Intelligence and Miscellaneous Articles. 471 



ON THE INTERFERENCE-BANDS OF GRATING-SPECTRA ON 

 GELATINE. BY M. CROVA. 



Photographed gratings applied on bichromated gelatine by 

 M. Izarn's * method may give rise to straight or curved inter- 

 ference-bands, sometimes very irregular, in the spectra which they 

 produce ; similar bands have been produced by Brewster f in other 

 circumstances. These phenomena are obtained with great beauty 

 On the spectra obtained by reflexion on gelatine -gratings on 

 silvered glass. 



M. Izarn^ in mentioning these interference-bands, expresses the 

 opinion that they are connated with the interference phenomena 

 by parallel gratings which I formerly investigated %. 



Sunlight reflected from a heliostat is caught on a very narrow 

 slit the image of which is projected upon a screen ; a very small 

 image of the sun is produced at the focus of this lens, which is 

 received on the striated surface of a grating photographed on 

 gelatine on silvered glass ; the real images of the slit and of the 

 diffracted spectra are received on a screen placed in the conjugate 

 focus of the slit in respect of the lens. 



The diffraction spectra are furrowed with large rectilinear black 

 bands parallel to the rays, and which are almost absolute minima, 

 the intensity of the rays reflected on the silvered surface being 

 very little less than that of the rays which fall on the gelatine. 



With a copy of a fine Brunner's grating, which I owe to the 

 kindness of M. Izarn, the spectra of the first order present a large 

 dark band in the green when the grating is very dry ; if the 

 surface is breathed on the band is displaced towards the violet ; 

 other and closer ones enter at the red end, and their number rises 

 to three when the deposit of moisture confuses the projection. 

 The same phenomena are produced but in the opposite direction 

 during drying, and the displacement of the bands becomes very 

 rapid if the evaporation is accelerated by blowing air over the 

 grating. 



If the incident light extends over the whole height of the 

 grating instead of only to a small portion of the surface which is 

 obtained by varying the distance from the lens, the fringes are 

 curved, become irregular, and are sometimes serrated. 



The phenomenon is due to the interference of two parallel 

 gratings ; the one real, situated at the surface of the gelatine in 

 the points in which the incident wave meets its discontinuous 

 part ; the other virtual, which is its image in the silvered mirror. 

 Their distance, which is virtually constant, is the optical path, 2 ne, 

 e being the thickness, and n the index of the gelatine. At the focus 

 of the lens, since the light only affects a small part of its surface, 

 the thickness of the gelatine is virtually constant. 



* Comptes Rendus, vol. cxvi. p. 506. 



t Phil. Mag. [4] vol. xxxi. pp. 22 and 98 (1866). 



% Comptes Rendus, vol. lxxii. p. 855, and vol. lxxiv. (1871-1873). 



