ASTRONOMICAL PHOTOGRAPHY 753 



connection with a sensitive galvanometer. A long-focus microscope is used for view- 

 ing the small diaphragm at stage center. The photographic plate is placed film down 

 in physical contact with the diaphragm, and the plate is so moved that a star image is 

 adjacent to the diaphragm. Radiation passing from the light' source through the 

 diaphragm to the thermocouple produces a given deflection D on the galvanometer. 

 The plate is then moved until the star image to be measured is seen to be central within 

 the diaphragm. The deposit of silver grains in the photographed image will then 

 obstruct a given amount of energy from the cone of light passing into the diaphragm 

 to the thermopile, producing a new galvanometer deflection D' . The relationship 

 holding between the magnitude of the stars and the galvanometer deflections has been 

 found to be 



— «;iH 



(13) 



where a. and /3 = plate constants; 



5 = the proportional fall in the galvanometer deflection represented by 

 the ratio; 



a = «^' 



Where 5 has been determined for two or more stars of known magnitude appearing 

 on a given plate, the quantities a. and /3 may be at once evaluated. The value of the 

 galvanometer deflection D for a given reading will depend, of course, upon the trans- 

 parencies of the plate film, the thickness of the glass, the voltage of the source, and the 

 resistance of the thermopile-galvanometer circuit. 



Since in the reduction, however, only the ratio (D — D')/D is involved, it will be 

 noted that any change in D due to a change in intensity of the light source or trans- 

 parency of the film will not affect the result, provided the conditions remain constant 

 through the measurement of D and D' for a given star. Since it is found that varia- 

 tions exist in the transparency of the glass and film over different parts of a plate, it is 

 necessary that measurement through the unexposed portion of the film be taken in 

 the immediate neighborhood of the star for the background readings D. In practice 

 two readings are customarily made on the star image and three readings made on 

 the background immediately adjacent, the respective means being taken for reduction 

 to magnitude. Instruments of the above design have been in use at Harvard, at the 

 Case School of Applied Science, at the Steward Observatory of the University of 

 Arizona, at Perkins Observatory, and the Argentine National Observatories. 



A modification of the thermoelectric photometer by Schilt has been in use at 

 Yale, Columbia, and elsewhere. In the Schilt instrument mechanical movements 

 of the plate in rectangular coordinates have been provided with appropriate scales for 

 recording the approximate positions of the stars measured. In the Schilt instrument 

 the plate is held in a vertical plane, whereas in the author's design the plate is allowed 

 to occupy a horizontal position. 



Visual Magnitudes and Color Index. — Since the ordinary photographic plate is in 

 general much more sensitive to the blue and violet end of the spectrum, high-tempera- 

 ture stars whose emission is strong in the region of short wavelengths will produce 

 larger and blacker images on the photographic negative than yellow or red stars that 

 to the eye give the impression of equal brightness. Magnitudes of stars therefore 

 determined from the photographic plate will differ considerably from magnitudes of 

 the same stars made with a visual photometer, on account of the large differences in 

 color. Such magnitudes are therefore referred to as photographic magnitudes as 

 distinguished from visvial or photometric magnitudes. The photographic plate, how- 



