SPECTROSCOPIC DETERMINATION OF THE SENSITIVENESS OF DRY PLATES.
After describing other methods of determining the sensitiveness of plates, Mr. Gf. F. Williams, in the Br. Jour, of Photo., thus explains his plan. I will now explain the method I adopt to ascertain the relative sensitiveness of plates to daylight. Procure a small direct vision pocket spectroscope, having adjustable slit and sliding focus. To the front of any ordinary camera that will extend to sixteen or eighteen inches, fit a temporary front of soft pine half an inch thick, and in the center of this bore neatly with a center bit a hole of such diameter as will take the eye end of the spectroscope; unscrew the eyehole, and push the tube into the hole in wood, bushing the hole, if necessary, with a strip of black velvet glued in to make a tight fit. By fixing the smaller tube in the front of camera we can focus by sliding the outer tube thereon; if we fix the larger tube in the front, we should have to focus inside the camera, obviously most inconvenient in practice. Place the front carrying the spectroscope in situ in the camera, and rack the latter out to its full extent; point the camera toward a bright sky, or the sun itself, if you can, while you endeavor to get a good focus. The spectrum will be seen on the ground glass, probably equal in dimensions to that of a quarter plate. Proceed to focus by sliding the outer tube to and fro until the colors are quite clear and distinct, and at same time screw down the slit until the Fraunhofer lines appear. By using the direct rays of the sun, and focusing carefully, and adjusting the slit to the correct width, the lines can be got fairly sharply. Slide your front so that the spectrum falls on the ground glass in just such a position as a quarter plate glass would occupy when in the dark slide, and arrange matters so that the red comes to your left, and the violet to the right, and invariably adopt that plan. It is advisable to include the double H lines in the violet on the right hand edge of your plate. They afford an unerring point from which you can calculate backward, finding Gr, F, E, etc., by their relative positions to the violet lines. Otherwise you may be mistaken as to what portion of the spectrum you are really photographing. The red should just be seen along the left edge of the quarter plate. When all is arranged thus, you utilize three-fourths of your plate with the spectrum, with just a little clear glass at each end. Before disturbing the arrangement of, the apparatus, it is desirable to scratch a mark on the sliding tube, and make a memorandum of the position of all the parts, so that they may be taken away and replaced exactly and thus save time in future.
To take a photograph of the spectrum, put a quarter plate in the dark slide and place in camera; point the camera toward a bright sky, or white cloud, near the sun--not at the sun, as there is considerable difficulty in keeping the direct rays exactly in the axis of the spectroscope--draw the shutter, and give, say, sixty seconds. On development, you will probably obtain a good spectrum at the first trial. The duration of exposure must, of course, depend upon the brightness of the day; but if the experiments are to have relative values, the period of exposure must be distinctly noted, and comparisons made for a normal exposure of sixty seconds, ninety seconds, two minutes or more, just according to whatever object one has in view in making the experiments. With a given exposure the results will vary with the light and the width of the slit, as well as being influenced by the character of the instrument itself. Further, all such experiments should be made with a normal developer, and development continued for a definite time. The only exception to this rule would be in the event of wishing to ascertain the utmost that could be got out of a plate, but, under ordinary circumstances, the developer ought never to vary, nor yet the duration of development. To try the effect of various developers, or varying time in development, a departure must be made of such a nature as would operate to bring out upon each plate, or piece of a plate, the utmost it would develop short of fog, against which caution must be adopted in all spectrum experiments.
On development, say for one, two, or three minutes, wash off and fix. You will recognize the H violet lines and the others to the left, and this experiment shows what is the sensitiveness of this particular plate to the various regions of the spectrum with this particular apparatus, and with a normal exposure and development. So far, this teaches very little; it merely indicates that this particular plate is sensitive or insensitive to certain rays of colored light. To make this teaching of any value, we must institute comparisons. Accordingly, instead of simply exposing one plate, suppose we cut a strip from two, three, four, or even half a dozen different plates, and arrange them side by side, horizontally, in the dark slide, so that the spectrum falls upon the whole when they are placed in the camera and exposed. There is really no difficulty in cutting strips a quarter of an inch wide, the lengthway of a quarter plate. Lay the gelatine plate film up, and hold a straight edge on it firmly, so that when we use a suitable diamond we can plow through the film and cut a strip which will break off easily between the thumb and finger. A quarter plate can thus be cut up into strips to yield about a dozen comparative experiments. When cut and snapped off, mark each with pencil with such a distinguishing mark as shall be clearly seen after fixing. The cut up strips can be kept in the maker's plate box.
The deep down underground electric railway in London has so far proved an unprofitable concern for its stockholders. It is 3½ miles long, touches some of the greatest points of traffic, but somehow or other people won't patronize it. The total receipts for the last six months were a little under $100,000, and they only carried seventeen persons per train mile. On this road the passengers are carried on elevators up and down from the street level to the cars. The poor results so far make the stockholders sick of the project of extending the road.
A New Catalogue of Value
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