(Concluded.)
Before leaving the subject of sonsilising, it may be mentioned that bichromate of potash is very poisonous with some people' when absorbed into the system through the skin, especially if the skin be broken or cut in any way. Many people do not find this so, but it is a8 well to use rubber fingerstalls when dabbling in the sensitising bath.
Practically no visible image is prodaced by exposure to light, so some method has to be adopted for timing the exposure. First exposures must necessarily be purely experimental, but asa guide carbon tissue, roughly' speaking, would be sufficiently exposed when P, O. P. under a similar negative to that being used for Carbon, were about half printed. This, however, would be a very unsatisfactory way of working. The best play is to use some kind of an actinometer. I have several here to night; the one I use is not only simple and easily made, but is very effect it Cut some fine tissue paper into strips and attach one to piece of clean glass, say quarter-plate size, with some colour Jess adhesive, Exactly over this, but within half an inch from the end of it, stick a second strip, a third within half an inch of the second and so on until you have @ strip of papers one layer thicker at each half-inch. In the centre of each half-inch print in black ink, letters or figures from A, or upwards, The rest of the glass may be blocked out, leaving only the graduated strip: of paper, place the whole in a printing frame as though it were a negative, the side with the paper strips representing the film side. Place a piece of P. 0. P. in contact, close the frame, and place it to print alongside the frame in which the carbon print is being made, 80 as to receive exactly the same amount of light.
'The first print must be more or less guess work, but as a guide, expose another piece of P. O. P. under s similar class of negative to the one you are making the carbon print from, and when that is about half printed, withdraw the' frames and turn your actinometer face down thus stopping printing any further.
Now develop the carbon tissue, and should it prove to have been correctly exposed, examine the paper in your actinometer, and you will probably find the image of the paper strips printed through to the third. or fourth strip, with the figure in each clearly visible. Make a note of the highest figure that has been printed and no matter what the light. be, on all future occasions when printing in carbon from that negative, you have but to examine your actinometer and when that certain figure is visible, you know that the print is finished. Should the first experi-
Text read from the printed page and cleaned; old type leaves the occasional error.