Light and lighting . thousand to tWO thousandtimes as Btrong. To our eyes it apj a little stronger, an impression which , subconsciously accentuated bythe fact that we naturally expect it to be so ; but a ratio of B thousand to two thousand to one seems absurd, untilwe try n ? ith s photometer or attempt to photograph in the two positions withthe same stop. A thousand foot-candles of daylight oi sunlight is less trying to our eyes thanlo foot-candles of artificial light. This is possibly due to the fact 1 we have only been accustomed to the use of the hitter for say 160 years o
Light and lighting . thousand to tWO thousandtimes as Btrong. To our eyes it apj a little stronger, an impression which , subconsciously accentuated bythe fact that we naturally expect it to be so ; but a ratio of B thousand to two thousand to one seems absurd, untilwe try n ? ith s photometer or attempt to photograph in the two positions withthe same stop. A thousand foot-candles of daylight oi sunlight is less trying to our eyes thanlo foot-candles of artificial light. This is possibly due to the fact 1 we have only been accustomed to the use of the hitter for say 160 years or SO—B periodwhich is extremely minute in comparisonwith the countless ages through which our have developed under natural light. Also, ii may be noted that the supremeindifference of our eyes to huge variations THE ILLUMINATING ENGINEER (jan.) 17 ZENITH SKY BRICHTNESS. DIURNAL VARIATION CHICAGO I838--UNCLOUDED SKY ONLY. MONTHLY AVERACES. [BASQUIN] £2000 1800 1600 1400 «! 1200 5 iooo £ 800Of t 600 400 < 200. FIG. 1. ZENITH SKY BRICHTNESS DIURNAL VARIATION LONDON ISOBCLEAR SKY ONLY £2000 1800 1600 1400 « 1200 6 1000 t 800 t 600 400 » / ft& £> ^^ j ^* ^Gec? 0^- RM. Fig. 3. 18 THE ILLUMINATING ENGINEER (jaw.) of ample natural illumination is replacedat low intensities by a sensitiveness tovariations which approximates to thatsupersensitiveness which obtains underartificial li_M, which bo many peopleregard ss its normal capacity Corappreciating degri illumination. The reason of this i-. possibly,hereditary. For many thousands, ibly million-, of years, increases of lowillumination must have heralded toour open-air ancestors the approach ofdawn and were signals to be noted ifthe valuable hours of daylighl were to beutilised. Similarly, variations of waninglighl indicated the approach of a stormor of nightfall, ami only by noting ihilly tin- intensity of such variationscould tin- available for reachingshelter be ganged. When the due observation
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