Miscellaneous Intelligence. 247 



conditions to cover the five months of March, April, May, Sep- 

 tember, and October, otherwise frequently too warm. At pres- 

 ent the change of temperature of the inner room between these 

 warmer and the coldest winter months is only a fraction of a 

 degree centigrade, and during an hour's observation it is gener- 

 ally less than one-tenth degree, the control being automatic. 



2. Many holographs of the infra-red solar spectrum have been 

 taken, which, in consequence of these improvements, have 

 yielded results threefold richer in " real " detail corresponding to 

 solar and telluric absorption lines than any hitherto obtained. 



3. About 40 of these holographs have been compared, as de- 

 scribed in the Appendix to the Secretary's Report for 1896, and 21 

 of the most perfect have been measured upon the comparator to 

 determine the positions of the deflections found to be " real," or, 

 in other words, corresponding to either solar or tell uric absorption 

 lines. These comparator measurements included about 44,000 

 separate observations. 



There have thus been found over 700 absorption lines in the 

 infra-red solar spectrum between wave-lengths 0*76 pi and 6-0 ju, 

 an increase of about 500 over last year's results. 



4. With the purpose of making a more accurate determination 

 of the wave-lengths corresponding to the well-determined posi- 

 tions of the absorption lines discovered in the rock-salt prismatic 

 spectrum, a very exact comparison of the dispersion of rock 

 salt and fluorite has been made. This comparison will allow the 

 indirect employment of certain recent and apparently very accu- 

 rate determinations of the wave-lengths in the fluorite prismatic 

 spectrum. Apparatus has been made ready and certain prelimi- 

 nary observations have been taken to directly measure the dis- 

 persion of rock salt. It is hoped that these steps will result in 

 furnishing the wave-lengths of the infra-red absorption lines to a 

 degree of accuracy corresponding to the exactness of the deter- 

 mination of their prismatic deviations. 



5. Many interesting instances of local variations in the absorp- 

 tion have been noticed. Among these by far the most striking is 

 a great decrease in the absorption at the longer wave-length side 

 of the great band ip at about 1*4 ju. This change occurred about 

 February 15, 1898, and caused the holographs to take on quite a 

 different form at the place in question. This new form continued 

 through the months of March and April, but in the month of 

 May the usual form was gradually restored. It is found, by 

 reference to former holographs, that this marked decrease in 

 absorption at this point takes place annually at about the same 

 period, which coincides (fortuitously or otherwise) very nearly 

 with that at which there is the greatest activity of growth in the 

 vegetable kingdom. This raises the question whether the growth 

 of vegetation does not abstract from the air great quantities of 

 some selectively absorbing vapor active in absorption at this wave- 

 length. 



Whether such be the case or not future investigation must 



