The Astrophysical journal . ifference were selected for measurement, togetherwith a plate exposed to the spectrum from the same sky, but withthe lens removed and a sheet of ground glass interposed in its place. Density measurement of these negatives gives the results asdetailed m Table III. When plotted, these results give the curves of Fig. 7. It will be noted that the difference between the secondary maximain the yellow-green, when expressed in density, =, and thisamount would be still further reduced if the blue sky negative had SENSITOMETRYTABLE III 133 Wave-Length 1)1 NSITV (Blue Sky)
The Astrophysical journal . ifference were selected for measurement, togetherwith a plate exposed to the spectrum from the same sky, but withthe lens removed and a sheet of ground glass interposed in its place. Density measurement of these negatives gives the results asdetailed m Table III. When plotted, these results give the curves of Fig. 7. It will be noted that the difference between the secondary maximain the yellow-green, when expressed in density, =, and thisamount would be still further reduced if the blue sky negative had SENSITOMETRYTABLE III 133 Wave-Length 1)1 NSITV (Blue Sky) (White Cloud) (Ground Glass) •4536 .7106 . 1172 .1218 .1636 .0952 .0494 .0:582 •3956 .6498 1 .2638 •2234 . 2012 ?2 739. 2619. 12 3880 1 2861 437° 478° ? 557o 573° 594o been exposed for a slightly greater length of time. The strong absorp-tion in the ultra-violet from X 3400 to X 3700 in the cloud and. X 6300 59°o Blue sky. continuous line. White cloud, x x. ; Ground glass, o o 4700 4300 39OO sky negatives is, of course, due to reflection and absorption bythe component parts of the lens-system. On the other hand, a 134 ROBERT JAMES WALLACE change in altitude of the observer would show a still greater difference,the blue of the sky becoming more intense as the altitude increased,and necessitating an increase in the exposure time. Comparison of these results, together with comparison of exposuresmade when the collimator formed an angle of 250 with the plane ofthe horizon (which of course showed considerably less difference),indicates that daylight from a low angle, when properly diffused, is asufficiently reliable guide for practical tests in selective second objection will be dealt with presently. HURTER AND DRIFFIELDS INVESTIGATIONS To correctly understand and appreciate the argument advancedfor the use of daylight as a standard in plate-te
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectspectru, bookyear1895
