The shutter to be described can be very easily made, and has some advantages over most shutters. It allows more light to pass through during an exposure. For instance, suppose a shutter is timed to take just one second to open and shut, it is obvious it may open so that at the half-second it is just fully open, and must begin to close immediately; or it may open in one-quarter second and remain fully open for half a second, and close in the remaining quarter-second. This latter will allow more light to pass through than the former. At the same time, it will not allow of more movement than the former, as immediately the shutter is opened a little, an image is shown on the plate, though not so bright as if fully open. It is an important matter in shutter work to get as much light as possible, particularly during dull weather. The shutter has, however, one drawback, and that is a slight kick during the exposure. This, however, is perfectly obviated by having a velvet bag attached to the back of shutter, with a running string to close it round hood of lens. It can then be held in the hand during exposure.

With light cameras this is always a good plan; with heavy cameras the slight shake would not be noticed.

The outside square in Fig. 61 is of wood 1/4 in. thick; any wood will do. Two holes must be cut - a, of any size to suit lens, and b large enough to allow the spindle of hard wood to revolve with the vulcanite flap. At the back of this spindle is fastened a watch-spring: - in such a way that, when the vulcanite is twisted round, it will spring back again in the direction of the arrow. d is a catch hinged as shown, and held in position by the spring e. The pneumatic release can be inserted between e and the bottom of shutter. The dot at the side of d is a pin inserted to prevent d from spinning round and interfering with the flap in shutting. / is a piece of brass projecting through the wood from the back, having a weak spring, making it press upon the edge of the vulcanite, so that when the vulcanite has passed it will come forward into the space dotted at n. This is the stop that prevents the vulcanite from doing more than one revolution. The side of the vulcanite at the side of d must be shorter than at A, measuring from the centre b, so that it will clear f in passing. The shock of the vulcanite striking f is very liable to split the wood; it is better to have a piece of brass g flush with the wood and turned over at the side and screwed.

A piece of sheet vulcanite 1/16 in. thick will do for the revolving piece. The more times the shutter is turned round, the greater will be its speed, as the spring at the back will be wound up. It will be seen that the hole a is fully open during the greater part of the time taken in completing one revolution. The lighting of the plate is very uniform with this shutter. - (G. G. P.) (See also iv. 424.) Chemicals. - Among the many chemicals used in photography, the alkaline salts are the most important, and often contain the greatest quantity of impurities. A number of these impurities are incidental to the processes by which the salts are manufactured, and not being easily eliminated in the course of purification, they may cause very serious trouble when used for photographic purposes. Taking sodium carbonate as the first, and, perhaps, most useful alkaline salt in the process of development, let us stop a moment to consider the several processes by which it is made, and the impurities it is likely to contain.

Fig. 61.

Shutter 10042

There are three processes from which the sodium carbonate of the American market may be obtained. The first of these is the old Leblanc process; the second, the Solvay or ammonia-soda process; and third, the cryolite process of the Pennsylvania Salt Company. It is probable that most of the soda crystals are made from products produced by the Leblanc soda process. Without going into details, this process consists in first converting sodium chloride (common salt) into sodium sulphate by the aid of sulphuric aid; then by roasting this sodium sulphate with coal and limestone, the sulphate is converted into sodium carbonate ("crude soda ash," as it is called); this last product after crystallising gives us the soda crystals of commerce. The sources of the impurities in this method of manufacture are numerous. The common salt used for the purpose is reasonably pure; at least, the impurities are generally only lime and magnesium salts, which do no harm so far as photography is concerned. But as soon as the sulphuric acid is added, we begin to introduce a number of impurities. This acid often contains arsenic and nitrous compounds, which, when brought into contact with the sodium chloride, form compounds With it that are by no means easily eliminated from the subsequent products.

When we remember the exceedingly minute quantities of material that affect the photographic image, we are constrained to believe that the traces of arsenic often found in soda crystals may have some important effect upon photographic results. Yet the quantity of arsenic is exceedingly small in even the crude soda crystals, and possibly its presence is unimportant; nevertheless, it is very active in its reactions, and may have an influence far exceeding our expectations. But this is not the most important impurity in soda crystals. The most serious trouble comes from the impurities introduced during the conversion of the sodium sulphate into carbonate by roasting with coal and limestone. Having made this roasting, the crude semi-fused mass is treated with water, which dissolves the sodium carbonate formed, and gives the liquors from which the sodium carbonate crystals are ultimately obtained. An examination of these liquors reveals the fact that in addition to the sodium carbonate which they contain, there is also present sulphite, hyposulphite, sulphide, and cyanide of sodium. It is obvious that unless the sodium carbonate is crystallised several times, traces of all these compounds find their way into the crystals.

Their effect upon the photographic work done with such crystals is the more dangerous, according to their amount.

It is too true that much crude soda crystals in use contains considerable quantities of sodium sulphate. This impurity is harmless, but is an index of imperfect purification, and such crystals should not be used for photographic purposes. If one impurity is present in large amount, it is reasonable to suppose that others of more importance, and not easily eliminated, are also present.

Fortunately, we are not confined to the use of soda crystals, and there are two products in the market that are admirably fitted for photographic use. Only they must be used with judgment. These products are the sodium carbonate made from cryolite, and a like product made from common salt by the ammonia process.

The first of these is perhaps the purest commercial sodium carbonate made, its only impurities being traces of alumina and lime. In using it remember that it is dry sodium carbonate without any water of crystallisation; consequently only about 37 per cent. of the weight given for the crystals should be used.

The sodium carbonate made by the ammonia process is perfectly free from the dangerous impurities incidental to the Leblanc process; but it contains -two products that result from its method of manufacture. One of these is ammonia salts, and the other is an excess of carbonic acid. Dry sodium carbonate made by the ammonia process often gives a faint odour of ammonia, due to the volatilisation of the ammonic carbonate which it contains. The presence of the excess of carbonic acid in this last variety of sodium carbonate is due to the imperfect heating of the sodium bicarbonate, which is one of the steps in the process of manufacture. There is no particular harmful influence exerted by this excess of carbonic acid, but it makes it very difficult to determine the true strength of the material. It is almost impossible to know how much real sodium carbonate there is in such a product, the sodium bicarbonate being of little use as a developing agent. - (Anthonys Photo. Bulletin.)