752 HANDBOOK OF PHOTOGRAPHY 



and small images in the same field. The fact that different observers using the same 

 method will show reasonably good agreement indicates at least the degree of reliability 

 of the results obtained in this way. The process is at best tedious and time consuming 

 so that other methods for calculation of magnitudes from stellar images on photo- 

 graphic plates are employed where a large amount of material needs to be studied. 

 One of these is the scale method, long in use at the Harvard College Observatory. 



In this method a scale of varying stellar images is obtained by making multiple 

 exposures at intervals on a single star. By increasing the time of successive exposures 

 in a definite ratio, a series of images varying in size by more or less constant steps maj^ 

 be made. Such a series of images forms a scale plate. By comparing the images on 

 this transparent scale plate with the images of stars of known magnitude on the photo- 

 graphic plate under investigation, it is possible to calibrate this arbitrary scale in terms 

 of magnitudes. With the constants thus determined for the scale plate, visual com- 

 parisons are made between the scale plate and stars whose magnitude it is desired to 

 determine. If for example a star image of unknown magnitude is found to fall midway 

 between two images of the scale plate which have been calibrated as 4.2 and 4.8 magni- 

 tudes, respectively, the magnitude of the star represented by the image under observa- 

 tion will be designated as 4.5. Such a method has the advantage of rapidity 

 of operation and is effective where estimates of a tenth of a magnitude gives the requi- 

 site precision. 



Another method extensively used in observatories is that of extra-focal images. 

 In this extra-focal method the plate is purposely placed either inside or outside the 

 focus of the telescope. In this position the converging pencils of light rays from the 

 star are intercepted so that a circular disk of more or less uniform blackening 

 is obtained for each star. If the position of the plate is sufficiently far removed from 

 the focus, all these circular disks will be of the same diameter but will vary in their 

 degree of blackening, depending on the brightness of the star for a given exposure. 

 The degree of blackening of the several images is measured by some form of densitom- 

 eter in which a calibrated photographic wedge such as is used in the Hartmann 

 microphotometer is frequently employed. 



In 1916 the author published an account of an apparatus and method for determin- 

 ing stellar magnitudes from photographic plates, utilizing a thermocouple in conjunc- 

 tion with a light source and galvanometer. The apparatus thus devised was called a 

 "thermoelectric photometer" and was found to be useful not only for measuring the 

 density of the extra-focal images but, through the provision of suitable diaphragms, for 

 the determination of magnitudes from focal images. The thermoelectric photometer 

 has the advantage over visual densitometers in that no auxiliary photographic wedge 

 is necessary. Furthermore a physical method of measurement eliminates eye fatigue 

 and the personal equation involved where visual estimates must be made. 



Various adaptations of the thermopile and photoelectric cell have since been made 

 in several forms of densitometers for measuring the blackening of appreciable areas. 

 The advantage of the thermoelectric photometer, herein described in its original form, 

 lies in the fact that it is equally adaptable to the measurement of focal images of stars 

 that vary very widely in size, the apparatus effectively integrating or practically 

 counting the total number of silver grains reduced by a given amount of starlight with 

 a given amount of exposure. 



The apparatus consists essentially of a light source consisting of an incandescent 

 lamp fed by constant voltage whose highly condensed filament is projected to the 

 stage of the instrument completely covering a small pinhole diaphragm that may be 

 varied from 0.1 to 1 mm. in size. The diaphragm occupies a position in the center of 

 the observing stage of the instrument. By a suitable optical system an image of the 

 illuminated diaphragm is projected on the surface of the thermocouple in electrical 



