742 HANDBOOK OF PHOTOGRAPHY 



and the introduction of meclianical devices which will render convenient its attach- 

 ment to the telescope. 



On account of the wide ranges of temperature encountered 'n\ the dome of an 

 observatory which, of course, is open to out-of-door conditions, the optical parts of 

 the spectrograph must be housed in a constant temperature case thermostaticallj' 

 controlled. In guiding the telescope for spectroscopic work, a guiding eyepiece is 

 provided to enable the observer to be certain that the stellar image falls on the slit 

 continuously. The guiding arrangement usually embodies a series of right-angle 

 prisms through which, by total internal reflection, light from the star image on the 

 slit is brought to a convenient observing point. 



For purposes of determining line shifts, means must be provided for comparing the 

 wavelengths of lines in the spectrum of a star with laboratory standards of line posi- 

 tions. To accomplish this, some auxiliary apparatus is needed for imposing a com- 

 parison spectrum on the same plate with the spectrum of the star. This is usually 

 performed by the use of a spark gap or vacuum tube near the slit end of the spectro- 

 scope, light from which may be brought into the spectrograph by means of an auxiliary 

 reflecting prism. By means of a suitable slit diaphragm the comparison spectrum is 

 photographed on portions of the plate not occupied by the spectrum of the star. By 

 making exposures for the comparison spectrum, both before and after the exposure 

 has been made on a star, a check is assured on the requisite registration. If, through 

 accident or change in flexure, the spectrum is disturbed during the process of exposure, 

 the second comparison exposure obviously will not register coincident with the com- 

 parison spectrum made on the first exposure. For the interpretation of the line shift 

 on spectrograms, recourse is made to the comparator or micrometer microscope. 



Details of the design of stellar spectrographs for the Mount Wilson, Yerkes, and 

 Dominion Astrophysical observatories have been printed in their publications and 

 serve as technical descriptions of this form of instrument. A recent modification 

 has been introduced at the Yerkes and Perkins observatories where the spectrographs 

 are of the autocoUimating type. In this form a single lens is used both for the colli- 

 mator and the camera of the instrument. In the case of the brighter stars the grating 

 may be utilized, producing a normal spectrum of relatively high dispersion. 



The Spectroheliograph. — The spectroheliograph is an interesting adaptation of 

 the spectrograph for the purpose of photographing the entire image of the sun in the 

 light of specific emissions from the lines of known elements in the sun. This was 

 originally designed by Hale in this country and by Deslandres abroad. The spectro- 

 heliograph utilizes the principle of a grating spectroscope by means of which the 

 absorption line of a single element in the sun may be isolated, such as the Ha line of 

 hydrogen. The camera consists of an arrangement by means of which a moving plate 

 follows an imposed motion of the image of the sun across the primary slit of the spec- 

 trograph. The resulting photograph consists essentially of a series of adjacent lamina- 

 tions, each of which gives a representation of the proportion of the image of the sun 

 covered by the slit in the moment of exposure of each lamination and in the light of 

 the chosen wavelength only. Since the plate moves with the same speed as does the 

 solar image across the primary slit, each elemental line section of the disk of the sun 

 occupies a fresh position on the photographic emulsion. If the apparatus is perform- 

 ing smoothly, the integrated image of the sun appears continuous, rendering beautiful 

 details of hydrogen clouds or flocculi that may be in evidence at the moment of expo- 

 sure. It is with the spectroheliograph that a continuous watch is now kept of erup- 

 tions of hydrogen and calcium on the face of the sun, which in many instances coincide 

 with disturbances in the ionosphere of the earth, causing fade-outs in radio reception. 



The adaptation of the spectroheliograph to motion-picture photography has been 

 accomplished by McMath and Hulbert of the McMath-Hulbert Observatory of the 



