350 smithsonian miscellanlious coi^lections vol. 52- 



Work with ths Spectroheuograph 



The spectroheliograph is an instrument for photographing the 

 Sun with the monochromatic hght of any of the vapors present in 

 its atmosphere. The instrument consists essentially of a spectro- 

 scope, on the slit of which an image of the Sun is formed. The 

 spectroscope analyzes the light of that portion of the Sun's image 

 which enters the slit, and spreads it out into a spectrum, crossed by 

 lines characteristic of the various elements. If a luminous cloud of 

 calcium vapor in the Sun's atmosphere happens to be intersected by 

 the slit, the dark calcium line of the solar spectrum will show a 

 ' bright line corresponding to a section of this cloud. Suppose the 

 eye-piece of a spectroscope to be replaced by a slit, and assume this 

 slit to be adjusted so that only the line of calcium passes through it. 

 If a photographic plate is placed almost in contact "with the slit, and 

 the spectroscope is moved at a uniform rate across the fixed solar 

 image, the second slit moving with it across the fixed photographic 

 plate, it is evident that an image of the Sun will be built up on the 

 plate from the successive images of the slit. The only light that 

 enters into the formation of this image is that of calcium vapor, and 

 the resulting picture therefore represents the distribution of this 

 vapor in the solar atmosphere. 



The advantages of using a fixed telescope are as great in the case 

 of the spectroheliograph as in that of the spectrographs already de- 

 scribed. The limitations in size imposed by the necessity of carry- 

 ing a spectroheliograph at the end of a moving equatorial telescope 

 do not obtain here, so that the instrument can be built of the dimen- 

 sions required to accomplish its purpose to the best advantage. Plate 

 XXXI represents the spectroheliograph constructed in the instrument 

 shop of the Solar Observatory for use with the Snow telescope. The 

 image of the Sun, 6.7 inches in diameter, falls on the first slit of the 

 instrument in about the position of the metallic disk shown on the 

 right of the plate (this disk is removed when the solar surface is 

 photographed). The light, after passing through the slit, falls upon 

 an 8-inch photographic objective, which renders the rays parallel. 

 They then meet the surface of a plane mirror, from which they are 

 reflected to two large prisms. The prisms disperse the light into 

 a spectrum, an image of which is formed on the second slit by a 

 second 8-inch objective. The prisms are so adjusted that the curved 

 second slit, which may be seen near the middle of Plate xxxi, coin- 

 cides accurately with the calcium line H„. The photographic plate is 

 placed in the supporting frame in front of the slit and the door 



