β Scribed by W. R. G. Atkins; Miriam Rothschild
No coin nor oath required. For personal study only.
Observations have been made with vacuum sodium and with rectifier selenium photoβelectric cells, under opal flashed glass diffusing plates. The former may remain constant save when exposed to intense tropical desert heat and full sunshine for a long period. The latter are far less constant, and in time the curvature of the lightβcurrent relation may increase; this is accompanied by a decrease in blue and green sensitivity so that the response at the long wave end of the spectrum becomes relatively greater. No serious temperature error is introduced when the current from a Ixeston selenium cell is measured with a 10 ohm galvanometer amid snowy surroundings.
Sodium cell measurements at about 0.41 ΞΌ give the reflection from the snow as being from 0.74 to 1.19 times the vertical component of mixed daylight. With a selenium cell, sensitive throughout the spectrum, observations ranged from 0.68β0.88. Calculated on the maximum values for mixed daylight the snow reflections were from 0.38β0.74 and from 0.28β0.38 respectively for the sodium and selenium cells.
Under Alpine conditions there is relatively less scattered light from the sky, consequently the sun/sky ratio is greater, in the violet, than in England or Ceylon for the same solar altitude; with the selenium cell the sun/sky ratio is even greater, with a maximum of 9.70, as against 3.55 for the violet, both determinations being made at Scheidegg (2,064 m.) with an absolutely cloudless very dark blue sky, solar altitude 48.1Β°.
The vertical component of daylight is higher in the violet, under Alpine conditions, than in Ceylon, for low or moderate solar altitudes, but the difference is less at higher solar altitudes. For measurements covering the whole spectrum the Alpine values are also noticeably higher. The physiological effects of Alpine sunlight are due partly to the intensity of the reflected light.
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