332 Intelligence and Miscellaneous Articles, 



the rays having passed through a centimetre of water. The lenses 

 of the apparatus were also of rock-salt. 



In these circumstances we have seen manifested distinctly, in the 

 dark portion of the spectrum, four cold bands, the distances from 

 which to the extreme red were 



19'-8, 30'-6, 42', ' 52'. 



These numerical indications have not, and cannot have, a degree of 

 precision equal to that obtained in determining the position of the 

 dark lines of the bright spectrum ; but, such as they are, they make 

 known very nearly the spot where the pile must be placed in order 

 to find the band on which we wish to operate. 



Most frequently, in our experiments, the pile was 30 centims. 

 from the axis of the prism ; and the breadth of the admission-slit 

 was half a millimetre : viewed from the axis of the prism, it sub- 

 tended an angle of 5'*7; and consequently each cold band made its 

 effect sensible within an augular space equal to its own angular 

 breadth increased by 5'* 7. -But the minimum of thermoscopic 

 effect was observed when the middle of the band nearly corresponded 

 with the middle of the aperture of the pile ; and we always took 

 for the position of the line that of the pile which corresponded to 

 the minimum studied. We will add that the illumination-slit had 

 also, in general, a breadth of half a millimetre. 



Subsequently to the researches of M. Lamanski on the cold lines 

 of the dark spectrum of the sun, one of us had sought to determine 

 the position of some of those lines ; and, according to his measure- 

 ments, four of them are situated at distances from the extreme red 

 sensibly equal to 



19'-1, 30', 44', 51'. 

 These positions are the same as those of the cold bands developed 

 in the spectrum of Bourbouze and Wiesnegg's lamp by a layer of 

 water of 1 centim. interposed in the path of the rays. The coinci- 

 dence here shown seems to assign to the water' of the atmosphere 

 a great part in the production of the cold bands in the dark portion 

 of the solar spectrum. 



"We afterwards made another series of experiments, with the aim 

 of comparing the actions exerted upon dark spectra by different so- 

 lutions, which consisted of a solvent nearly inactive with respect to 

 the development of the lines and of a dissolved substance which, on 

 the contrary, was capable of determining their formation. The 

 active substance was iodine ; the inactive solvents were chloride of 

 carbon, chloroform, and sulphide of carbon. These three liquids 

 dissolve iodine copiously; and all three solutions have the same 

 appearance. Interposing them in the path of the rays, in layers of 

 1 centim. thickness, we obtained some results which we will put 

 together in the form of a Table. In these fresh experiments the 

 prism and lenses were of flint glass. 



