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172 THE PHOTOGRAPHIC NEWS. [April 9, 1868. paper keeps its qualities very well for some weeks, and is as sensitive as silver paper, and it is darkened by the light to a tint deep enough for photometrical purposes. If such a strip of bichromated paper be exposed under the transparent scale, the strip is darkened by the light, at first under the thin end of the scale, and this colouration will advance to the thick end of the scale, in ratio corresponding with the actinic quality of the light. For recognizing exactly the point to which the action of light is advanced, numbers are printed on the transparent scales in opaque ink, which retards the action of light, and causes the numbers to appear white on the sensitive paper. The instrument is like a printing-frame, but with an arrangement to open it from the glass side for observing the number made visible by light on the bichromate paper. The bichromate paper is cut in strips corresponding with the length of the inner case C, which is, for placing in the strips, opened from the lower part; the dotted piece of wood, h, is re- X moved, and the sensitive strips are laid one on the other in the case, which is then closed. In this manner the paper strips are pressed by the elastic (dotted) springy against the upper side (open in the figure), and if the cover Dis closed (by aid of the crotch a), the strips are pressed against the trans parent graduated scale. This scale is fixed on a glass strip which forms the upper part of the cover D. A second cover, L, of wood, serves for exposing the transparent scale to the light when it is necessary to make an observation or examine the bichromate strips by lamp-light. For employment in carbon printing, this photometer is brought to the light at the same time as the printing-frame with the negative, which is allowed to remain exposed till the instrument indicates 10, 12, 14 degrees, according to the thickness of the negative and the sensitiveness of the carbon paper (16° in employing Swan’s paper for a medium negative). The method employed for fixing upon the number of degrees requisite for printing any negative is very simple. My own negatives I can at once, on inspection, divide into three classes, viz., thick, thin, and medium ; and in order to fix the degree of intensity I take a carte-de-visite negative containing three or four pictures of equal density, and ex pose it to the action of light with a piece of carbon paper. When the photometer, which is exposed at the same time, indicates 10°, I cover up one of the pictures ; at 12° I cover a second ; and at 14° a third. Whichever degree of intensity gives the best picture on development is the one best suited for the negative under examination, and others of a similar thickness. To ascertain the printing degree in other nega tives I proceed in the same manner, a small strip of sensi tized paper only being required for each experiment, just sufficient to cover a thin section of the negative. When the degree of a negative has been fixed upon, I mark the latter with the number. A little experience in the use of the photometer and the carbon process will enable the operator to ascertain the requisite degree of any negative by simple inspection ; and when this has been ascertained, the process is very much simplified, for, as Wilson remarks very truly, “ if the exposure is correct, the print cannot fail to be a success.” It may be supposed that one photometer is required for every copying-frame ; this is, however, by no means neces sary, if a systematic method of working be adopted. A number of frames are furnished with sensitized paper, and brought into the light at the same time, together with a photometer ; and when the instrument indicates the degree of intensity necessary for the printing of the thin negatives, the latter are either turned over or removed, the thicker nega tives being treated in the same manner when the photometer indicates that they have also been sufficiently exposed. In favourable weather the whole batch will be printed in a few minutes, and attention must therefore be paid to the photo meter, in order that ‘the negatives may be covered up at the proper time. As many as ten prints may sometimes be ob tained from a thin negative in one hour. If the prints are re quired in a hurry, and it is necessary that no time should be lost in the printing, it is as well to have one photometer for each of the three descriptions of negatives (viz., for the thick, thin, and medium); but even by this arrangement only three instruments are necessary. There is one more point still to be considered, and that is, the sensitiveness of the carbon paper, which differs greatly according to its manufacture, that exported from America being almost double as sensitive as that made in England. To ascertain the sensitiveness of a new paper, it is necessary to make an experiment, in the manner above described, with a negative of which you already know the printing degree. For instance, if, with American paper, a negative is known to have the printing degree of 10, and with the new paper the degree is found to be 12, then the degrees for all other negatives must be added when printing with the new paper. The reason of this simple calculation is, that the degrees of transparencies of the scale are in an exact mathematical proportion, the proof of which I will give to you as shortly as possible. Imagine a series of transparent strips of absolutely the same quality—for instance, mica, glass, paper, &c.: if the light goes through a series of such strips, its intensity is diminished by reflection and absorption. This diminution will be the greater if the number of strips is larger. Suppose the intensity of light which falls upon the upper strip will be = 1, and the diminution by going through one strip is such that the original intensity is reduced to —, then the n diminution will be, after gone through— the second strip = 1 X A = 1; n n n- the third strip = 1 X 1 = 1 ; n" n n 3 the fourth strip = 1 { 1_1; 3 n n4 therefore, if you construct a layer of strips like a staircase, the intensity of light 1 2 3 2 1 1 n n- n 3 after gone through will be—■ 4 5 6 ... ... 9 strips 1 1 1 t n* n 5 76 ’ "n that is, the intensities of light gone through such a layer of strips form a geometrical series. Now, it the intensity of light is reduced in such a manner whilst going through the strips, it is very easy to conceive that the quantity of chemical (actinic) light which must fall upon the layer of strips for making a visible impression on chromate paper, which is situated under the strip, must be larger in the same proportion as it is reduced whilst going through the strips. It is demonstrated that the diminutions are in the proportions— 1 . j ; 1 . 1 . 1 1 1 n » 2 n 3 ’ ! 76' 76 n’