is in every way suitable for the purpose. It may be well to call to mind the principle on which it is based. A solution of ferric oxalate is mixed with a proportion of chloroplatinite of potassium and spread over a sized paper and dried. When light acts on this mixture it reduces the iron salt to the ferrous state proportionately to the intensity of the light and to the time during which it acts. When such an exposed paper is placed on a hot solution of potassium oxalate (neutral) the salt reduces the platinum salt, and metallic platinum (platinum black) is deposited. The black or grey produced by this means is most suitable for measuring in the manner which will subsequently be described.
The other desideratum is also found in this process. The prints are absolutely permanent, and the records obtained by it can be referred to at any time, and re-measured if necessary. Further, the paper keeps well before exposure, and after exposure and before development. When kept dry in an air-tight box with calcium chloride in contact with the air, in fact, it will keep unaltered in sensitiveness for several months. As an example of this it may be mentioned that two pieces of paper cut from the same sheet were each exposed to various and similar intensities of light for equal times. One was developed on the potassium oxalate solution immediately after exposure, and the other was not developed till about six weeks had elapsed; on measuring the greys on the two papers their darkness was found to be precisely the same, and the unexposed parts equally white in the two cases.
It was, for these reasons, determined to use the platinum process for the experiments detailed.
XXVII.—Instrument used in the Observations.
The instrument which was designed for use with this sensitive paper was of the simplest form; one of the conditions which it had to fulfil was whilst exposing the paper it should allow but a small amount of light from the sky to reach it. The latter light is anything but negligible, for absolute measures, in some cases, show that if the photographic intensity of sunlight be called , that of the sky might be as much as .
To admit only a practical minimum of sky-light, and such as would be negligible in proportion to the sunlight, the instrument was constructed so that only about ° of skylight fell upon the paper with the sunlight. The diagram, fig. 1, will give an idea of the instrument. is a frame, in the back and front of the sides of which are cut deep continuous grooves. A wooden shutter, , formed of narrow laths, glued on to a leather back, runs in these grooves also at the back of the box. This flexible shutter carries an oblong block , pierced with four square apertures, as shown, which are closed by covers, , , , . The covers can be opened by turning the mill-headed buttons, , , , , which are connected by long pins with the covers.
In order to confine the photographically active spectrum to smaller limits than would be given if the light were unchecked, two thicknesses of cobalt-blue glass were
