414 



The value of the widlli of llie slit for tav.g. u^l is (hat for 

 which the first extinction of the horizonlal vibrations becomes per- 

 ceptible. 



A second series of observations has been made with a slit 'also 

 of platinoid) belonging to tiie collimatoi' of an echelon spectroscope 

 and for two different colours. 



red light 



Interesting is the difference in absolute width at which for the 

 two slits the same phenomena occur. For, though the measurements 

 iwAy not claim great accnracj, yet the different behaviour in the 

 two cases seems to be beyond doubt. Very probably the form of 

 tiie edges of the slit is here of much importance. The variation 

 with wavelength has the direction we should expect. 



We also made some experiments with white light. When the slit 

 is gradually narrowed the image formed by the horizontal vibrations 

 becomes fainter and at the same time of bluish hue. 



So we come to the result that with widths of slit often used 

 with spectroscopes in laboratories, polarisation phenomena are already 



of .some importance. The greater the I'aiio '- is taken, the less 



these appearances will be noticed. So with the 75 feet spectrograph 

 of the Mount Wilson Solar observatory we surely shall not see anything 

 of the mentioned polarisation phenomena. 



Recently a problem comiected with the passage of light through 

 a narrow slit has been treated theoretically by Rayleigh in a paper : 

 "On the Passage of waves through fine slits in thin opaque screens" '). 

 But as is observed by Raylkigh: "It may be well to emphasize 

 that the calculations of this paper relate to an aperture in an infi- 



1) Rayleigh. Proc. R. S. London. Vol 89. 194. 1914. 



