Page:Philosophical Transactions of the Royal Society A - Volume 184.djvu/6

given, that is, if the unit time of exposure be doubled, the same result will be obtained if the intensity of light acting be doubled. This is not a mere theoretical assumption, but one which the writer has amply proved with very numerous experiments. To obtain such a scale, what is known as Spurge's sensitometer was employed. This is an instrument which was described in detail before the Photographic Society of Great Britain by the inventor. It is only needful to say that it consists of thirty small chambers, 2 centims. in height and 1 centim. square section, placed in six rows, of five chambers each. A brass plate covers one end of these chambers, and in it holes are pierced, of such dimensions that the area of each hole is exactly ${\sqrt[{3}]{2}}$ greater than that of the next to it in one row. One of these holes is pierced in the plate exactly over the centre of each chamber. Thus we have a series of graduated intensities of $1,2^{\tfrac {1}{3}},2^{\tfrac {2}{3}},2,2^{\tfrac {4}{3}},2^{\tfrac {5}{3}},4,$ &c., falling on the bottom of these chambers when the light falling on them comes from a large and equally illuminated surface.

Paper was exposed in this instrument to the light reflected from a large flat card, uniformly illuminated by sun and sky light, and after development the blackness was measured. The paper used was cut from sheets specially prepared by the kindness of Mr. W. Willis, of the Platinotype Company, and the sensitiveness of various parts was tried, and if found to be constant, the rest of the sheet was taken to be comparable with those tried. The care with which this paper is prepared is shown by the fact that with several varieties of paper, procured at intervals of several months, the sensitiveness showed no variation in gradation of tints. Latterly the formula seems to have been a little changed, and the sensitometer curve has slightly varied from that found at first; but this is unimportant, as the results are so reduced as to be comparable.

The term "developing" when applied to this paper may be a little misleading. It must not be confounded with the development of plates containing silver salts. In the case of this sensitive paper the amount of platinum deposited is exactly dependent on the amount of the ferric oxalate reduced by light, and if it be exposed to the developing solution after the ferrous salt has reduced the platinum salt in contact with it, no more platinum will be deposited. Hence the deposit of platinum is an exact equivalent of the iron salt reduced, and is a measure of the intensity of light multiplied by the time of exposure. With a silver salt the deposit of silver is increased according to the time during which the developer is in contact with it, and there would, therefore, be a necessity of exposing and developing on the same plate a scale of density together with that produced by the light to be measured.

It is evident that with the platinum process one scale will suffice for a batch of paper which is prepared at one time.

To measure the blackness of the paper a modified Rumford method of photometry was employed. A source of light illuminated a small square of white paper (the same as that used in the platinum process to hold the sensitive salt), and one of the blackened squares placed alongside it as in fig. 2.

A rod was so placed that its shadow fell on the white square. A mirror was placed