. Memoirs of the American Academy of Arts and Sciences. age 211, gives a directmeasure of the ratio of the light of two stars of the same color, if we can assumethat the brightness of the image is proportional to the area of the object-glass. Theprincipal objection to this method arises from diffraction, which enlarges the imageswhen the aperture is very small. A defect in the portion of the plate on which thestandard is photographed might affect all of the measures. This should be testedby measuring all the stars on a plate which has received several exposures withvarious apertui-es. The resu
. Memoirs of the American Academy of Arts and Sciences. age 211, gives a directmeasure of the ratio of the light of two stars of the same color, if we can assumethat the brightness of the image is proportional to the area of the object-glass. Theprincipal objection to this method arises from diffraction, which enlarges the imageswhen the aperture is very small. A defect in the portion of the plate on which thestandard is photographed might affect all of the measures. This should be testedby measuring all the stars on a plate which has received several exposures withvarious apertui-es. The results also give a good means of studying the effect ofaberration at different distances from the centre of the plate. Instead of varyingthe aperture, we may vary the time of exposure. This may be accomplished byvarying the rate of motion of the image. For stars in the vicinity of the pole, thevelocity is proportional to the polar distance. If, as on page 187, we assume thatthe brightness of a star capable of producing a given impression will vary as the. Plate 2 r = lD cm — TRAILS DF CIRCUMPDLAH STARS STELLAR PnOTOGRAPHT. 223 time of exposure, we may hence obtain a method of determining relative intensitiesof light. If two stars give equally distinct trails, their intensities must be inverselyas their polar distances. The last column in Table VII. shows that this conditionis nearly fulfilled. The difference in the photographic magnitude of 8G 269 and cis magnitudes, and the photometric difference is magnitudes. The image ofthe first star in this case moves nearly seventy times as fast as the image of thesecond star. Individual stars show a greater discordance, but the entire series failsto indicate any appreciable systematic variation. The evidence is not conclusive, onaccount of the small number of stars and the color of some of them. A much bettertest is afforded b} Plate 245. This photograph was taken on January 5, exposures were made of the vicinity of the
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