macerated for at least half an hour, in a liquid prepared by mixing one part of the already described acid nitrate of mercury, with nine or ten parts of alcohol. A bright lemon yellow precipitate of basic hyponitrate of the protoxide of quicksilver falls, and the clear liquor is preserved for use. The macerated paper is removed from the alcoholic solution, and quickly drawn over the surface of diluted muriatic acid (one part strong acid to seven or ten of water), then quickly washed in water, and slightly and carefully dried at a heat not exceeding 212° of Fahr. The paper is now ready for being bleached by the rays of the sun; and in order to fix the drawing nothing more is required than to steep the paper a few minutes in alcohol, which dissolves the free bichloride of mercury. I must confess, however, that in my hands the process has not been so successful as it is described to have been by the author of it

It is perhaps necessary to remark, that we cannot multiply designs from an original hydriodated photograph. The yellow colour of the paper is of itself fatal to transfers, and independently of this, the wet hydriodic solution would immediately destroy any superposed photograph.

On

We have seen in a former chapter that the white photographic papers are darkened by the blue, indigo, and violet rays. the dark papers washed with the hydriodic salts in solution, the bleaching is effected most energetically by the violet rays: it proceeds with lessening intensity to the blue, while all the rays below the yellow have a darkening influence on the paper. This effect will be best illustrated by figure 78, in which is shownsomewhat exaggerated for the sake of distinctness-the very remarkable action which takes place; clearly establishing the fact first noticed by Wollaston, that the two extremities of the spectrum have different powers.

The remarkable manner in which the point of greatest intensity is shifted from the blue to the violet, when papers have but a very slight difference in their composition or mode of preparation, is an extremely curious point of philosophical inquiry. It will be evident from what has been said, that it is necessary the focus of the violet rays should be always chosen in using the hydriodated papers in the camera.

CHAPTER X.

GENERAL REMARKS ON THE USE OF THE CAMERA OBSCURA.

THESE remarks will apply with equal force to all the processes by which views of external objects can be obtained; but they have more especial reference to those highly sensitive ones, the Daguerreotype, the Talbotype, and the Collodion processes. It has already been stated

that a single achromatic lens, producing a large image, should be employed for motionless objects,where time is not of consequence. For a building, a statue, or the like, it is not of much consequence whether one minute or ten may be con

79.

sumed in the operation of obtaining its impression. With the human figure and animals the case is very different: the utmost concentration of the solar radiations is therefore required to ensure rapidity of action. This is effected by the double combination of lenses, which are usually mounted and adjusted as shewn in the above figure, 79. In Fig. 80, the single lens arrangement is shewn.

It is often of the utmost importance, to obtain definition of the objects, that all extraneous rays should be cut off; this is effected by means of a diaphragm of stops, which can be obtained to fit any lens. With this adjustment any sized aperture can be obtained.

80.

SECTION I.-BUILDINGS, STATUES, LANDSCAPES, AND FOLIAGE.

The great defect in nearly all the photographic pictures which are obtained is the extreme contrast between the high lights and

the shadows, and in many an entire absence of the middle tones of the picture.

In the very beautiful production of Mr. Buckle, of Peterborough, which we displayed in the Great Exhibition, there was a very remarkable degree of fine definition, united with a beautiful blending of the respective parts which constituted the picture. There was no glaring contrast between the lights. Those parts which were the most brilliantly illuminated were softened into the middle tones of the picture, and those again faded gradually into the deep shadows. In the works of M. Martin and M. Flacheron, whose processes I have given, the same harmonising of lights and shadows was generally found to exist.

The usual mistake with amateurs is that of selecting bright sunshine as the period for operating. It is thought, when a cathedral, for example, is brilliantly lighted up by sunshine, is the time for obtaining a photographic copy of it. A little reflection will convince the operator that this is the case only under particular conditions.

When the projecting parts of the building are flooded with sunshine, they cast the deepest possible shadows; consequently, in the photographic picture the prominent points would appear brilliantly white, and the shadows intensely dark.

It will be understood that I refer always to the positive, or completed picture.

A clear blue sky, reflecting its light upon a similar structure, produces less prominent illumination of the bold ornamental parts, and gives more light to those parts on which the shadows are cast. A photograph taken under such conditions of light and shade will be far more beautiful than the spotted productions which ordinarily result from the practice of operating when the sun is shining brightly on the object.

In the same manner, when the sun shines brightly on the leaves of trees, a very large quantity of light is reflected from their surfaces, the other parts appearing by contrast in almost absolute shadow. Hence, nearly all photographic views of forest scenery have more the appearance of scenes which have been sprinkled with snow than foliage glowing with sunshine.

An artist studies in his productions the most effective disposition of the lights and shadows, and it is by the harmonious, disposition of these that he succeeds in giving a peculiar charm to his productions. Nearly all photographic pictures, although they have the merit of strict truthfulness, appear to want this great beauty of art. This has mainly arisen from the circumstance that intense illumination has been sought for under the idea of producing the sharpest picture; and it is true that thus we do

obtain a very perfect definition of outline. Many productions are remarkable for this, and, indeed, reproduce with unnatural exactness all the minute details of the objects copied; whereas the human eye never sees this extraordinary sharpness of outline in nature; upon the edge of every object there are fringes of light which soften off their outlines, and subdue the general tone of objects, blending all harmoniously. Perhaps there is more than ordinary difficulty in producing this in a representation of nature which is effected by means of a lens. The artist may, however, do much all times, even of bright illumination, are not fitted for producing a picturesque photograph. Nature should therefore be looked at with an artist's eye, and the happy moment chosen when the arrangements of light and shade give the most picturesque effects, and when these are in a condition to be correctly reproduced according to the laws by which actinic influences are regulated.

:

SECTION II.-PORTRAITS FROM THE LIFE.

It is important for the production of a correct likeness that as small an aperture as possible should be used. By doing this there is great loss of light, and consequently the necessarily prolonged time must be compensated for by greatly increased sensibility in the plates.

It is also important that arrangements should be made to cut off from the lens all light proceeding from extraneous objects: this is best effected by the modes adopted by M. Claudet.

81.

The camera is placed, as shewn in section, Fig. 81, within an arrangement of curtains which, as will be seen in the vignette heading to this part, page 187, is capable of adjustment, so as

to have any required opening in front of the camera. The whole of this screen being mounted on rollers is easily moved; therefore the operator has it in his power to adjust the opening, and to shut off all adventitious radiations, thus securing the effectiveness of the rays proceeding directly from the sitter, or the object to be copied.

The sitter should be placed in the easiest possible position compatible with the arrangement of the body as nearly as is possible in a vertical plane. This is necessary, as the parts which are nearest the glass suffer a very considerable degree of distortion and enlargement. Of course great steadiness is required on the part of the sitter during the few seconds he submits to the operation of the photographer. It is usual to support the head by a rest fastened to the back of the seat, as shewn in fig. 82; but where the person can maintain a steady position without this, the result is generally the most satisfactory, the "rest" not unfrequently giving an air of stiffness to the sitter. In a great number of portraits a dark and unnatural shade is thrown under the eyes: this arises from the employment of a "top light." The light falling vertically produces the shadow of the brow over the eye, and gives a sombre character to the face. This is objectionable also, as being annoying to the sitter, who assumes in consequence a somewhat painful expression.

82.

Those who have attended to the analyses of the spectrum, included in the second division of our subject, have become aware that the radiations from all coloured objects are not alike. A long description would not render this so apparent as a single illustration. The Frontispiece represents, therefore, a female figure, to which purposely a blue face has been given, who carries on her head an earthenware vessel which has a general yellow colour, and whose dress consists of the lightest colours, yellow, red, and green; an exact copy of a photograph taken from such a figure is placed beside it, and the result is, a very white-faced female, from the intense action of the blues, clothed in dark dress from the want of chemical action in the radiations proceeding from the gay dress of the original. Hence it is of the utmost importance, particularly to ladies, that they should be directed to avoid in their dresses, when about to sit for their portraits, such colours as would produce darks for lights, and the contrary.