280 M. R. Ruhlmann on the Alteration produced by Heat 



lines with Fraunhofer' s has been proved by Grailich*. A later 

 observation of A. Weiss has shown that, on altering the tempe- 

 rature or density of the vapours, displacements occur in the lines in 

 such a way that some of them disappear and new ones are formed. 

 The method is also not much to be recommended, because on 

 employing artificial light, the more refrangible part of the spec- 

 trum beyond F contains too few rays. To procure another sort 

 of homogeneous light, such as Fraunhofer suggested and Dutirouf 

 employed, requires rather a complicated arrangement. The reader 

 is therefore referred to the original communication. 



The simplest means which appeared to me to remain was to 

 make use of the bright lines which appear in the spectrum-appa- 

 ratus caused by the flames of incandescent metals. The sodium- 

 line appeared at once for the Fraunhofer line D. All other me- 

 tals, such as potassium, strontium, calcium, &c, gave lines which 

 were too weak to be seen at a distance of 8 metres by means of 

 my best telescope; so that there only remained the red lithium- 

 and the green thallium-line. The brilliant lines of caesium, in- 

 dium, and other newly discovered metals could not be employed, 

 on account of the dearness of their preparations. 



The three lines of hydrogen gas incandescent in the induc- 

 tion-current of a Ruhmkorff s coil passing through a Geissler's 

 tube, such as Landolt used, could not be employed, on account 

 of practical difficulties. 



As in calculating the formula of dispersion it was necessary to 

 know the undular length of the homogeneous light, an attempt 

 was made to estimate this quantity under the hypothesis that 

 the undular length for the line D (Ad = 0*0005888 millim.) was 

 known. I made use of a Fraunhofer's lattice, and measured the 

 distance of the first refraction-images from one another. Ac- 

 cording to the theory of the refraction-spectrum, the sines of the 

 angle of deviation must be proportional to the lengths of the 

 waves. Unfortunately it was not possible to measure exactly 

 to less than 10" with the theodolite. Moreover it was rather diffi- 

 cult to catch the feeble lithium-light line, and the only momen- 

 tarily appearing line of the volatile thallium . The measurements 

 obtained are the following : — 



Lithium- 



line. 



Sodium-line. 



A 



Thallium-line. 



1° 50 55 



-6 



1° 37 15 



+ 1 



1° 28 8 +13 



1 50 45 



+ 4 



1 37 17 



- 1 



1 28 25 - 4 



1 50 47 



+ 2 



1 37 18 



- 2 



1 28 25 - 4 



1 50 43 



+ 6 



1 37 15 



+ 11 



1 28 27 - 6 



1 50 48 



+ 1 



1 37 18 



- 2 



1 28 18 +3 



1 50 57 



-8 



1 37 22 



- 6 



1 28 22 - 1 



1 50 49 



mean 



1 37 16 



mean 



1 28 21 mean 



* Krystallograpli. opt. Untersuchunrjen, p. 21. 



f Ann. de Chim. et de Phys. S. 3. vol. xxviii. p. ISO". 



