ADVISORY COMMITTEE ON ASTRONOMY 14.1 



have been r€i:)eatedly observed at the Yerkes Observators^ in full 

 sunlight. It should not be forgotton, moreover, that the prismatic 

 camera or objective grating, though perfectly adapted for eclipse 

 work, may give results which are not always capable of certain in- 

 terpretation. Thus a faint chromospheric arc, whose depth is suffi- 

 cient to require some time for the Moon to pass over it, may produce 

 in a photograph an effect similar to that given b}- a much shallower 

 but more brilliant stratum. With sufficiently powerful apparatus 

 such questions could be easily settled by observations made in full 

 sunlight. A large grating spectroscope (for many such purposes 

 the astigmatism of the concave grating need not seriously interfere 

 with its employment) , used with a coelostat and a long focus ob- 

 jective or .speculum giving a large focal image of the Sun, would 

 probably bring to light many new phenomena, and permit those 

 now known but not j^et understood to be rigorously investigated. 



From recently published photographs it appears that at rare inter- 

 vals the Sun's reversing layer is temporarily so completely changed 

 in character as to render the solar spectrum corresponding to the dis- 

 turbed region almost unrecognizable. In order to secure observa- 

 tions of other such phenomena — the only one so far recorded lasted 

 onl}^ a few moments and involved a region whose length was at least 

 one-sixth of the Sun's diameter — the solar spectrum corresponding 

 to the most active regions of Sun-spots should be kept continuously 

 under obsei-vation during the spot maximum. A series of photo- 

 graphs of the spectrum, taken at brief intervals of time, may be 

 needed for this purpose. On account of the large number of photo- 

 graphs required to give a fairl}' continuous record, some plan of 

 cooperation ought to be adopted. A single photograph of such a 

 phenomenon would well repay all the time and troiible required to 

 obtam it. 



As the field of solar research is almost unlimited, many other prob- 

 lems requiring investigation will suggest themselves. If modern 

 methods had already been applied to the solution of solar j:)roblems, 

 thus diminishing the chances of securing new and striking results, 

 a reason, though an inadequate one, for the present neglect of solar 

 research might be found ; but, with a few notable exceptions, such 

 applications have 5'et to be made. 



The Advantages of Reflecting Telescopes. 



During the nineteenth century the development of the telescope 

 in all countries except England was confined almost exclusively to 



