MAKING A GRADUATED SCREEN.
Take an ordinary dry plate, immerse it in a 1 per cent. solution of potassium bichromate, and dry it in the dark in a good current of air. (The same precau tions must be taken in drying as are necessary in the carbon process.) When the plate is dry it should be exposed beneath a graduated scale of densities prepared as described on page 07 ; about five or six densities on the plate will be sufficient, and the width of each section should be about three-quarters of an inch. A half-plate cut through lengthways will provide a suitable shape and size. This bi chromated plate is exposed with the glass side against the graduated deposit for a suitable length of time to be ascertained as small as possible, then cut some quar ter-plates with a diamond to a convenient size and place one in a quarter-plate carrier with a piece of cardboard on either side to hold it in position. Now, by means of a cap made as in Fig. 596, the graduated colour screen is arranged so as to pass in front of the diaphragm of a single lens. An exposure is first of all made of correct length in the ordinary way ; next an exposure is given with the lightest tint in front of the lens, allowing a sufficiently increased time, as far as may be judged, substituting, of course, a fresh piece of plate. Another section
is pushed forward, a third piece inserted in the slide and a third exposure made, and so on until all the tints have been used. Each plate must be carefully by the actinometer. The plate is then washed in hot water to dissolve away the gelatine to the extent to which it may be soluble in the different portions. When this preparation is complete, the plate, on being held up to the light, will show various thicknesses of film which may be detected by the varying densities of the white deposits still left in it. The plate may now be immersed in a clean solution of hypo. and carefully washed ; it is then bathed in a solution of aurantia until the thinnest deposit but one just shows a per ceptible yellow tinge. A scale of colour deposits roughly prepared in this way will do much towards making clear the influence which the depth of the screen has upon the rendering of contrast in the result.