504 Mr. B. Stewart on the Solar Spectrum. 



the influence of aqueous vapour on that small but interesting 

 region of the spectrum embraced by the two D lines. 



The observations made at Kew, in which your most powerful 

 arrangement of prisms (the sulphuret-of-carbon set) was used, 

 may be considered comparable with any of the observations of 

 Mr. Cooke. There may possibly be a small preponderance of 

 power of the one instrument over the other, although I do not 

 know which possesses it ; but evidently, from the sketches given, 

 these two arrangements are of the same order as regards power. 

 The sulphuret-of-carbon prisms were in operation at Kew for 

 about four months, until it was found that a glass train of infe- 

 rior power was really more useful for your main object of ob- 

 taining an accurately-measured map of the solar spectrum ; and 

 during these four months the region between the two D lines 

 was repeatedly observed, especially by Mr. Loewy. 



These observations were made under varying atmospheric con- 

 ditions, but in all these the same number of fine lines between 

 the two broad D lines was invariably observed. The appearance 

 presented by this region was that you have sketched in a paper 

 communicated to the Royal Society on March 17, 1864, and it 

 differs very little from fig. 4. given by Mr. Cooke in the Philoso- 

 phical Magazine for May 1866, as representing an observation 

 of this region made by him when the weight of vapour in one 

 cubic foot of air was 6*57 grains. 



When, however, we analyze the state of the atmosphere at 

 Kew at the time (from midday to 3 p.m. of March 12, 1864) 

 when the observation recorded and sketched by you was made, 

 we find that the weight of vapour in one cubic foot of air was 

 only 1'98 grain. In as far as we can judge of the vaporous 

 condition of the whole atmosphere by a hygrometrical observa- 

 tion, the vapour of the atmosphere was much less at Kew at the 

 time of this sketch than it was at the time when Mr. Cooke 

 observed the similar appearance which he has represented in 

 fig. 4; indeed the air at Kew at this moment probably contained 

 less vapour than at the moment when he observed the appear- 

 ance given in fig. 2, where there are only two lines between the 

 lines D. But your sketch exhibits thirteen intervening lines. 



Our Kew experience therefore does not quite accord with Mr. 

 Cooke's results ; for we have obtained — 



(1) In all our observations, extending over the/period of per- 

 haps one month, the same number of lines between the two D 

 lines. 



(2) One of these observations was made at a time when there 

 was probably little vapour in the atmosphere, and yet the spec- 

 trum obtained at Kew is similar to that obtained by Mr. Cooke 

 at a time when there was much vapour. 



