( 730 ) 



here however, it was not ueccssai-y either to pay attention to great 

 brightness. 



The combination of the last «nentioned condilion witli a conside- 

 rable valne of dispersion naturally leads lo the application of the 

 principle of auto-collimation, preferably with 2 "halfprisms". In using 

 the instrument as a secondary spectral source of light immobility of tiie 

 entrance slit cannot be dispensed with, and for the exit slit it is 

 also required — perhaps with the exception of small sources of 

 light, easily moved. We ai-e, however, aware of the drawbacks 

 which attend this principle, viz. a higher degree of false reflexes, 

 and the so-called "vignettalioji" of the beam of light due to the neces- 

 sity of placing the slits at different heights. 



The utmost brightness is specially required for experiments on 

 polarisation, in which a nearly crossed i)Osition of the nicols allows 

 only a very small fraction of the light to pass. It follows that in 

 such cases polarisation by the apparatus itself does not give rise to 

 difüculties ; it may even prove advantageous that refraction should 

 in each case take place at the angle of polarisation; for then there 

 is absolutely no loss by rellection of light [)olarized i)arallel to the 

 refracting edges of the prisms. So in its usual position the apparatus 

 would allow light to pass which shows a strong partial polarisation 

 along the vertical. 



From Brewster's law it follows that the angle of refraction of the 

 whole prisms must then amount to (180' — 2 arc tg. n) ; we shall by 

 preference choose prisms of HO^ (res|). 30°), corresponding to 

 n = yd = 1,732. For this case, with 2 whole and 2 half prims, 

 the simple scheme of tig. 1 is naturally evolved, wliere evidently 

 all the angles of incidence amount to 60^. 



Now the glass must meet the following principal requirements:!) 

 index of refraction for a mean colour about 1,73 ; 2) no strong absorption 

 of violet light; 3) homogeneity and absence of bubbles; 4) resistance 

 against atmospheric influence ; 5) sufficient dimensions of the rough 

 blocks. In spite of the present ample choice it proved impossible 

 as yet to satisfy all these 5 conditions to a sufficient degree. 

 In the instrument constructed in the spring of 1907 by C. Zeiss we 

 therefore used heavy flint N". 1771 of the firm of Schott & Co. at Jena, 

 for which /«/)= 1,794; according to what precedes an angle of 

 refraction of somewhat more than 58^ (or 29^) corresponds to 

 this. The value of (in between C~F amounts to 0,0309; from this 

 follows a disi)ersion for every whole prism of 4°4' ; hence for the 

 whole course of the rays 2 X ('/, + 1 + i +VJ X 4°4' =i 25^ neai-ly. 



In order to keep the system in the minimum of deviation, every 



