difficult to take landscapes, while portraits would be out of the question. We therefore resort to what are termed development processes. In these a change in the chemicals is induced by an exposure of a few minutes or seconds, or sometimes of half or quarter of a second, depending on the amount of light, and the description of lens employed.

We are now considering the process for taking negatives which are on glass. In order to hold the sensitive substances on the glass, the glass is coated with collodion, that is, a solution of gun cotton or pyroxyline in a mixture of ether and alcohol. As this substance only plays a passive part, holding the sensitive salt of silver in a finely divided state on the surface of the glass, it is unnecessary to enter into its chemical composition.

In the collodion process, after having exposed the plate, on taking it into the dark room, we find no visible change on any part; a change has been impressed by the action of light, but it may be only a molecular change. This change is continued by the action of the developer, that is, a liquid is poured over the plate and gradually blackens those portions on which the light has acted. The image appears and becomes more and more intense as the development proceeds.

The developer is a compound liquid, the components of which, by their chemical reaction, are gradually decomposing one another, and depositing a darkening substance, which, in consequence of the attraction of cohesion, falls only on those parts that have been changed or modified by the action of light. The developer also, in some cases, continues or assists the change which has been commenced in the sensitive film, and then deposits the silver on the altered parts.

There is a radical difference between the chemistry of sun-printing and that of the development processes. In the sun-printing, light was gradually causing certain bodies to decompose, and reunite to form new compounds; in the development processes, light almost immediately impresses an image, and the developer merely throws down dark matter on that image. Thus, if you touch a dry plate of glass with your finger and then breathe on it, the breath acts as a developer, for it will bring out the markings which were invisible before, by depositing only on the other parts. And again, a photograph which has been partially developed, may be fixed, washed, and dried, and kept any length of time, and the development again proceeded with by a fresh application of the developer; for the silver, while being precipitated from it, will fall only on that already formed. Light has the same power of so modifying the surface of bodies that vapours are precipitated differently on the parts which have been under its influence from those which have not. If a plate of mirrorglass be exposed in the camera and then breathed upon, the breath, by settling more strongly on the parts affected by the light, will develope an image. This image, which is invisible till developed, is commonly called the latent image.

You will understand from this, how the sensitive surface of iodide of silver, when exposed in the camera, has parts of its surface modified by the light, and a change, perhaps only a molecular change, but sufficient to develope upon, induced in the chemicals, though no visible image has appeared.

The action of the developer is seen if we mix protosulphate of iron and nitrate of silver; the nitrate of silver is decomposed, in consequence of the strong affinity of the protosulphate of iron for oxygen, which then becomes persulphate, precipitating silver and setting nitric acid free. And again, nitrate of silver is decomposed by pyrogallic acid, which, being a powerful deoxydizer, or absorber of oxygen, precipitates silver by depriving it of its oxygen.

The nitrate of silver would undergo a general decomposition, and blacken the whole plate if there were nothing present to check the action of the developer, and retard the decomposition of the nitrate of silver. organic acid, such as citric or acetic, is therefore added to render the precipitation of silver more gradual.

Now, the particles of silver, which in a very finely-divided state are being precipitated, from their cohesive attraction for one another have a great tendency to unite together while they are floating about in the developer, and to accumulate on any particles already formed on the plate. And as the salts of silver in the sensitive film, if they have not been already reduced to the metallic state where acted on by the light, have been modified by it, and a change induced, these points act as centres of attraction for the falling particles, which, uniting together, fall on them and continue accumulating as long as there is nitrate of silver in the developer to be decomposed and supply fresh silver.

But to return to the salt of silver employed in this case for the sensitive film, it is not the chloride of silver that we employed for the paper, but the iodide or bromide of silver. Iodine and bromine, as I mentioned, were affected in the same manner as chlorine by light, that is, in having their affinities for hydrogen increased, though not to the same extent as chlorine. And it appears curious at first why, when we want a more sensitive substance, we employ one which is less affected by light.

The theory is rather complicated, and not very certain, but it is probably this:

Pure moist iodide of silver is not affected by light, for the chemical affinity for hydrogen induced in iodine by light, is not so great as that induced in chlorine. Thus, iodine will not liberate oxygen from water in the light; chlorine will, forming hydrochloric acid with the hydrogen;† whereas hydriodic acid (iodine and hydrogen) is decomposed on exposure to the air by the oxygen of the air taking the hydrogen, to form

water.

But though iodine has not sufficient power to displace hydrogen, if the brown solution of iodine in water§ be exposed to the light, the tendency is so far increased, that the solution becomes colourless, by the iodine combining with both elements of the water to form iodic acid with the oxygen, and hydriodic acid with the hydrogen of the water, which in the dark decompose one another, reforming water and iodine. Hence, though the

tendency given by light to iodine to combine with hydrogen, is not sufficient to cause iodide of silver to be decomposed in the presence of water alone, and form iodic and hydriodic acids; if there be free nitrate of silver present, the silver from which can take the iodic and hydriodic acids, to form iodate and iodide of silver, then the change is rendered much easier, and the light can affect it.* Especially if we suppose, as is most probable, that the iodide is reduced only to the subiodide.

In the dark, without organic matter, iodide and nitrate of silver are reformed. But if there be any organic matter in the collodion which has a tendency to absorb oxygen, and combine with the subsalt, the change is rendered more complete.†

The developer, having a tendency to absorb oxygen, would act in the same way. Thus, when iodide of silver is employed, light causes merely a rearrangement of the elements, which it has just sufficient power to prevent returning to their former state; whereas, when chloride is employed, the release of one of the elements is effected. But the change, being less complete, is more easily impressed. And this change having been impressed, is sufficient to develope upon. The particles of silver which are being precipitated from the developer falling, as I before explained, on the places where the change to subiodide, though not perceptible to the eye, is commenced.

To impress this change sufficiently to develope a good negative upon, a certain definite length of time of exposure is necessary, varying according to the intensity of light in the camera, though a slight amount of under exposure may be compensated for by more development, and over exposure by less; yet this latitude does not extend beyond very short limits, and no amount of development will compensate for under exposure, as then the feeble lights not having had time to impress themselves, the photograph is deficient in half tones; and in over exposure, not only do the feeble lights, by acting too long, impress themselves too strongly, and, overtaking the high lights, appear as such; but the high lights themselves go back as it were, and do not leave a sufficient change to develop on. For it must be remembered that I am speaking of negatives; and where the greatest amount of silver forms, the least light will be able to pass through; and acting very slightly, or not at all, will there leave the print lightest.

The manner in which we obtain the collodion film charged with iodide of silver is something like the way in which we sensitized or charged the paper with chloride of silver.

The collodion is mixed with iodide or bromide of potassium, of cadmium, or of ammonium, or a mixture of these. The glass plate is then coated with it; when nearly dry the plate is put into a bath of nitrate of silver. A double decomposition then takes place similar to the one with chloride of sodium and nitrate of silver, and iodide of silver is formed in the film.

After the negative has been sufficiently developed, it is fixed by removing the unchanged iodide with hyposulphite of soda, or, as is generally done,

* The following is a probable explanation,—6Ag,I+6AgO,NO =AgO,IO + 5Ag,I +6 NO。 +6Ag; or more probably=AgO,IO5 + 5Ag2I+6NO¿( + Ag).

† AgI+AgO,NO5+ M=Ag2 I+NO+ MO. But rendering the change more omplete, makes it more difficult to accomplish, and therefore gives a less sensitive film.

by using cyanide of potassium, which is a more rapid solvent, but acts in tho same way, that is, forms a soluble double salt of cyanide of silver and potassium.

We must, however, leave this portion of our subject. I have already kept you too long.

With respect to the application of photography to military purposes, the first necessary is portability in our apparatus. Captain Fowke, of the Royal Engineers, who has fitted out most of the parties of Engineers who have taken photographic apparatuses with them, invented this form of camera, which is extremely portable, collapsing into this size. It can be easily carried in knapsack.

With this camera and chemicals, &c., carried in boxes on pack-saddles, the different photographs which were exhibited had been taken by parties of Sappers.

1. A series by Corporal Lawson, who formed one of the party with Lieutenants, now Captains, Gordon and James, R.E., on the Asiatic boundary in Asia Minor, between Russia and Turkey.

2. A series by Sergeant Church, who accompanied Colonel Stanton when he went to verify the reports on the projected line of railway across the Isthmus of Panama-the Honduras line.

3. A series which were done in India, and at Singapore.

4. A series in China; from which Mr. Burford took his panorama. 5. A series by Sergeant Mack, R.E., at Moscow, whither he accompanied Lord Granville.

6. And, lastly, two which were done at Varna. The photographers who came out to the Crimea were unfortunately lost on board the Prince. One of the photographs was a copy of the plan showing the order of landing; Photography having been employed to obtain a number of copies of it.

Hence, we see that photography can be applied under very difficult circumstances, such as on long and rapid journeys, and that, in fact, the photographer could accompany the army in the field; and, what is more, that the art can be learned and practised by our men.

Many of these photographs are certainly not such as Mr. Fenton or Mr. Thurston Thompson would care to exhibit at the Photographic Exhibition. Still, some of them are very good as photographs, all of them, I think, are very creditable when we consider the circumstances under which they were executed.

Photographs of country give a most truthful and accurate idea of it. They would do more to give a correct idea of any particular position than yards of description on foolscap. They might be found of great service in illustrating a report on a country, as, indeed, they have been employed by Colonel Stanton; and in this way they might be of great service to the general commanding an army in the field.

Photographs are of great service in providing the engineer with a ready and rapid means of making progress-plans, showing the state of the works on any particular day; in fact, so good and so truthful a way that I believe it was first adopted by the Russian Government, who have often

* Captain Fowke's camera, made by Ottewill, was exhibited.

had considerable difficulties in this direction with their engineers and contractors. For though one may draw the plan and elevation of an imaginary fort or castle in the air, one cannot photograph it.

(Some progress-plans of the works at Aldershot for the War Department were shown.)

Photography can also be employed in copying and multiplying plans; as in the case of this plan of the position of the ships for landing the troops in the Crimea.

I have here a number of photographs executed at Chatham, which show what an admirable means they afford of conveying descriptions of various operations-bridge-making, and so on-taken at various stages of progress, which could only be done by expensive lithographs, and then not so well.

We are enabled to obtain a perfect picture of any size we wish. I could not give a better example than these photographs of the Cartoons at Hampton Court. They are done on five different scales. (The different series were shown.) You will, perhaps, think that they have little to do with the application of photography to military purposes. My reasons for exhibiting them are, that they are admirable specimens of photography, and that they were executed, with the assistance of Sappers of the Royal Engineers, by Mr. Thurston Thompson, who has instructed most of our men at South Kensington.

I have lastly to call your attention to some other applications of photography. They are scarcely for a purely military purpose. They are the reductions of the maps on the Ordnance Survey from one scale to another, for engraving. By employing photography for these reductions, the Survey Office at Southampton saves £1,600 per annum, and the whole saving on the survey will be about £32,000; and this with increased rapidity and accuracy. Formerly the reductions were made by the pentograph, a long and tedious operation, in which, as the hand and eye were employed, the accuracy was dependent to a great extent on the skill of the operator.

Now, by merely fixing the camera at different distances from the plan to be copied, it can be reduced to any scale desired by an operation of a few minutes, and with the greatest accuracy.

The scales of the maps are for

The 10-inch is reduced to the 25-inch, and the 25-inch to the 6-inch, by photography.

But here the perfect truthfulness of photography militates against it. In reducing from the 6-inch to the 1-inch scale, the photograph is too crowded with details, so that at present in a portion of this operation the pentagraph is still employed.

The photographs are at once transferred to the copper plates for the engraver, or the zinc plate for the zincograph process, in this manner: instead of printing from the negative on to ordinary printing paper, they employ printer's tracing paper, sensitized by being washed over with a