They really require a warm place and should be left three weeks in a flat position, so that as soon as the first lot of exuded balsam has been wiped off they should be placed flat on a glass or shelf with a piece of paper underneath them, as the balsam that exudes will cement them to any thing that they touch. Balsam is one of the stickiest of all sticky substances and the best way to clean the fingers is to well wet newspaper with denatured alcohol and rub well, using as much clean paper as possible and then benzol and paper, followed by plenty of soap and hot water.

An alternative method is to leave the hinge on, place the filter on a thickness or two of blotting paper on a level surface and place a card on top with a good-sized weight, four pounds not being too heavy for a 12 x 12 cm filter. Direct heat cannot be used for drying, as this causes the edges to dry first and gives rise to distortion.

At the end of the three weeks the exuded balsam should be scraped off, and the glass cleaned with alcohol and newspaper, then with bits of cloth and finally polished. Do not try to be sparing with the cleaning cloth, or use one large cloth; little bits and each piece thrown away as soon as it gets sticky is the easiest way. The final polishing should be done with tissue paper and alcohol, following the same plan, that is, fresh pieces continually. Benzol, xylol or chloroform should not be used, as they are energetic solvents of balsam and will almost inevitably creep in between the edges, in which case the job will have to be done all over again.

Those who would like to make their own preparation of balsam may purchase some dried Canada balsam from a lens worker or optician. This should be roughly powdered, which is most easily done, though it is rather wasteful, by tying it up in a cloth and hammering it with a heavy hammer; a fine powder is not wanted but the big pieces should merely be broken up. Then place this in a wide mouthed bottle, place in the water bath and bring the latter slowly to a boil, stirring the balsam all the time; add about one fifth of its weight of xylol, stirring well and then letting it get cold in the water bath. This preparation requires a much higher temperature to melt and must be used hot. It then sets very quickly and at a pinch a filter thus cemented may be used the next day. The only difficulty likely to be met with is the setting of the balsam before an even film is obtained, but warming the glasses, or keeping them on a hot plate for some time under pressure will soon make the balsam spread out.

There are four possible positions for the filter; in front of the lens; between the combinations close to the diaphragm; behind the lens; and immediately in contact with the sensitive surface. Between the lenses is the very worst place to choose, although this requires the smallest filter, as is obvious. In the first place, it is very likely to upset the corrections of the lens, particularly with the later forms of anastigmatic lenses, and with these there is often not enough room to insert any other than a film filter. Secondly, it is not easy to change the filters in this position without some special fitting, so that we can dismiss this at once. Either in front of or behind the lens may be chosen, which one being a matter of indifference, provided focusing is always effected through the filter, a matter that we shall have to deal with later on when talking of screen plates (See Chapter XII (Screen-Plates)). In either case some sort of sliding fitting is advisable, although this is not convenient in some cases inside the camera, as not only may the rear lens protrude beyond the lens board, but one has to have some means of shifting the filter between exposures, which necessitates a light-tight fitting.

It is possible in many cases to arrange a frame to slide over the camera front, and to fit the ordinary lens panel on this, so that the filters will be behind the lens. The sliding frame can be made on the same lines as the usual lantern slide carrier, and if velvet is used to line the outer frames there will be no trouble in making it light-tight. Or it may be possible to fit such a frame on the lens barrel itself, but here it must be so securely fastened that there is no chance of its slipping off. Really the simplest plan is to obtain one of the square slip-on cells, which, fitting on the lens hood or barrel, may be always retained in position and the filters merely lifted out and inserted as required. It is advisable, if possible, to remove the lens hood and fit the holder on the barrel, as this means not only slight reduction in size, but as a rule a firmer hold. In order to obtain the correct size of fitting, the diameter of the lens tube should be taken with a pair of sliding calipers. Failing these, the next best plan is to take a narrow strip of hard writing paper and wrap round the lens barrel so that the ends overlap by about half an inch, then with a sharp penknife cut right through both pieces of paper midway of the overlap, not at the end.

Placing the filter close to the plate means that the filter must be of the same size as the plate. Defects in the filter, such as want of absolute parallelism of the surfaces, are here of the least consequence; but local defects, such as coating striae or bubbles, are more apparent on the negative image, though only locally.