io6 POPULAR SCIENCE MONTHLY. 



position on the sun's limb. It was exceedingly unfortunate that three 

 cameras of the forty-foot pattern could not have been working in har- 

 mony, at three stations widely separated, to determine what changes, 

 if any, were taking place in the disturbed coronal area. Under excel- 

 lent atmospheric conditions, cameras still larger than those referred 

 to should record more minute details of coronal structure, and thus 

 lead to valuable results; but such observations would reach their full 

 value only in case comparisons could be instituted with photographs 

 taken under similar conditions at distant stations. However, as al- 

 ready stated, cameras of the forty-foot pattern give greater promise 

 of cooperative usefulness, taking into account the average atmospheric 

 conditions which must be expected at some of the stations. 



The spectra of recent coronas have led to most interesting results. 

 They leave no doubt that, at those eclipses, the spectrum of the inner 

 corona contained no perceptible dark lines. Perrine's Sumatra pho- 

 tographs seem to establish that the spectrum of the great outer portion 

 is substantially a copy of the solar spectrum. The simplest interpreta- 

 tion of these observations is that the outer corona is largely composed 

 of minute particles which reflect and diffract the sunlight falling upon 

 them, whereas the portions near the hot solar surface are mostly incan- 

 descent, shining by their own light. Polarigraphic observations are in 

 harmony with this theory. Opposed to the idea of the incandescence 

 of the inner corona stands, alone, the thermographic observations by 

 Abbot in 1900, of a corona less hot than the instrument with which he 

 worked. While it is difficult to assign such a low temperature to par- 

 ticles near the solar surface, and one should perhaps look for other 

 interpretations of the thermographic results, yet there is an urgent 

 demand for a repetition of all the preceding observations bearing upon 

 the nature of the corona. 



The polarigraphic observations of recent coronas have been very 

 interesting — leading to the knowledge that the light of the corona is 

 strongly polarized, except, apparently, in close proximity to the sun's 

 surface; and strengthening the view that the corona is very largely 

 composed of minute particles of matter which receive their light from 

 the photosphere. Unfortunately, the photographs do not permit the 

 making of quantitative measurements of the amount of polarization in 

 and across the solar radii ; and future programs for eclipse observations 

 in this line should make provision for securing comparable unpolarized 

 coronal images for standards of reference. 



Special interest will be taken in determining whether the com- 

 paratively shallow inner stratum of the corona which yields a bright- 

 line spectrum, is more extensive at the sunspot maximum of 1905 than 

 it was at the minimum of 1898-1901. The chances are that it will 

 be both thicker and more uniform in thickness. Should it be brighter 



