In taking the plate from the bath, raise the dipper with the same steady motion as in in serting, as any stoppage will surely leave a mark across the plate. Allow to drain for a few seconds, and then set against the wall or other support, and wipe the back with blotting paper, and place in the camera plate-holder, which has been previously set to fit the plate and screen. As heretofore arranged the screen and its frame or holder is of size to fit the size of plate used ; but recently a plate-holder has been invented accommodating any size of screen or plate, and with a device by which the separation of the screen and plate may be regulated to a nicety, which must prove a val uable adjunct to the working plant of the pro cess worker (Fig. 366).
The Screen.—At this point it may be well to consider the screen and its use. It is an indispensable accessory to half tone work, for without its assistance it would be impos sible to etch plates from photographs or wash drawings capable of being printed by the typographic process. By its influence the picture is broken up into minute points, which present a surface to the inking rollers of the press, without filling up the interstices' between, and so preserving the picture. Ac cording to the fineness of the ruling of the screen will be the fineness of the resulting print, but, on the other hand, it will call for superior paper being used and greater dexterity in workmanship in the printing. The fineness of the screen should therefore be commensurate with the ultimate intention in the use of the engraved plate. If it is to be printed on a common quality of paper, then a screen of coarse rul ing— say of 8o lines to the inch—should be employed, while screens of from 132 lines and up to 175 will necessitate the finest coated paper and the most skilful presswork. For more ordinary work screens of i2o to 124 lines to the inch will be found most serviceable, but to get the best results out of these a good coated paper and an experienced workman at press is needful.
Distance of Screen from Plate.—But a matter of much importance in working with the screen is the distance of separation between it and the sensitized plate. This distance will range from one-sixteenth to one-eighth of an inch, to be regulated according as the desire of the operator is to get the high lights up without using a larger stop, or giving longer time of exposure. The use of the screen in this respect will be understood when it is pointed out that the influence of the wider separation is to allow the light so much more space in which to spread, and thus fill up the high lights. With this fact kept in mind a wide field is
open for experiment, and the acquisition of practical knowl edge, which can only be thus attained.
Cleaning the is of the utmost importance that the screen be absolutely clean, and to get it so is more difficult than might be expected, the smallest stain from the fingers or other source being observable on the negative.
Perhaps the simplest way to clean is to fix in a screw vice, which is made for the purpose, and not costly (see Fig. 367) Keep a little of the finest French chalk mixed with alcohol at hand, and apply a few drops, rubbing over the surface, afterward polishing with a little pure alcohol or distilled water, applied with some absorbent cotton, and lastly rubbing up with a clean bit of soft, old silk. It takes a close scrutiny to discover streaks, and a careful examination in all lights is necessary.
The inserted the sensitized plate and closed the holder, the next operation will be with the camera. This should be of size to accommodate the screen and plate chosen, and be erected on a platform on which it can be made to slide far enough for considerable reduction being made. There must be perfect vertical align ment between the camera and the picture to be copied, and this is best attained by the copy holder being placed on the same platform, and also made to slide on it (Fig. 368). A platform about io feet long will be found a use ful length for ordinary work. It may be conveniently erected on a table with castors that it may be readily moved about, and springs that may counteract any vibration that may occur during exposure. The camera and copy-holder being placed on the same platform, any vibration occur ing will be imparted to both plate and copy alike. Another method of counteracting any vibra tion is to suspend the platform with camera and copy-holder from the roof ; but unless the operator's light is from a steady source the former plan will be found most convenient, as it can be shifted about so as to suit the direction of light at any time of day. Fig. 369 will explain what is meant.
The lens is perhaps the most important item of the pro cess worker's outfit. There are many good lenses on the market, and those of any of the reputable makers will afford a guarantee of suitability. It will therefore be the truest economy to procure the best the intending operator can afford to purchase, and he has a wide choice. It should, however, be of the rectilinear class.