Light and lighting . is then 13650x3 8*2 foot-i-andles. 4x39x39 The working benches are - feel 9 inchesfrom the floor. Assuming the ceilingto be ;i flat rectangle, the illuminationimmediately underneath its centre andat a depth of 9 feel 3 inches u given • 2 x 8 _ E = (sinOtan hut Binrtan .-in) where Bin 9 0*90 Bin <t^0865tan sin e = 0733 tan sin <r I THE ILLUMINATING ENGINEER (april) 211 substituting these values, the illuminationis 16-4 (0-643 + 0-635) - 67 foot-candles. The actual readings obtained are shownwithin the circles drawn on the benchesin Fig. 23. The mean of the observedval
Light and lighting . is then 13650x3 8*2 foot-i-andles. 4x39x39 The working benches are - feel 9 inchesfrom the floor. Assuming the ceilingto be ;i flat rectangle, the illuminationimmediately underneath its centre andat a depth of 9 feel 3 inches u given • 2 x 8 _ E = (sinOtan hut Binrtan .-in) where Bin 9 0*90 Bin <t^0865tan sin e = 0733 tan sin <r I THE ILLUMINATING ENGINEER (april) 211 substituting these values, the illuminationis 16-4 (0-643 + 0-635) - 67 foot-candles. The actual readings obtained are shownwithin the circles drawn on the benchesin Fig. 23. The mean of the observedvalues at the four positions nearest to no allowance has been made for thepresence of the beams. Th3 presenceof cross reflections from the verticalsides of one beam to the other would tendto reduce the effective reflective co-efficient of the ceiling as a whole. Theengineers responsible for the lay-outof this installation were using certainspacing rules and reflection coefficientswhich gave them a figure of 7*5 foot-. FIG. 24 POLAR CURVE OF LAMP ANDFITTING USED in LIGHTING ROOM FORMANUAL TRAINING (FIG. 23) the centre of plane of reference is 53foot-candles, a disi repancy of about 21 percent. There is no doubt that this is in themain due to a shrinkage in the polarvalues on service as compared with thoseobtained in laboratory tests, inevitablewith arc lamps. Possibly also a toosanguine view has been taken of thereflective capability of the ceiling, and candles for the mean horizontal illumina-tion as the maximum possible under thebest conditions, but their experience ofthis type of lighting enabled them toquote as a figure for actual service 50foot-candles, which is within 3i per cent,of the figure found. The formula for the luminous rectanglecan be used to predict the variation of D 2 212 THE ILLUMINATING ENGINEER (april) illumination disclosed by the observa-tions on benches 9,10,11,12, as follows :—-The illumination at 0 already cal-culated can be considered as made up offo
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