It is naturally impossible to give even an outline of all the many and varied applications to which photography can be and has been applied. In this article it is proposed to give a few of them, more for the sake of informing the student what has been done, than for teaching him the practical method of working them. The method of securing the automatic registration of barometers, thermometers, and magnetometers should command our attention first. It will be necessary to divide these into two classes which require different treatment. A mercurial thermometer may be taken as the representative of the first class.

Supposing we have a darkened chamber, in the side of which is a slit of just sufficient dimensions to allow the bore of the capillary tube to fill it, and that light can only have access to that chamber after passing through that slit when so closed, it is manifest that if a strip of sensitive paper be caused to pass gradually behind such a thermometer tube the different height of the mercury will be registered, owing to the opacity of that fluid to light. If the supply of paper be properly regulated it is also manifest that the height of the mercury at any particular instant will be known. Since daylight is not always available, resort must be had to artificial light to impress the sensitive paper, and a suitable process of development employed.

Such a method exists for registering the movements of this class of instruments, the details of apparatus and manipulation being altered to suit each individual case. There are, however, other instruments to which such would be totally inapplicable. As an example, we may take the magnetometer. The oscillations of the suspended magnet as used for measuring the horizontal or vertical components of the earth’s magnetism are very minute, so minute indeed that they can scarcely be perceived by the eye. If to one of these magnetometers, however, we attach a very small and light mirror, the plane of which is at right angles to the axis of the magnet, and cause a beam, proceeding from a source of light, to pass through a small aperture, thence to a fixed lens on to the mirror, which reflects the beam of light on to a screen so placed that the image of the aperture is in the focus of the lens, any small deviation of the magnetometer will cause the beam of light to deflect on the screen. The amount of the deflection will be dependent on the focal length of the lens, and the distance of the aperture and screen from the mirror. Suppose the screen to be opaque, and that a slit is cut in it in the direction that the deviation of the beam would take, and lying in the same plane as the deviation, and that a strip of sensitive paper moves behind that slit in a direction at right angles to its length, then at each instant the position of the beam of light will be registered on the paper. On developing the image we shall have a sinuous line corresponding to the deflections of the magnetometer at every time of day and night, the reading of the time being dependent on the rate at which the paper travels.

For meteorological purposes we may also hope that photography will be more utilized than it has hitherto been. Mr. A. Mallock, at the meeting of the British Association at Plymouth, has shown a way in which it may be made subservient to ascertaining the heights of clouds.

In military science it is only necessary to call to mind the service that the pigeon post performed during the siege of Paris. A large series of letters were printed on one sheet, and then photographed to a very small scale on collodion pellicle. Such pellicles, measuring about 6×2 centimeters, were tied to pigeons, which when liberated carried the dispatch to Paris, where they had been trained. On arrival the collodion pellicle was detached from the pigeon, placed in a lantern, and the letters transcribed and sent to the various addresses. Of so much use was this pigeon post that the German military authorities have established a regular service of pigeons in the chief fortresses of the empire, which would be used in case of investment or siege by a hostile army.

During the investigation of the action of torpedoes the use of photography was also largely brought into requisition by the writer in order to ascertain the work that was expended by different charges of gun cotton. The method adopted was roughly this: A mine having been laid down at a known depth and position in water, a scale was placed over it, and photographed from the position the camera was to occupy. On the explosion of the gun-cotton or powder an instantaneous exposure was given to a specially sensitive plate, and the height, breadth, and general form of the resulting column of water was obtained from the photograph after comparing it with the photographic scale.

At Shoeburyness, again, a regular staff of photographers is kept in order to photograph all the experimental work carried on by the artillery against iron shields, &c., and the series of such pictures has been able to convey more to the minds of committees than elaborate drawings could do.

We can not conclude these applications of photography without recalling the fact that it has proved exceedingly useful in the repression of crime. The portrait of every convict is taken by an authorized photographer in each convict establishment, and when necessity arises prints from such negatives are produced by the hundred and distributed, in order that the various police authorities may be enabled to identify a criminal who may have happened previously to be placed under their surveillance.