Ferric potassium oxalate 1 oz. or 5 parts[5]

Sodium sulphite 1 oz. or 4 parts Oxalic acid 1/4 oz. or 1 part Hypo solution (25 in 100) 5 oz. or 25 parts Water 20 oz. or 100 parts

[5] The formula in "parts" does not strictly correspond with that in ounces, but the difference is immaterial.

The constituents must be dissolved in water in the order given. The solution can be used at once and it keeps fairly well if protected from light, in well corked bottles filled up to the neck.

INTENSIFICATION.

Intensification is a process in which the opacity of the image is increased by adding some fresh matter, metallic or otherwise, to the reduced silver that constitutes the developed image.

The usual plan is to bleach the image by means of a solution of mercuric chloride (mercury perchloride or corrosive sublimate), which converts the dark-coloured silver into a white mixture of silver chloride and mercurous chloride, and this is subsequently treated with some re-agent which will reconvert the image into a dark product of greater opacity than the original.

It is absolutely essential to successful intensification that the negative be completely fixed and completely washed after fixing, for any trace of hypo left in the film will give rise to brown stains.

It is also important, in order to prevent stains of another sort and to secure uniform action, that the mercuric chloride solution be mixed with a small quantity of hydrochloric acid. Too much acid will cause frilling. If the negative has been dried it must be immersed in water for, as a rule, not less than half-an-hour, in order that it may be thoroughly and uniformly wetted.

MERCURIC CHLORIDE SOLUTION.

Mercuric chloride 1 oz. or 5 parts Hydrochloric acid 1-1/2 drachms or 1 part Water to make up to 20 oz. or 100 parts

When uniform intensification is required the negative is allowed to remain in this solution until it is completely bleached. If, however, it is desired to intensify the shadows more than the high-lights, the plate should be removed from the solution as soon as the shadows have bleached, and should be rapidly washed in order to stop the action on the more opaque parts of the image.

In either case the negative must be thoroughly washed after bleaching, and the water used must be soft water. Hard water tends to produce a precipitate of the mercury salt in the film, which may subsequently lead to stain or fog.

Perhaps the best plan of all, when constant results are desired, is to treat the bleached negative with the ferrous oxalate developer, which will gradually convert the white image into a black one, after which the plate is thoroughly washed and dried. It is recommended that the first water used for washing should be slightly acidified with oxalic acid.

Instead of using ferrous oxalate the bleached plate may be treated with a weak solution of ortol or metol to which some sodium carbonate (soda crystals) solution has been added, but _no sulphite_. After the image has blackened completely the plate is washed.

With any of these methods if the first intensification is not sufficient, the plate may be again bleached with the mercury solution and the process repeated.

An old method, frequently used, is to treat the bleached plate with dilute ammonia, which converts the white image into a dark brown one of very considerable printing opacity. The results are often very good, but are somewhat uncertain, since the precise effect obtained depends on the strength of the ammonia solution and the time during which it is allowed to act. With somewhat strong ammonia, allowed to act for a fairly long time, part of the intensification first produced is removed. This affects the shadows more strongly than the lights and the result is to increase the contrast of the negative, which is very useful for certain purposes.

The negatives intensified with mercury solution followed by ammonia are more liable to spontaneous change and deterioration than those intensified with mercury solution followed by one of the developers.

The latter, in fact, if properly washed, may safely be regarded as permanent.

URANIUM INTENSIFIER.--A very considerable degree of intensification can be obtained by the use of the uranium intensifier, which is very different in its mode of action, and is a little uncertain in its results. A solution containing potassium ferricyanide and a uranium salt, generally the nitrate, is applied to the negative, and a deposit of a deep orange-red colour is formed upon the silver image and very greatly increases its printing opacity. The great difficulty is to prevent this deposit forming on the whole of the film, and it is absolutely necessary that every trace of hypo should be washed out of the film. The addition of acetic acid to the solution not only promotes uniformity of action, but also helps to keep the shadows of the image clear.

FERRICYANIDE SOLUTION.

The same as for the ferricyanide reducer.

URANIUM SOLUTION.

Uranium nitrate 1 oz. or 10 parts Water to make up to 10 oz. or 100 parts

THE INTENSIFIER.

Uranium solution (1:10) 1 drachm or 5 parts Ferricyanide solution (1:10) 1 drachm or 5 parts Acetic acid (glacial) 2 drachms or 10 parts Water to make up to 2- oz. or 100 parts

The negative is placed in this solution and allowed to remain with occasional rocking until the degree of intensification is sufficient, which can only be learnt by experience. If it is seen that the deposit is beginning to form on the clear parts of the negative, the plate should be at once removed. After intensification the plates are well washed. If the water is "hard" the intensification will be slightly reduced during washing, and this is often useful in removing a slight stain over the whole of the plate. Treatment with water containing a small quantity of ammonia or sodium carbonate removes the whole of the deposit, but leaves the original image slightly reduced and also partially altered in composition.

VARNISHING.

A negative after been thoroughly dried may be used for printing without any further treatment, especially if only a few prints are required and the ordinary ready sensitized papers or emulsion papers are used. It is, however, better to protect the negative from mechanical as well as chemical injury by means of a film of hard varnish or collodion.

Many excellent negative varnishes can now be purchased, and the general mode of application is the same. The negative must be thoroughly dry, and in order to secure this and to make the varnish flow more easily, the negative is very carefully heated in front of a fire or over a small stove until it is just warm, but not hot. The negative is best supported by means of a pneumatic holder held in the left hand, and a fairly large pool of varnish (the exact amount can only be learnt by experience) is poured on the plate somewhat towards the right-hand top corner, and by carefully tilting the plate it is made to run first to the nearest corner, then along the edge to the further left-hand corner down to the nearer left-hand corner, and back to the right-hand bottom corner, from which it is poured into a bottle. The plate is gently rocked whilst it drains into the bottle, and as soon as the varnish ceases to drop the plate is again carefully warmed until the back of it is just too hot for the back of the hand to bear, after which it is placed in a rack to cool.

It is necessary that the varnish should be quite clear and free from any solid particles, and if necessary it must be filtered through a plug of cotton wool moistened with alcohol and placed in the apex of a glass funnel which is resting in the neck of a clean and dry bottle. Since dust may fall into the varnish whilst it is on the negative, it is the best plan to pour the excess of varnish off the negative into a second bottle instead of back into the first, out of which it was poured. To put it in another way, one bottle should be kept for the clear varnish, and a second bottle for the varnish poured off the plate. When the second bottle is full, its contents are filtered into the first bottle for use again.

Instead of varnish, a film of collodion, toughened by the addition of a few drops of castor oil, and known as "leather" collodion, may be used. The collodion is applied to the plate in the same way as varnish except that the plate is not warmed.

_C. H. Bothamley._

[Illustration]

_Lenses._

[Illustration]

Photographs of flat objects such as leaves, lace, drawings, etc., can be made by simply putting the object on the sensitive surface and exposing the arrangement to light. But this method will not serve if the photograph is wanted of any other size than the original, nor with solid objects of any size, except perhaps in the production of full-size profiles of faces. It is therefore quite the exception in photography to "print" directly from the object itself, and the only alternative is to produce an image on the sensitive surface.

All illuminated objects reflect light and so become for practical purposes sources of light, just as the moon shines, as we say, although it only shines because it is shone upon by the sun. The simplest source of light to consider is a point of light, and if we can get a dot of light on a white surface from a point of light we have at once an image of that point of light. The smaller the dot the sharper or more perfect is the image, the larger the dot the more diffused or fuzzy is the image. It is impossible by any known means to get the dot so small that it is an actual point, that would be absolute perfection, and on the other hand there is no size of the dot at which it can be definitely said that it ceases to be an image. Every point of an illuminated object is a point of light, and fine definition consists in keeping these points separate in the image. So far as the dots overlap they are confused. Confusion, or diffusion, or fuzziness is sometimes desirable, as for example in a portrait, which may be excellent although it is impossible to distinguish in the picture the individual hairs on the person's head.

[Illustration: Fig. 1.]

The simplest means for getting an image is a small hole in an opaque screen. In fig. 1, two points of light, A and B, shine through the hole in the screen S and produce two dots of light, _a_ and _b_, on the surface T. The two pencils of light do not practically interfere with each other although they pass through the same small hole, nor would any greater number; so that an illuminated object, which may be regarded as consisting of an infinite number of points of light, would give an image on the surface T. The disadvantages of a small hole, or "pinhole," for the production of images are (1) it must be so small that it lets very little light through and therefore gives a very feeble image, (2) that it can never give a sharp image. The first disadvantage is obvious. With regard to the second, a little consideration will show that the image of a point must be larger than the hole itself, it is always larger though it may have a central brighter part that is smaller. If the hole is reduced in size beyond a certain limit, it gives an increased spreading of light on the surface, so that a sharp image can never be produced.

[Illustration: Fig. 2.]

Now the function of a lens is to obviate these drawbacks as far as possible; namely, to let more light through and form a brighter image, and to give sharper definition. In figure 2, the lens L collects all the light that falls upon it from the point B, and condenses it to the point _b_ on the surface T. The light from the point A that falls on the lens is also condensed and would be brought to a point or "focus" at _a_ beyond the surface T, but on the surface the light forms a patch of considerable size. Suppose that the lens is thirty times the diameter of the pinhole its area is 900 times as large, and the light that falls upon it is 900 times as much as the light that passes through the hole. Such an enormous gain of light is worth so much that photographers willingly put up with the very many imperfections of lenses for the sake of it, and if to this gain there is added the superior definition that is possible, it will be seen that lenses are indispensable to the photographer. To take a Daguerreotype portrait with a pinhole might have required several days if not weeks exposure of the plate and therefore would have been impossible, so that the gain in brightness of image is a great deal more than a mere convenience.

It will be observed in figure 1 that both points of light, A and B produce images on the surface T, although they are at different distances from it, but in fig. 2, although the effect of the lens is to concentrate the light from both points to two other points, one of these is beyond the surface T. This is a disadvantage inherent in lenses. They have so many other imperfections or "aberrations" that it is desirable to consider these separately. The reader should bear in mind that the one aim of opticians in perfecting lenses is to concentrate as much light as possible from each point in the object to a corresponding point, or as small as possible a dot, in the image, and the image should be flat because the plates used in photography are flat.

[Illustration: Fig. 3.]

_Spherical Aberration._--The surfaces of lenses are always ground to spherical curves, and this fact makes it impossible for a single lens, such as that shown in figure 2, to bring to a point all the light that falls upon it from a point. If a pencil of light passes through a piece of glass with sloping sides it is bent or "refracted" towards the thicker part of the glass, and the greater the angle of inclination of the two sides the more is it refracted from its original path. In figure 3 it is clear that the two sides of the lens shown in section are inclined to each other at a continually increasing angle as they approach each other at the edges of the lens. The refracting effect of the lens increases from the centre outwards, and it increases to a greater extent than is necessary to bring the incident light to a point. The focus of the pencils of light that pass through the edges of the lens is nearer to the lens than the focus of the pencils that pass through its central part. In the figure two foci are shown, _a_ and _b_, but of course, in fact, intermediate parts of the lens produce intermediate foci, and what should be a point in the image, is spread out into a line on the axis of the lens, and all along this line is surrounded with the light that either is coming to a focus or that has come to a focus and has spread out again. On a screen placed at _b_ there would be a point of light surrounded by a halo, while at _a_, nearer the lens, the central focus or point is surrounded by a brighter or more condensed light, and the appearance is of a circular patch of light with a brighter boundary. This is positive spherical aberration. Negative spherical aberration is due to over correction, the focus of the light passing through the margins being furthest from the lens, and the appearances on a screen are of course reversed.

_Chromatic Aberration._--When light is refracted, that is bent out of its original path by a single piece of glass, it is not refracted as a whole, but each constituent behaves as if none other were present. Ordinary white light or daylight is a mixture of many coloured lights as seen in the rainbow, and when refracted, the blue is bent more than the green, the green more than the yellow, and the yellow more than the red. So that using a single lens the focus of the blue light is nearer the lens than the focus of the red light and the others come in between. In figure 4 this is represented in an exaggerated degree to make it more distinct. It will be observed that a screen placed at the focus of the blue light will show a reddish margin and if removed further from the lens the margin or halo will be bluish.

[Illustration: Fig. 4.]

These two aberrations, spherical and chromatic _were_ the principal faults that opticians had to deal with, because they affect the whole of the image, even the very central parts. But in photography it is necessary to get an image of a very large size as compared with the focal length of the lens, and there are some faults that only begin to show themselves at a little distance from the centre of the image and increase as the distance from the centre is greater. These aberrations were, practically speaking, incurable until a few years ago, but as recent optical advances have provided kinds of glass by the use of which they may be eliminated, or nearly so, they have become of practical importance. They are astigmatism and curvature of field.

_Astigmatism and Curvature of Field._--If a diagram of suitable size is made with a series of concentric circles and radial lines upon it, and the centre of it is arranged exactly opposite the centre of the lens, and in a line with the centre of the focussing screen, the screen and diagram being parallel, then if the lens suffers from astigmatism it will be found impossible to get the outer circles and the radial lines where they cross them simultaneously focussed.

Where this difficulty begins the astigmatism begins, and the greater the difference there is between the focal planes of the radial lines and the circles, the greater is the astigmatism. It will probably be found with any of the older types of lenses that neither is in focus at the same time that the centre of the diagram is, but that the screen must be racked in; this is due to curvature of field, and the difference between the curvature of field for the circles and the radial lines is due to astigmatism. In the older lenses a flatter field could only be obtained by the introduction of astigmatism, but now by the employment of the new glasses made at Jena, it is possible to practically eliminate astigmatism, and still keep the field flat.