I06 CARNDGIE INSTITCTTION 



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 variability of the ' ' solar constant ' ' ? 



(e) V/hat differences are there in the radiation of different portions 

 of the Sun's disCj such as spots, facniae, and prominences, and what 

 evidence may 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 inx'olves the use of the spectroscope, photogra- 

 phy, and probably electrical apparatus. 



2. 'PLJ\.i^ OF Observations. 



Nature of Ike Work,. — Nearly all these re^earchf'S 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 reffectors 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 regiot? of the solar di.sc is 

 selected from this image by receiving it upon a screen with a ?:ma11 

 aperture, v,'hich serves also as the slit of a spectroscopic trairs. 

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

 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-iengtb after wave-length, at a rate fixed 

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

 exlnbiting 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 such Spectrum Energy Work to Particular Objects. — 

 I do not purpose here to give a minute scheme of obsei'vations or a 

 detailed statement of the apparatus and accessories which they will 

 require, bat 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. 



Tke Atmospheric Absorption, — It will be apparent that question {c) 

 (of page 105), relating to the amount and variabilit3- of terrestrial 

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



