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752 S. K. GTJTH AND E. A. LINSDAY illustrated in Figure 6. The relative footcandles for equal visibility showlija Table 1 are in agreement with this. In other words, dark, low-reflectance t' can be given a reasonable visibility level and can be performed with re'lativefi i if higher levels of illumination are made available. However, these added fc,, candles will not produce excessive brightnesses because of the inherentlyilji reflectances of these more difficult tasks. READING BLACK PRINT ON WHITE PAPER FOOTCANDLES ON READING MATTER I 10 100 NERVOUS MUSCULAR TENSION WHILE READING FOR 30 MINUTES DECREASE IN CONVERGENCE RESERVE OF OCULAR MUSCLES AFTER READING FOR ONE HOUR DECREASE IN HEART RATE AFTER READING FOR ONE HOUR INCREASE IN BLINK RATE AFTER READING FOR ONE HOUR Figure 6. A summary of the results of several researches relating to.ease of.seeing by four of the effects of visual effort. IV. Quality of Lighting As the brightness of a visual task is increasedJjy higher fpg^gjdj^j more sensitive foveally, they should become more sensitive peripheral^^S fore, the quality of lighting--the distribution of light upon the task' anHui^.. roundings and the distribution of brightness throughout the visual field--hej increasingly important. Our tolerance of brightness in areas outside of .tlfef LIGHTING FOB SEEING 753 ases as footcandles go up.26*28 These brightnesses .and the .relationships :m contribute favorably or unfavorably to the seeing. opn<|itiqns...They Ittifluence the visibility of the task or they may result in decreased''ea|e; oh 'PfjBrightness and color are primary factors in perception, and; brightness rates very generally of major importance in the visibility and-recognition, of 4Msp,fibject8. In other words, within the visual task, brightness, contrastuis a. ^fyipMactor in seeing. However, when, the brightness of the centraUfield or 'nsiia'llask differs greatly from that of the surrounding field, unfavorable effects on `^glcONTRAST SENSITIVITY BRIGHTNESS OF SURROUNDINGS' J 100 5 TIMES THAT OF TASK SAME AS TASK % THAT OF TASK ..BLINK RATE 5 TIMES THAT OF TASK SAME AS TASK B '/s THAT OF TASK UKJurt^fThe effects of'brightness of surroundings upon contrast sensitivity and blink rate, indicating thedesiraEiE'ty of uniformity ofTbrigHtness.",' produced. Large brightness variations in the phripheral field also are In this respect, brightness contrast is. an unfavorable factor in seeing. .M;E|Figure ^ are shown the effects, of the brightness.pS|iiS^oundings upon con^^^^srasitivity29 (ability-to-see) and blink. raie81.;,'(easS|Mleifig)'.,1'Sensitivity of j||j;al sense is greatest when the brightness of as She task. Similarly, ease of sheinglalhbtSMC&^^M^Sjfb^B^lratoouiid Jesses are the same. Studies ..bf',>VMual>`a`&mt#f,l'&^^eiaioiSi7cifr.aeeingM- ^^Splladayi^Hfeifun^^|p^Ta^?^|S^siHty,s3A^0p^SSi5ftfilBri?:'`i!2r27l7-- 1% .-Harrison, What is. wrong.with.our.50rfootcandle..installations?,Varans.,/Hum. Eng. 32i.208 (1937). ' ...'' a',.. . Cobb, The effect on'fdyeal'.visioh'idfibi^ht^suhdtihHmgs--Psychol., Luckiesh and F. It. Moss-Brightness contraata ln seeing, Trans, Ilium. Eng. 'Soc., ... ,.f;....... Lythgoe, The Measurement of Visfi&li'Aciiity,-Special Report*NbY;-173, Medical raihCouncil, H. M. Stationery Office,'vLon'dpn, T932, -: . W. Cobb and F. K. Moss, The effect-of brightness on the precision of visually coh- m'lcupperations, 7. FrankKn/nst., 199, 507 (1925). 1 /. uoU)*uir>W.'SXsWj*