24 STOKES. — SPECTROSCOPIC OBSERVATIONS. 



2. Six spare steel wires (three to go with each instrument). 



3. Supply of matches ready made. (The slower the match 

 burns, the better. If those supplied burn too fast, steep them in 

 water and dry them again.) 



4. White blotting paper and nitrate of lead to make more matches 

 when wanted. (Moisten the paper with weak solution of nitrate 

 of lead, and roll into matches with thin paste made with a very 

 little nitrate of lead in the water.) 



5. Six spare pumices (three for each electrometer); India rubber 

 bands to secure pumice in lead case. 



6. Eight small stoppered bottles of prepared sulphuric acid 

 (four for each electrometer). 



7. Tin foil and fine wire. 



v.— OPTICS. 



1. Spectroscopic Observations. By Prof. G. G. Stokes, 



Sec. RS. 



(1.) Observations of the Spectrum of the Sun with a view to 

 Terrestrial Absorption. 



It has long been known that when the Sun is near the horizon 

 additional lines and bands are seen in its spectrum, which are 

 either not observed when the Sun is high, or are found to be 

 much narrower. These are referrible to terrestrial absorption, 

 and maps have been made of them by Brewster and Gladstone, 



o 



by Angstrom, and by Hennessey, which are sent with the Expedi- 

 tion. Kecent researches appear to show that the greater part at 

 any rate of these additional lines are due to watery vapour, but it 

 is still a question whether some of them may not be due to some 

 other constituent of the earth's atmosphere, to some substance 

 present in the atmosphere in such minute quantity as to elude 

 chemical tests. 



In the extreme cold of the Arctic regions the quantity of 

 water present in the elastic state in the atmosphere must be com- 

 paratively small, and consequently the absorption due to aqueous 

 vapour at a given small altitude of the Sun, might be expected to 

 be considerably reduced as compared with what is observed in 

 warmer, and especially in tropical countries ; and, as we have no 

 reason to suppose that the other absorbing constituent or con- 

 stituents of the atmosphere, if such there be, would be similarly 

 affected by cold, the comparison of the absorption- spectrum 

 obtained in Arctic countries with that observed in more temperate 

 climates might afford means of detecting bands of absorption, if 

 such there be, of other than aqueous origin, and thereby perhaps, 

 by subsequent researches at home, of leading to the discovery of 

 some other constituent of the atmosphere present in quantity too 

 minute to admit of direct detection. Besides, the length of time that 



