Meteorological conditions.

These are by far the most important factors with which we have to deal in exposure, and as they are as variable and uncertain as nature herself, so must exposures vary and be uncertain until meteorology shall be perfected. Even the perfect actinometer which we are promised will not settle the matter, for there are so many subtle conditions to consider besides the mere chemical power of light. For instance, for artistic reasons of light and shade, it may be absolutely necessary to work against the readings of the theoretical perfect actinometer. That a perfect actinometer may be of use in scientific photography we do not doubt, but that is a matter which concerns only scientific specialists.

Bouquet.

A few examples showing the protean aspects of nature, and the difficulties of dealing with it, will illustrate our meaning. Bouquet has calculated that the sun at an altitude of 50° above the horizon is 1200 times brighter than at sunrise. If we, then, apply the ordinary chemical law, that the chemical action is proportionate to the illumination, noon would be the time to give the least exposure; but such is not our experience, for the period of greatest intensity is often an hour or so before or after noon, because the angle of reflection is more favourable to us in England. Again, another factor to be considered is the presence of clouds; white clouds needing less exposure, as they reflect light to a powerful extent. Again, in sunrise and sunset light we have to consider refraction, the warm colours predominating. Another point to consider is our altitude, for there is less atmosphere in high altitudes; therefore, as any Alpine traveller knows, the sun acts more powerfully on the peaks than in the valleys. Dr. Vogel tells us that the light of the blue sky is chemically active and powerfully so. It will be seen, then, from previous remarks, why winter light is so feeble. Bunsen has worked out the chemical power of light, and expressed it in degrees thus:—

1212345678
(noon).p.m.p.m.p.m.p.m.p.m.p.m.p.m.p.m.
June 138°38°38°37°35°30°24°14°6°
Dec. 2120°18°15°9°0°0°

Thus at noon on June 21st the light is nearly twice as powerful as on December 21st, and when we couple with this fact the moisture generally found in the atmosphere at mid-winter, we see how deceiving are appearances. Again, it is acknowledged by many that the light in autumn is one and a half times as great as it is in spring; but we cannot act on this knowledge alone for outdoor work, for the conditions of vegetation are quite different, for, as Tyndall has shown, “in delicate spring foliage the blue of the solar light is for the most part absorbed, and a light mainly yellowish-green, but containing a considerable quantity of red, escapes from the leaf to the eye: ... as the year advances the crimson gradually hardens to a coppery red.”

Another complication is the east wind. It certainly sweeps away the moisture from the air and dries everything up, giving all things a black hue and bringing them up closer to view, at the same time dwarfing distant objects; and while an east wind does all this by taking away moisture from the atmosphere, the actinic value of light is at the same time lowered. On the other hand, after rain, the light acts quickly, probably owing to the numerous reflections from moist leaves, and from the fact that they do not absorb so much light under these conditions. That the warm colours require a longer exposure than others is too well known to need dwelling on. The presence of water in the foreground, on the other hand, necessitates a shorter exposure: even the amount of sky included in the picture will affect the length of exposure. The existing temperature, too, strongly affects the negative.

No rule for exposure.

It is perhaps necessary here to state that there is no set key or scheme of lighting to work by. Some untrained persons have preached that no photograph should be taken when there is no sun, or that sunlight is the best time for taking a photograph: such statements are as absurd as childish, one might as well ordain that all music should be played in one key. As beautiful pictures are to be obtained on the grey dull days of November as in sunny June. We remember once reading a statement that all paintings were of sunshine subjects. We quite forget by whom this extraordinary statement was made, but at any rate the writer must have been very ignorant of his subject; he could never have heard of half the great pictures of the world; but surely the name of Rembrandt might have occurred to him. A photograph must be true in sentiment, and true to the impression of the time of day, just as a picture must be. There are some subjects which in sunshine look beautiful, and which on grey days are worthless, and vice versâ. Therefore, here again there is no rule, each subject must be judged by itself.

|Sensitometer.|