472 Intelligence and Miscellaneous Articles. 





If, on the contrary, the light extends over a considerable sur 

 face, the thickness of the gelatine varies at different parts, 

 especially if the plate has been placed vertically while drying ; the 

 bands are then bent while diverging, and their greatest divergence 

 is at the part where the layer is thinnest. 



"When the grating has been prepared, the distortion of the bands 

 is very irregular ; but after a great number of hydratations followed 

 by dryings the phenomenon becomes more regular. After fixing 

 the grating in water and drying, the gelatine possesses, as is known, 

 a kind of temper which is manifested by its accidental double 

 refraction ; but when it has been hydrated and dried slowly a 

 great number of times its structure becomes more homogeneous, 

 and the bands no longer possess serrations. It is possible that 

 such alternations injure the good keeping of the gratings, and it 

 is thus desirable to keep them in a dry place. 



Observing the band-spectra in the goniometer they appear like 

 broad and very dark spaces, but if sun-light is used condensed on 

 the slit, by a cylindrical lens, the pivot lines of the spectrum are 

 defined in these spaces with marvellous precision. The production 

 of these parasitical bands does not affect the accuracy of measure- 

 ments made with these gratings. 



With gratings in gelatine on transparent glass these phenomena 

 are scarcely perceptible by reflexion or by transmission, owing to 

 the almost total identity of the refractive indices of gelatine and 

 glass. 



If the index of gelatine is taken at 1*52, it is easy to calculate 

 the thickness of the layer of gelatine as a function of the number 

 of bands contained in the spectrum reflected on silver ; I have thus 

 found that in the copy which I use the thickness of the layer is 

 0-04 millim. when it is dry, and about 0*16 inillim. when it is at its 

 maximum hydratation ; this number is only approximate, as the 

 index varies with the quantity of water it contains. 



M. Izarn's gratings are of admirable sharpness, and examined 

 in the microscope they do not differ from the original ; in a Fro- 

 ment's grating, a hundred one, which I possess, the opaque interval 

 is virtually equal to a fifth of the transparent interval : this is also 

 the case with M. Izarn's copy ; this is not a negative but a posi- 

 tive. The transparent intervals are the bands of insoluble gelatine, 

 while the opaque intervals are the places where the soluble gelatine 

 has been dissolved away by the water ; but owing to the extreme 

 fineness of the intervals, the water by capillary action has hollowed 

 out cylindrical grooves which to a plane wave behave like an opaque 

 body. When the opaque interval is very great compared with the 

 transparent one, the opposite might take place ; but it is easy to see 

 that even when the two intervals are transparent, the difference of 

 the refractive indices of gelatine and air is sufficient to produce 

 phenomena identical with those of the grating. This question calls 

 for new investigations. — Gomptes Rendus, March 27, 1893. 



