best for producing a portrait by the Daguerreotype.
From my own experiments, I have found that a plate, by being galvanized, can be rendered more sensitive to the operation of the light in proportion of one to five, viz.: if a plate as furnished by the market, be cleaned, polished, coated and exposed in the camera, if the required time to freely develop an impression be ten seconds, a similar plate prepared in like manner and galvanized, will produce an equally well-defined image in eight seconds. In connection with this subject, there is one fact worthy of notice; a plate with a very heavy coating of pure silver, will not produce an equally developed image, as a plate with a thinner coating, hence the thin coating, providing it entirely covers the surface, is the best, and is the one most to be desired. The experiment is plain and simple. Let the slate receive a heavy or thick coating by the electrotype, then polish, coat, expose in the usual manner, and the result will be a flat, ashy, indistinct impression; when, on the other hand, the thin coating will produce a bright, clear and distinct image, with all the details delineated.
The style of battery best for the purpose has been, and now is, a question of dispute among operators; some preferring the Daniell battery to Smee"s. Some claim the superiority of the first from its uniformity of action; others, of the latter, for its strength. I consider either good, and for the inexperienced would prefer the Daniell. This is more simple in its construction, while it has certainty in action. The more skillful electrotyper would prefer Smee"s, and this is the one most generally in use. I would remark that the plan of galvanizing plates should be followed by every operator, and when once thoroughly tested, no one will abandon it.
SILVERING SOLUTION.
To any desired quant.i.ty of chloride of silver in water add, little by little, cyanide of pota.s.sium, shaking well at each addition, until all the cyanide is dissolved. Continue this operation, and add the cyanide, until all the precipitate is taken up and held in solution.
This solution is now ready for the plate-cup. Enough water may be added to cover any sized plate when held perpendicular in the cup. The strength of the solution may be kept up by occasionally adding the chloride of silver and cyanide of pota.s.sium. There should alway be a very little excess of the cyanide.
The plate should be well cleaned and buffed, and the solution well stirred before it is immersed. Care should be observed to keep the solution clean, and allow no particle of dust to come in contact with the surface of the plate. The plate is now to be attached to the pole of the battery.
After remaining a short time, it a.s.sumes a blue color; take it out, rinse freely with pure water, then dry with a spirit lamp, and it is ready for buffing. Buff and coat in the usual manner. Some operators are in the practice of immersing the plate in the solution and buffing twice. This additional silvering is no improvement wherever there has been a proper first coating.
Sometimes the operator is troubled with streaks or sc.u.m on the plate.
This may arise from three causes, all of which experience must teach the experimenter to avoid; first, too great an excess of cyanide in the solution; second, a lack of silver; third, the current too strong.
Another annoyance arises from the solution being dirty and the dirt collecting on the surface. When this is the case, the dirt is sure to come in contact with the surface of the plate as it is plunged into the solution, and the result is a sc.u.m that it is difficult to dispose of.
This can be prevented only by frequent filtering. One thing should always be borne in mind in electrotyping Daguerreotype plates--that in order to secure a perfectly coated surface, the plate should be perfectly cleaned. In this point, many who have tried the electrotype process have failed, attributing their ill success to other than the proper cause.
DAGUERREOTYPES WITHOUT MERCURY.
The following process possesses some interest, and is worthy a trial from operators. M. Natterer, of Vienna, discovered a process for obtaining proofs on iodized plates with the chloride of sulphur, without the use of mercury. A plate of silver is iodized in the usual manner, and then placed on the top of a vessel six or eight inches high, having at the bottom, in a small cup, a few drops of chloride of sulphur; it should remain exposed to the action of the vapor until the sombre yellow color is changed to a red, after which it is brought to a focus in the camera, where it is exposed to the light in the camera, for about the time necessary to produce an ordinary daguerreotype. The plate is then taken out and examined in the camera by the light of a candle. It often occurs that no trace of the image is as yet perceptible, but if the plate is heated by placing over a spirit lamp the unprepared side, or if left for some time in the dark, or, lastly, if exposed only a few seconds to a weak, dimmed light, the positive picture then appears with all its shades. Of these three modes of bringing out the image, the second is superior to the others.
MANAGEMENT OF CHEMICALS.
It is necessary, first of all, to know that you have a chemical which is capable of producing good results when in skillful hands. For this reason it is best to prepare your own quick, after some formula which is known to be good. Those quick-stuffs which contain chloride of iodine are noted for their depth of tone while they probably operate with less uniformity than those which are dest.i.tute of it. For operating under ordinary circ.u.mstances, especially with an inferior light, probably no accelerator is more quick and sure than Wolcott"s.
It also produces a very fine, white pleasing picture, though lacking that depth of impression so much to be desired. The dry quick operates with surety, and its use is simple and easy, producing an impression much like Wolcott"s. For those having a good and permanent light, however, we would recommend a chemical giving more body to the impression.
There is a cla.s.s of accelerators called sensitives, claiming to work in from three to ten seconds, which, however, will be found very little, if any, more sensitive than this. We frequently work it with the ordinary coating in twelve and fifteen seconds. The manner in which the sensitives are worked is by coating very light. In this way, a flat, shallow picture is obtained in a few seconds; and the same can be done with any of the more volatile quicks.
It is a fact not generally known, that a plate coated in a light chemical room is more sensitive than when coated in darkness. By admitting a free, uniform light, and exposing the plate to it a few seconds after coating, then timing short in the camera, a very light, clear impression is obtained. The time in the camera is reduced in proportion to the previous action of light. The shades, of course, are destroyed, and the tone injured; still, for taking children, we have succeeded better by this method than by the use of "sensitives." The discovery of this principle was accidental, while operating where the direct ray s of the sun, entering the window just before sunset, fell on the curtain of our dark room, rendering it very light within.
The selection of iodine is not unimportant. Reject, at once, that which has anything like a dull, black, greasy appearance; and select that which is in beautiful large crystalline scales, of a purple color, and brilliant steel l.u.s.tre.
Solarization, and general blueness of all the light parts of the picture, were formerly great obstacles to success, though now scarcely thought of by first-cla.s.s artists. Beginners in the art, however, are still apt to meet with this difficulty. It is occasioned by dampness in the iodine box, which causes the plate to become coated with a hydro-iodide of silver, instead of the iodide. The remedy is in drying your iodine. If in summer, you can open your box and set it in sunshine a few minutes; or if in winter, set it under a stove a short time. The true method, however, is to dry it by means of the chloride of calcium. It has such a remarkable affinity for water, that a small fragment placed in the open air, even in the dryest weather, soon becomes dissolved.
Take one or two ounces of this chemical, heat it in the drying bath, or in a hot stove, to perfect dryness; place it in a small gla.s.s toy dish, or large watch crystal, and set it in the centre of your iodine box.
Take this out and heat to dryness every morning. Adopt this process, and with your mercury at a high temperature, you will never be troubled with blue pictures.
Young operators are apt to impute all want of success in operating to their chemicals, even though the cause is quite as likely to be elsewhere. Failure is quite likely to occur from dampness in the buffs, or in the polish; it is therefore necessary to be constantly on the guard in this quarter. With a view to this, always sc.r.a.pe your buffs with a dull knife, or with one blade of your shears, the first thing in the morning, and after brushing them thoroughly, dry them, either in the sun, by a stove, or in the buff-dryer. It is equally important that the polish and the brush should be kept dry.
Want of success may arise from vapors of iodine or bromine in the camera box, mercury bath, or even in the buffs. It is incredible how small a quant.i.ty of these vapors will affect the effect of light when coming in contact with the plate, after or during the exposure in the camera. It is therefore necessary to be cautious not to mix chemicals, nor open your boxes or bottles in your room, but take them out to do it. Never hurry the operation through from lack of confidence in the result. The fact of anything being out of order, forms no excuse for slighting the process. If unsuccessful, do not pursue the same course every trial, but vary with a view to detect the cause of the difficulty.
In case of a long series of failures, inst.i.tute a regular course of investigation, after this manner, commencing where the trouble is most likely to occur:
1. Are the plates well cleaned?