( 581 ) 



could thus study the .slnudtaneou,s influence of the two factors, 

 temperature and magnetisation. 



§ 3. For the observation or measurement of the absorption spectra 

 we used, besides a hand-spectroscope, according to circumstances: 



1. A Raps ^) spectrometer with a prism of heavy tlint glass; 

 dispersion C — F ai)out 7.5'^. 



2. An autocoUimator made according to our directions by the tirm 

 of C Zeiss, a description of which will soon appear; dispersion C — F, 

 about 25°. 



3. The concave grating presented by Rowland to Berlin university, 

 and kindly placed at our disposal by Prof- Rubens ; the radius is 

 about 4 m., the number of lines 5684 per cm. The arrangement was 

 as usual in a right-angled triangle with movable constant hypotenuse; 

 a unit of the scale in the spectrum of the first order (half mm), 

 corresponded to 0.23 nn. 



The calibration of these instruments in wave-lengths was made by 

 means of lines of hydrogen, helium, potassium, and those of a mercury 

 arc lamp between the limits 434 and 770 mi. 



The sources of light were according to the required strength of 

 illumination 1. a "Neriist lamp", 2. a "Lilliput" arc lamp (2 Amp.), 

 3. an arc lamp with horizontally directed and slowly rotating positive- 

 carbon (25 Amp.), 4. sunlight. 



The polar pieces of the large ring-electromagnet had slit ted or 

 rectangular openings, to which attention was paid in the dioptric 

 determination of the path of the rays along the field's axis. 



The cryomagnetic arrangement was that used before"^); the level 

 of the liquid air was kept at the lower edge of the openings; the 

 sample could be eiiclosed in a thick copper frame in order to let it 

 have a temperature as uniform as possible. 



Outside the field a small vacuum vessel was used with unsilvered 

 strips for observation ; the variations of temperature had to take 

 place as gradually as possible to [)revent the samples from bursting 

 and cracking. 



The use of higlier tem[)eratures up to about 200' does not give 

 rise to any difllicidty with this apparatus which may also be arranged 

 pyromagnetically ; we hope to revert to this (juestion later on. 



Results. 

 § 4. We reserve the detailed description of the measured absorption 



1) A. Raps, Zeitschr. f. Instr. Kunde, 7 p. 269, 1887. 



2) H. Du Bois & A. P. Wills, Verb. D. Phys. Ges. I'p. 169, 1899. - F, C. Blake. 

 ibid. 9 p. 295, 1907. 



