6 
CAPTAIN W. DE W. ABNEY ON THE TRANSMISSION 
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 Photo¬ 
graphic 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 ^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% 2% 2, 2^, 2*, 4, &c., falling on the bottom of these cham¬ 
bers when the liglit 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. WiiiLis, 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 com¬ 
parable 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 ecjuivalent 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 Bumeord 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 
