754 HANDBOOK OF PHOTOGRAPHY 



ever, with an auxiliary yellow filter such as the Eastman Minus Blue may be used to 

 record magnitudes on a visual scale by the photographic process. When the yellow 

 color screen is used, plates sensitive to yellow light, often referred to asorthochromatic 

 plates, are employed. Stellar magnitudes on such a visual scale determined from 

 orthochromatic plates with yellow filters are termed "photovisual" magnitudes. 



If one compares photovisual magnitudes with the ordinary photographic magni- 

 tudes determined from the usual blue-sensitive plate, the difference is connotated 

 "color index." This quantity is obviously an index of the color of a yellow or red star 

 as compared to a white or blue star. Sometimes a red filter is employed in conjunction 

 with a panchromatic plate in determining the degree of redness a star possesses. A 

 careful comparison of color indices w'ith respect to spectral types of stars yields a 

 rather close correlation, so that stars of such faint magnitudes that a spectroscopic 

 analysis is impossible may roughly betraj^ the type of spectrum from a determination 

 of the color index alone. From the color index the black-body temperature of the 

 stellar surface may likewise be inferred. Bj^ the use of plates especially sensitized for 

 the infrared, such as the Eastman M or P, Hetzler of the Yerkes Observatory has been 

 able to photograph dull-red stars not previously rendered visible. 



Precautions. — Since photographic photometry is in a sense a quantitative process, 

 certain precautions should be mentioned in the use of star images for photometric 

 purposes. 



When focal images of stars are used, stars off the optical axis will produce images 

 that increase in diameter as the distance from the optical center increases. This is 

 due to the fact that the field of any telescope has a certain amount of curvature, and 

 unless, as is done in some instances, provision is made for curving the plate to the 

 known curvature of the field of the lens or mirror, images of stars off the optical axis 

 will be slightly extra focal; hence they are somewhat enlarged. 



It is possible to make a magnitude correction for this off-the-axis effect. To do 

 this, the driving clock of the telescope may be stopped just as a bright star is coming 

 into the field of the plate and exposure made. For a bright star a few seconds suffice 

 to gain an impressionable image. After a few moments w^ait, a second exposure is 

 made, and this process repeated until the star has passed over the entire plate due 

 to the diurnal motion of the sky. If the telescope has been properly focused, the 

 images of the stars at the axis will be true focal images and therefore appear to be of 

 smaller dimensions than the outlying images. By any one of the various means of 

 measurements previouslj'' mentioned, the magnitude differences of each of the off-axis 

 images may be determined and plotted against the linear distance of each image from 

 the plate center. From these data a correction curve can be drawn from which a 

 magnitude correction may be deduced to reduce any determined magnitude of a star 

 off the axis to the photographic magnitude which it would have if it had been exposed 

 in the position of the optical axis itself. 



In developing plates that are to be used for photometric measurements, relatively 

 weak solutions are employed for a developer, and a development time of 5 min. is 

 desirable. Care should be taken to see that the plate is completelj'- covered with the 

 developer and the tray rocked manually or mechanically throughout development. 

 This insures uniformity in the chemical treatment of the emulsion and also tends to 

 reduce to a minimum the difficulties attributable to the so-called Eberhard effect. 



Eberhard Effect. — The Eberhard effect is particularly conspicuous on plates which 

 have been overdeveloped, especially where there is considerable sky fogging in the 

 background. It is noticeable as an aureole or light ring immediately surrounding the 

 stellar image. Its appearance is explained by assuming a slight dilution of the devel- 

 oper in the immediate neighborhood of the star image where reduction of the silver 

 grains draws most heavily upon the constitvients of the developing agent. Rocking of 



