The camera and stand must be perfectly rigid. A large tripod head and a bigger clamping screw than usual are advantageous. When long extensions of camera are employed, either a strut attached to the tripod, or an independent support should be given to the back end of the camera to prevent vibration during exposure. The importance of guarding against vibration cannot be insisted upon too strongly. Any slight movement of the camera corresponds to a displacement of the short end of a lever, and the definition is impaired by a corresponding movement of the long end.

When it is possible to choose atmospheric conditions, the clearer and quieter the atmosphere the better. The clearest appearance of the atmosphere is not always to be relied upon in photographing distant objects ; tremor due to differences in the temperature of air currents, evaporation of moisture, etc., may frequently interfere with the definition given. If these effects are not visible to the eye, a careful examination of the image on the ground glass may reveal them, showing a slight appearance of "dancing." These drawbacks are most likely to occur in hot weather, when the atmosphere is seldom homogeneous.

Focusing must be performed with the greatest care and patience. When a considerable magnification is given, focusing is best carried out by means of the rack and pinion on the lens mount, which adjusts the separation of the elements of the system. A very slight movement of this will rapidly throw the image in or out of focus, and it is best to use a magnifier upon the ground glass. Focusing should always be performed with the largest aperture in the positive lens consistent with good definition, even if a smaller stop is finally employed.

The greater the magnification given, the less crisp in itself will the image appear on the ground glass, as compared to images given by ordinary positive lenses, but details which are invisible in the smaller image are now clearly visible in the magnified image, and can be photographed. The separate bricks, or blocks of stone, for example, in a building may each not be visible in a small image given by a lens of short focal length, yet when the primary magnification has taken place, we are able to obtain an image of sufficient size to render this separation possible. It is evident that calculation of the requisite magnification (or the equivalent focal length of the system necessary) is always possible when we know the size of certain details in the object, and its distance, in order to render the necessary details visible in the image. The scale of the image is determined by the focal length of the lens, and if we ascribe a limit to the actual size of the image for distinctness, the calculation is simple. As a numerical example, say a church clock 6 ft. in diameter is 500 yards distant, To determine the focal length of the lens necessary to give an image 1/4 of an inch in diameter, the scale is 1/4or 1/288 ; so that the focal length of the combination must be 500/288 yards, or roughly, 60 inches. Hence, if our positive element has a focal length of 10 inches, the image must be magnified 6 times by the negative element, or the screen must be placed at a distance of 5 times the focal length of the negative lens from it.

A focusing cloth of sufficient size should be employed to allow the corners of the plate to be examined, after the stop with which the photograph is to be taken has been inserted. If the corners are not fully illuminated, a greater extension of camera must be employed, and the operation of focusing repeated. When considerable extension of camera is employed, it may be found necessary to use a "Hooks Joint" handle in order to focus by an adjustment of the separation between the lenses.

If the positive lens has a high intensity, and only moderate magnification is given, the position of sharp focus may be roughly arrived at by means of the rack and pinion as before, the final focusing being carried out by the camera rack, which will now act as a "fine adjustment" in determining the plane of the sharpest focus. In the previous case the pencils will probably be too attenuated to permit of this method of focusing, as the screen may be moved a considerable distance without much alteration in the definition being traceable.

Fig. 66.

Focusing by means of Hook's universal joint; the handle may be made of any length.

Focusing by means of Hook's universal joint; the handle may be made of any length.

In focusing, attention must be paid to the varying "curvature of field" caused at different camera extensions ; the plane of the image being adjusted to equalise any difference between the planes of best definition for centre and edge of plate, when uniformity throughout is desired. If emphasis is required for any particular plane in the object, naturally the sharpest possible definition is given to this plane.

Where orthochromatic methods are advantageous in ordinary photography, they will be pronouncedly so in Telephotography. The utility of proper light filters (yellow screens, etc.) in the lens used in conjunction with orthochromatic plates is of the highest importance for distant work. When an object is very far off and brightly illuminated, the exposure will be found to be considerably shorter than is necessary for a comparatively near object: - The author has found that the increase of exposure presumed to be necessary when using an orthochromatic screen (when the theoretical increase is not more than 2 to 2J times), may be ignored; that is to say, normal exposure, as indicated by the exposure meter without the screen, may be given.

Generally speaking, shorter exposures are necessary in photographing distant objects than would be expected ; while longer exposures are necessary with the Telephotographic lens than for lenses of ordinary construction of the same focal length, when near objects are photographed, the increase of exposure being proportional to the square of the distance between lens and object.

The chief difficulty in distant Telephotographic work is the adequate rendering of contrast in the negative. At great distances we have not only to contend with the lack of homogeneity in the atmosphere already referred to, but with dust and other impurities in the air which reflect light, and contribute to a general haziness. Backed plates should always be used and slow emulsions are to be preferred.

It is advisable to develop slowly (the plate being carefully protected from light), adding the accelerator very gradually, but development should be carried a little farther than is necessary for a negative taken under ordinary conditions. As the tendency in using all Telephotographic lenses is towards inequality of illumination, or a falling off in the intensity of the image towards the edges of the plate, it is always advisable after first flooding the plate, to develop from the edges towards the centre, by allowing the developer to continually move round the edges in a circular manner in the developing dish.

The requirement of emphasising contrasts will suggest many kinds of developer to the photographer. The following formula given by

Front of the Church of San Zeno taken with an ordinary lens.

Plate XIX

Front of the Church of San Zeno taken with an ordinary lens.

{Copyright of and kindly lent by J. IV. Cruickshank, Esq., Haslemere.)

Working Data 121

Plate XX

Wheel Window, San Zeno, from same point of view as in Plate XIX., taken with 6-in. Ross Universal Symmetrical with 3-in. negative aitachment, 15 in. from back of lens to plate, stopped down to f/22. 38 seconds exposure.

[Copyright of and kindly lent by J. W. CruUhshank, Esq.)

Mr. Marriage has been found to produce admirable results in architectural work:

No. 1. - Pyrogallic acid ....

1 ounce

Sodium sulphite

3 "

Citric acid ....

1/4 "

Water to make ....

10 „

No. 2. - Washing soda ....

8 „

Sodium sulphite

10 „

Water to make ....

80 „

No. 3. - Bromide of potassium

I „

Water to make ....

10 „

No. 4. - Carbonate of ammonia

...... 1 ,,

Water to make ....

10 „

For use take 30 minims No. 1, 1/2 ounce No. 2, and 10 minims No. 3 make up to one ounce with water. If the plate is over exposed, add equal parts of 3 and 4 to the developer (say 15 minims of each per ounce of developer).

Comparative results by an ordinary and a Telephotographic lens of all ancient and interesting buildings, landmarks, etc, printed in a permanent process, stating the standpoint from which the photographs are taken, date. etc, should be sent to Sir Benjamin Stone, M.P., for the National Photographic Record collection. The ordinary photograph will show the intervening country, or buildings as the case may be, when taken from a distance, between the standpoint and the chief object of interest. These records in pairs will have an added interest in years to come.