. The Astrophysical journal. changes in seeing, atmospheric trans-parency, and small errors in the equality of the times of curves are all similar and express the variation of 5* withdistance from the optical axis for four different-sized images corre-sponding to four different values of 5* for any given distance fromthe axis. Within the range of images measured, no particular THERMOPILE IN PHOTOGRAPHIC PHOTOMETRY 275 dependence of the form or size of correction upon the size of theimage seems apparent. It must be admitted that corrections madein this manner seem at best artific
. The Astrophysical journal. changes in seeing, atmospheric trans-parency, and small errors in the equality of the times of curves are all similar and express the variation of 5* withdistance from the optical axis for four different-sized images corre-sponding to four different values of 5* for any given distance fromthe axis. Within the range of images measured, no particular THERMOPILE IN PHOTOGRAPHIC PHOTOMETRY 275 dependence of the form or size of correction upon the size of theimage seems apparent. It must be admitted that corrections madein this manner seem at best artificial, and at present it seems exceed-ingly doubtful if corrections taken from certain plates used experi-mentally can be apphed generally to any other plate without largeassumptions as to the order of accuracy maintained. In applyingcorrections from the curves of Fig. 9 to the magnitudes of R 531,Fig. 8, the constant ^ of equation (4) was so chosen as to reduceto a minimum the residuals of the corrected magnitudes of R 531. o. 40 20 o 20 Distance from center in minutes of arc 40 Fig. 9.—Calibration curves for 2-foot reflector from Plate R 2594 This gives a value of 0^^026 for a change in 5* of unit. Stars24 and 27 (Bessel) receive the largest corrections, being 17 and 26from the axis, respectively. The whole matter of finding suitable methods of correction tothe center of the plate is one of the most obstinate difficulties in theway of exact photometry on an absolute scale. A high degree ofprecision in the measurement of photographic images will thereforebest be rewarded in the investigation of relative magnitude-changesor in the photometry of small areas where the field under investiga-tion is restricted to the vicinity of the optical axis. 270 IIARLAX TRCE STETSOiY Other diiriculties in the way of more accordant results fromvarious observers is the lack of uniformity of equipment and spectralsensitiveness of the plates used. Photographic magnitudes, asat present publish
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectspectru, bookyear1895
