"PARTS," FORMULAE IN The system of stating formulae in " parts " has some advantages. A " part " may mean any thing : a drop to a gallon in liquids, or a grain to a ton in solids ; and as long as one keeps to the same unit and multiplies this by the number in the formula it is impossible to go wrong. The following table will be found useful, as it will save some calculations when quantities are ex pressed in " parts " : This table may be used to some extent for roughly converting amounts in one system to the other. Say, for example, it is required to convert the following or any similar " part " developing formula into one for a solution containing to oz. of water : Kachin . . . . 16 parts Sodium sulphite . . 24 Sodium carbonate . . 5o Water . . . . t,000 The water is to equal to oa., then x part is equal to 4.8 mins., or 4.5 grs. (precisely, 437), for 1 oz. is equal to 48o mins. or 437'5 grs.
Kachin . .(16 x41) = grs.
Sodium sulphite .(24 x 4 f) =1o8 Sodium carbonate .(50 x4}) ,, Water . = to oz.
It is near enough in photographic formulm to take a "part " as meaning I gramme or I cubic centimetre. Thus, the formula above given will work out as follows : Of course, there Is a percentage of error, but this is of small importance in the generality of _photographic solutions.
This is do cheap and very effective method of preparing prints for the wall, and is a good sub stitute for the more orthodox method of framing. The chief drawback is that the absence of the support given by a stout wooden frame results in fragility ; so that while passe-partouts are quite satisfactory for home use, they run serious risks if sent to exhibitions. Briefly, the method consists iu binding together by the edges a sheet of glass, a mounted print, and a backboard, and providing a means for hanging up. The actual work is carried out with many variations, but the outline of one course of procedure will be sufficient to indicate the general lines to be followed.
The mount of the print should be sufficiently stout to ensure its lying flat, as with a thin paper mount it is difficult to secure such close pressure against the glass as to prevent buckling. A sheet of glass the size of the finished picture is cut, taking care that the edges are as clean-cut as possible, and the shape truly rectangular. This glass is used as a template for trimming the print and for marking out a rectangle on a piece of very stout strawboard. If this latter is of the right kind and thickness it will have less " give ' in it than a similar piece of wood. It is extremely hard to cut with a knife, and those who have a fine-cut saw, or even a fret-saw, available will find it an advantage. Glass, mount, and back
board should now be of the same size, with the edges flush and even all round.
Before binding up, the backboard is fitted with a hanging arrangement. Tapes are some times glued on for this purpose, but a better method is to use paper fasteners and curtain rings. The former should be of large size, say about if in. or if in. long ; the latter are hollow brass rings about f in. in diameter. The back board is pierced with two holes, a ring is slipped between the forks of the fastener and drawn tight up to the head, and the fastener is then pushed through the hole, opened out, and ham mered down flat. If the rings are so held as to occupy the best position for taking the pull of the cord, it will be found that the flattened paper fasteners incline towards each other, and are not parallel to the sides of the backboard. They will resist a pull very much greater than will be put upon them by the weight of the finished picture.
Excellent binding for the edges can be bought in rolls, and of different colours. Grey, black, and brown are the most useful. The strips are of tough " pebbled " paper, already gummed with a strong adhesive. The glass having been thoroughly cleaned the three component parts are put evenly together, and laid, glass side up, so that one side projects very slightly over the edge of a table. A piece of edging the length of this side is cut from the roll, drawn under a straight-edge or through the fingers to remove the curl,, and the gummed side moistened with a brush, being careful not to take off the adhesive. With a little practice this strip can be laid evenly along the edge of the glass, using the thumbs at each end to adjust it, and keeping about one third of the width on the glass. The strip is then thoroughly rubbed down into close contact with a duster. The whole thing is now turned carefully over, and the free edge of the strip drawn tigntly over and rubbed down on to the backboard. The opposite edge is done in the same way, and then the other two edges, allow ing time between each for the last strip of edging to set. Some prefer to mitre the corners of the strips, but if one is placed over another the difference cannot be detected at a little distance. Should alternations of wet and dry weather subsequently cause the edging to leave the glass in places it can be re-damped and rubbed down again. In extreme cases it is a simple matter to renew the binding altogether.