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A very suitable instrument for timing the exposure of carbon tissue is Sawyer's actinometer.

The actinometer consists of a large cylindrical thermometer bulb, with a scale considerably lengthened, so that minute changes may be easily seen.

The actinometer of M. Violle is, in principle, a sphere of copper, blackened externally, and having inside a thermometric apparatus which registers some distance away.

With daylight, however, the light is constantly fluctuating, and this renders the use of an actinometer a necessity for correct exposure.

Another object was, to bring down specimens of air from different altitudes, for analysis; to try the effect of the actinometer at great elevations; and to note the hygrometric condition.