ox CEUTAIX DIFFKAC'I'ION PHENOMENA. 



821 



Ob.servation shews that the small additional term is of very small 

 eÜ'ect. Calculating the minima of C' + S' by means of Knochenhauer's 

 series, I find that the agreement of caleiilation with observation is 

 quite close, except when the point considered lies outside the 

 geometrical shadow. 



In order to test the result of calculation with observation, the 

 following experiments were made with a slit of 90^ aperture, cut on 

 a right circuit) r cylinder of 5.0 nun. i-adius. Sunlight was admitted 

 into a darkened room. After passing tlirough a small vertical slit, 

 and a lens, it was analysed by a prism. The spectrum thus formed 

 was projected on the slit of a spectrometer. The slit, howcNer, was 

 cl(jsed by thick paper, and only a small liole was pierced, throng] i 

 which light was passed to the slit under examination. The spectriun. 

 was so distinctly formed, that one could easily make the light c<jrres- 

 ponding to any one of the principal Frauidiofer's line illuminate the 

 slit. The following oljservations were made for the positions of zero 

 intensity of the fringes formed by the slit. 



Width of the slit l>^ = 0.5745 mm. 

 Wave leno'th of liHit X = 0.000-1861 mm. 



