756 HANDBOOK OF PHOTOGRAPHY 



constants whose value must be determined from simultaneous equations covering at 

 least three known wavelength measurements. 



The problem of determining the abundance of elements from the intensities of the 

 lines in stellar spectra now forms an important study in astrophysics. The theory of 

 ionization phenomena and the method of measuring line contours by means of tracings 

 made with a suitable microphotometer is similar to the problem of the physicist work- 

 ing in a laboratory in spectroscopy. The wide variations in stellar spectra with the 

 varying densities of stellar atmospheres, together with their temperatures, afford 

 intriguing problems for the astronomical spectroscopist. 



A problem of astronomical spectroscopy not frequently encountered in ordinary 

 laboratory technique is that of the determination of the radial velocities of stars and 

 nebulae from the displacement of lines in the spectra of celestial objects from their 

 normal standard positions. If Dopper's principle is utilized, the velocity may be 

 determined from the formula 



V = ^ (16) 



where 6X = the changed wavelength due to the approach or recession of the object 

 under observation; 

 X = the standard wavelength of the known line; 

 c = the velocitj^ of light; 

 V = the radial velocity or velocity in the line of sight. 



The quantity v obviouslj^ is plus if S\ is plus, i.e., if there is an increase in wave- 

 length in the star line from that of the normal position. Similarlj'' v will be minus if 

 the wavelength decreases. In the first instance the celestial object is receding from the 

 earth and in the second instance approaching the earth at the time the spectrogram 

 was made. 



Since in general the radial velocity of the star will be desired with respect to the 

 sun, corrections must be made for the orbital motion of the earth and, for greater 

 accuracy, corrections for the movement of the observer on account of the rotation of 

 the earth. A third correction, depending upon the small amount of motion of the 

 earth about the center of gravity of the earth-moon system, must be made for still 

 greater refinement. The reduction of observations involving such corrections and 

 the detailed procedures employed at various observatories are beyond the scope of 

 this handbook. The reader is referred to Campbell's "Stellar Motions," Andre's 

 "Astronomie Stellaire," and similar works. 



Caution sometimes has to be used in interpreting apparent plate velocities for 

 actual velocities of the celestial objects themselves in the line of sight. Pressure and 

 relativity effects also serve to displace lines from their normal positions. It is perhaps 

 patent here to remark the question of the theory of the expanding universe rests upon 

 interpreting plate velocities of the extra-galactic nebulae in terms of the Doppler effect 

 alone. Should there occur reasons for believing in a change of frequency of light 

 through astronomical distances and large units of time, doubt would at once be cast 

 upon the validity of interpreting plate velocities as actual velocities in the line of 

 sight. 



Experiments made by McCuskey at the Harvard Observatory hold considerable 

 promise for utilization of objective prism plates for the measurement of radial veloc- 

 ities. Such a method has the advantage of collecting a large amount of data from 

 many stars on a single photograph without recourse to the refinements of the more 

 complicated temperature-controlled stellar spectrograph. 



Astronomical Photography Applied to Special Objects. — Much of what has pre- 

 viously been written in this section concerns chiefly stellar photography. For photog- 



