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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1921 Excerpt:...exactly between wedge readings 20 and 40, and very nearly between 40 and 60. Above 60 the divergence is rapid, amounting to.09 magnitude for reading 70. The difference between the two curves is seen by comparing Tables II and IX. In actual practice we have rejected star images having densities greater than 70. Moreover, most of the wedge readings are below 60, and in determining magnitudes of variable stars a number of comparison stars are selected giving images of different densities. Thus far we have used the corrected reduction curves. The uncertainty due to selection of the curve should not exceed.02 magnitude for an individual determination of any variable star on the program. Diameters of Extrafocal Images. The star images used in the present discussion have diameters ranging from 0.3 to 0.5 mm. Mean diameters have been measured, where possible, on the 1914 plates by Cummings; on the 1915 plates by Baker. From a number of duplicate measures a difference in setting between the two observers is shown, that causes the following difference in the measures: A Diameter, Baker minus Cummings =-0.007 mm. This is taken as a correction to Cummings' diameters. Diameters have been plotted against distance from center and a mean curve drawn for each plate. Frequent large residuals show that the diameter is not a function of distance from center alone. The effect of density on size of image is noticeable on the plates and may now be studied. Diameter vs. Wedge Reading. Residuals from the curves above described are plotted against wedge readings. The normal places are shown in Figure 7. As we have anticipated, the images increase in size as their density increases; but there is a definite halt between wedge readings 32.5 and 45.0. Otherwise the function appears to b...