Wooden developing frames or racks placed successively in developing and fixing baths must be cleared of all traces of the hyposulphite which has penetrated into the wood before being used again. This can be done by allowing them to remain for some time in a solution of about 8o gr. to the gallon of potassium per manganate (followed by a rinse in pure water). The same precaution can be taken with metal frames,' but they need not remain so long in the bath.
Metal can, if cleaning with water is insufficient, be rinsed occasionally with hydro chloric acid diluted with about ten times its volume of water, but the vessel must not be allowed to remain in contact with this acid bath too long.
Too great emphasis cannot be laid on the necessity of carefully cleaning every new vessel before taking it into use. For the first cleaning of metal goods (except for aluminium, which can be cleaned with soap or with a solution of trisodic phosphate) hot soda solution should he used to remove the greasy material which has been employed for polishing or for the protec tim of the surface.
267. Various Accessories. The racks sold for drying negatives are usually of bad design. As a tradition from the days of wet collodion, when the photographer had to carry his entire apparatus to the place of operation, these accessories are of folding pattern, to the detri ment of their stability their grooves are also too close together, and are of rectangular section, whereas they should be of V-section, so as not to allow the wet gelatine to stick to the sides of the groove. For professional work,
and in all cases of large sizes, the racks should be rigid, with well-spaced grooves, allowing free renewal of the air round the drying gelatine.
As regards balances, it is best to choose the Roberval type, thus permitting the weighing of substances in a weighed vessel without inter ference of the usual stirrups supporting the pans. The sensitiveness of a balance being about a two-thousandth of the maximum weight it will carry, it is well not to buy a balance which is capable of dealing with greater weights than are necessary. If, in a large establishment, it is necessary to have balances for heavy weights, it will be best to provide a separate balance for light weights. A Roberval balance, having a very long needle, will weigh 3 oz. to 7 oz. to the nearest grain." A very useful accessory in the photographic dark-room is a small clock having only a minute and second hand, both being visible in the semi-darkness of the dark-room, or, better still, a metronome, set to beat half seconds or seconds.
A final item of equipment is a thermometer, fixed at some distance from the wall, to indicate the temperature of the dark-room also a small thermometer, graduated on the stem, for placing in tanks or dishes to ascertain the temperature of the baths. 2