REPORT OF COMMITTEE ON OBSERVATORIES 67 



upon the question of site. Estimates of the cost of construction are 

 based on the assumption that such means of access will be available. 



Plan of IVork. 



The general plan of work which we believe should be undertaken 

 at this observatory would comprise the following classes of observa- 

 tions. This program is of course subject to such modifications as 

 experience may suggest. 



(i) Frequent measurement of the solar constant, together with 

 studies on the absorption of the solar atmosphere and the radiation 

 of different portions of the sun's image, such as spots, faculae, and 

 prominences. The principal instruments needed for this research 

 are a 16-inch coelostat and large spectro-bolometer for the solar con- 

 stant work, and a 30-inch coelostat, with concave mirror of about 200 

 feet focal length, for providing a solar image suitable for detailed 

 radiation work. 



(2) Systematic observations, with large spectroscopes and spectro- 

 heliographs, on such problems as the solar rotation, the structure and 

 nature of sun spots, faculae, etc. , and other problems related to the 

 solar constitution. For this work there will be required two 30-inch 

 coelostats, used in conjunction with objectives and mirrors ranging 

 in focal length from 64 feet to 200 feet ; two large plane grating 

 spectroscopes, having focal lengths of about 21 feet and 42 feet 

 respectively, provided with auxiliary apparatus for work with the 

 spark and arc ; a three-prism spectroheliograph of about 10 inches 

 aperture, and a three-prism spectroheliograph of 8 inches aperture 

 and about 33 feet focal length. In order to secure the best defini- 

 tion of the solar image, the coelostats should be mounted at a con- 

 siderable height above the ground, and electric fans should be 

 provided for stirring the air by Langley's method. 



(3) Astrophysical researches on stars and nebulae with a large 

 reflecting telescope, provided with a three-prism spectrograph, and 

 also with a concave grating spectrograph mounted in a constant 

 temperature laboratory, for use with the reflector arranged as an 

 equatorial coude in photographing stellar spectra with very high 

 dispersion. 



(4) Laboratory investigations, mainly of a spectroscopic nature, 

 on problems arising in connection with the solar and stellar work. 

 Much of the apparatus provided for this purpose should be mounted 

 and used in connection with the solar spectroscopes. 



