TRANSACTIONS OF THE SECTIONS. 11 



Oil fJie Adaptation of Bisulijliide-of-Garhon Prisms, and the itse of Telescopes 

 of Long Focal Distance, in the Examination of the Sun's Sjiectrum. By 

 J. P. Gassiot, F.R.S. 



The dispersive power of sulphide of carbon lias caused it to be generally 

 used for producing the spectrum ; and some time since the author had a battery 

 of eleven prisms constructed, which has been used at the Kew ObserAatory in ob- 

 serving the spectrum of the sim. By means of this apparatus, the double D line 

 was observed in a very remarkable manner, presenting an angular separation of 

 3' 6", while at the same time eleven other associated lines were counted which had 

 not pre-\-iously been detected. But although this great augidar separation is a 

 proof of the power of the instnuneut, spectrum observations made with sulphide- 

 of-carbon prisms are attended with dithculties of the most perplexing and often 

 annoying character. One of these is the necessity of readjusting their positiou 

 whenever ditlerent portions of the spectrmu have to be examined, the time thus 

 occupied being often fatal to the seeming of true and faithful residts. Another 

 and serious difficidty arises from the changes of temperature taking place during the 

 period of observation. The author had been informed by Mr. Browning that Prof. 

 Cook, of New York, who had a battery of nine prisms, made by Alvan Clarke, 

 had found the influence of temperature so great as to render the batteiy unser- 

 viceable for long-continued and exact investigations. In order to test the effects 

 of changes of temperature upon Mr. Gassiot's batteiy, a careful observation was 

 made of the lines discovered in the space between the double lines of D. A tin 

 vessel containing hot water waa placed on the plate in the centre of the battery, 

 the heat from which soon affected the iluid in the prisms, and the spectral lines 

 gi-adually became confused and indistinct, travelling at the same time rapidly 

 across the field of view ; several hours elapsed before the prisms resumed their 

 nomial state. The fluid prisms are, notwithstanding all diflicidties in using them, 

 the author considered, an indispensable and most valuable adjunct to a complete 

 spectroscope, as, by the enormous dispersive povrer of this sulphide, observations 

 of Imes in the spectrum are obtainable that otherwise would probably entirely 

 escape notice ; and if, as in the battery which Mr. Browning had constructed 

 for him, the prisms have surfaces so perfectly plane as, at equable temperatures, to 

 give such satisfactory definition, they become highly valuable for the purposes of 

 comparison, and thus of determining in a remarkable manner the coincidence of 

 certain lines, the acciu-acy of the results not being interfered with, as it is evident 

 both sets of lines Avould be equally affected. The observers at Kew believe they 

 have noticed the coincidence of several bright gold lines with corresponding dark 

 lines in the solar spectrum, fi'om which the presence of that metal may he infeiTed 

 in the sun's atmosphere. If confirmed by further obsoi-vations, this \nl\ be an im- 

 portant addition to our present knowledge. In order to increase the power of the 

 author's unequalled battery of flint-glass prisms, Mr. Browning has recently adapted 

 a pair of telescopes of three feet focal length, in place of those of two feet, for- 

 merly used. The improvement thus effected is very striking; and on the only 

 opportunity for using it, the solar spectrum assumed an appearance f;ir more nearly 

 resembling that obtained by the batteiy of fluid prJsms. This single observation, 

 however, has been enough to show that the employment of telescopes of long 

 focal length may be indispensable for minute and reliable research on the lines 

 in the solar spectmm. 



On the Transmission of the Red Ray by many Coloured Solutions. 

 By Dr. Gladstone, F.R.S. 

 The author has been in the habit of observing the absorption of the different parts 

 of the prismatic spectrum by coloured liquids, by allowing a line of light to pass 

 through the varying thicknesses of the liquid contained in a hollow wedge of glass 

 and analyzing it by a prism. From the diagrams representing the phenomena thus 

 produced, it was evident that in many cases the extreme red ray was capable of 

 penetrating very far, while the less refrangible red or orange ray was almost imme- 

 diately absorbed. The following instances were given : — Solutions of chromium 



