I06 CARNEGIE INSTITUTION 



through what may be termed the Sun's atmosphere, and is this ab- 

 sorption constant, or is it, as has been suggested, variable and the 

 cause of the supposed variabilit}^ of the ' ' solar constant ' ' ? 



(e) What differences are there in the radiation of different portions 

 of the Sun's disc, such as spots, faculse, and prominences, and what 

 evidence ma)' be afforded from this as to the nature of those phe- 

 nomena ? 



(/) Another large class of inquiry is not directly connected with 

 heat radiation, but involves the use of the spectroscope, photogra- 

 phy, and probably electrical apparatus. 



2. Plan of Observations. 



Nature of the Work. — Nearly all these researches require a study 

 of the intensity of radiation of selected solar rays of all wave-lengths. 

 The most suitable method for this involves several steps. The first 

 is to obtain by the aid of one or more plane reflectors and appropri- 

 ate mechanism a fixed horizontal beam. Second, a large solar 

 image must be formed, preferably by a single concave reflector of 

 great focal length. Third, the desired region of the solar disc is 

 selected from this image b}' refceiving it upon a screen with a small 

 aperture, which serves also as the slit of a spectroscopic train. 

 Fourth, a large fixed arm-prism spectroscope, with concave reflectors 

 for collimator and objective, forms the spectrum of the selected solar 

 beam. Fifth, this spectrum falls upon a highly sensitive tempera- 

 ture-measuring instrument and is caused to march uniformly over 

 the sensitive surface, wave-length after wave-length, at a rate fixed 

 by an accurate clock. Sixth, the indications of this heat measurer, 

 exhibiting themselves as to-and-fro rotations of a suspended mirror, 

 are caused automatically to record a curve upon the photographic 

 plate, itself driven at a uniform rate before the mirror by clock- 

 work. 



Applications of stick Spectricvi Energy Work to Particular Objects. — 

 I do not purpose here to give a minute scheme of observations or a 

 detailed statement of the apparatus and accessories which they will 

 require, but merely to indicate the main features. A more minute 

 description, both of procedure and of apparatus, has been drawn up 

 and will be submitted later if desired. 



The Atmospheric Absorption. — It will be apparent that question (r) 

 (of page 105), relating to the amount and variabihty of terrestrial 

 atmospheric absorption, must be solved before or at the same time 



