100 



SCIENCE. 



[N. S. Vol. VII. No. 160. 



instrument to many of the problems of solar 

 physics is dwelt upon at length. A chapter is 

 also devoted to the various simple observations 

 which can be made without the use of elaborate 

 apparatus. Following this preliminary discus- 

 sion is an account of the eclipse expedition to 

 Kio Island, with a description of the arrange- 

 ment of the camp and apparatus, and an ac- 

 count of the development of the latent obser- 

 vational powers of the officers and crew of H. 

 M. S. ' Volage,' which had been detailed to as- 

 sist in the expedition. Then comes the story 

 of clouds, failure and the retreat. A chapter is 

 now devoted to the success at Novaya Zembla, 

 where Mr. Shackleton succeeded in obtaining 

 the spectrum of the chromosphere by means of 

 a prismatic camera. This finishes what has 

 been referred to as the first part of the work. 



What follows is devoted to the bearing of 

 eclipse observations up to date upon the ques- 

 tion of the composition and distribution of the 

 solar atmosphere. It is stated that the ' flash 

 spectrum ' of the chromosphere is radically dif- 

 ferent from the ordinary absorption spectrum 

 with which we are familiar, and that therefore 

 the chromosphere is not the seat of most of the 

 absorption. Comparisons are made with arc 

 and spark spectra and that of ' hot stars ' with 

 a view to showing that the chromosphere is 

 hotter than the absorbing media, which must 

 therefore be situated higher up in the solar at- 

 mosphere. The step from this proposition to 

 dissociation is a short one, and, with the satisfied 

 conclusion that "The eclipse work strengthens 

 the view that chemical substances are dissoci- 

 ated at solar temperatures," the author closes 

 his book. 



In brief, it may be said that the features of 

 the work are the stress laid upon the importance 

 of the prismatic camera in eclipse work, the 

 account of the volunteer corps of the ' Volage,' 

 and the exposition of the vindication of the 

 dissociation hypothesis by all the phenomena 

 of solar and stellar spectroscopy. 



While there is no denying the fact that in the 

 slitless spectroscope we have one of the most 

 powerful instruments for the prosecution of 

 eclipse work, it seems doubtful whether it will 

 accomplish all that our author, its warmest advo- 

 cate, expects from it. It is hoped to get a defini- 



tive spectrum of the corona, by means of sub- 

 tracting from the spectrum of the whole eclipse, 

 obtained with an integrating spectroscope, that 

 portion which is due to the chromosphere alone. 

 This latter is to be determined by the prismatic 

 camera. It is not impossible that a line might be 

 common to both chromosphere and corona, but 

 shine so feebly in the latter that its presence 

 would be masked by the continuous spectrum. In 

 such a case the line would be assigned to the 

 chromosphere alone. It would, therefore, seem 

 as though the true solution of the problem is to 

 be expected from the slit spectroscope, part of 

 the slit being made to lie on the moon's shadow 

 In order to make such an attack complete many 

 parts of the corona should be covered. With 

 an instrument of the probable dimensions of 

 that described by Sir Norman Lockyer the field 

 of the collimator should be flat enough to allow 

 several images of the sun to be used. These 

 could be twisted by means of reversion prisms so 

 that any portion of the corona could be brought 

 upon the slit. In this manner the regions sur- 

 rounding the sun could be well commanded. 



It will be seen that in the case of the inte- 

 grating spectroscope the full efficiency can not 

 be developed, as the central part of the lens 

 will be covered by the dark cone of the moon. 

 Again, and this is more or less in the same line 

 of argument, the brightness of a line will be an 

 average of the brightness of that color over the 

 entire field, while with the instrument provided 

 with a slit we have maxima and minima, which 

 is important in the case of faint lines. For 

 these and other reasons it seems doubtful 

 whether the great power the instrument de- 

 scribed might not be used to better advantage in 

 some manner other than the one proposed. 



It is expected, by means of the prismatic 

 camera, to decide between the two contending 

 hypotheses regarding the distribution of gases 

 in the solar atmosphere. Do the vapors all rest 

 upon the photosphere, and thin out at difiierent 

 heights, or are they arranged in concentric 

 layers ? One of the methods suggested is as 

 follows, to quote Sir Norman Lockyer : " There 

 is a very definite way in which the photographs 

 taken with the prismatic camera may indicate 

 the presence of layers of vapors concentric with 

 the photosphere, but not reaching down to it. 



