A reference to the spectrum curve, fig. 5, will show that this wave length of 4244 is close to the maximum sensitiveness. This is what was to be expected when it is remembered that the equivalent ray for the optically observed spectrum was close to the place of maximum luminosity.
We are now in a position to reduce all the photographically measured coefficients of intensity to those which would have been observed optically.
XXXII.—Places of Observation.
It has been the object of these observations to determine, as far as possible, the different coefficients of absorption at different altitudes, and with this object in view the instrument has been taken to various places at different altitudes. Thus, South Kensington, Weybourne, Oxford, Derby, which may be taken as places at sea level. Grindelwald, in the valley, and on the various hills surrounding it, up to 8900 feet on the Faulhorn.
Como, Perarola, Cortina, and Interlaken, also Chamounix and Zermatt, with their surrounding mountains, have also been observing stations.
Fig. 5.
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These stations are at various altitudes above the sea, and the results obtained are such as to show that the loss of light varies enormously, according as the altitude of the observing station is increased, This is a result which of course may be looked for. The observations of Langley on Etna, and at Pike's Peak, show such to be the case; but it is necessarily more marked in the case of these observations owing to the part of the spectrum employed being that which suffers most loss by the scattering action of small particles. The blue end of the spectrum for this purpose has evidently a greater advantage over the red end. and darker portions, more particu-
