714 Dr. G. W. C. Kaye on a Method of 



It may be found necessary to prop the upper plate in position 

 until the surface in contact with the glass has " set." The 

 cylinders which are made of gelatine and water are now to 

 be immersed in glycerine, the glycerine jelly cylinders in 

 cold water. The glycerine should be stirred occasionally, 

 as the layers in contact with the jelly take up the displaced 

 water. The action will be found to be well under way in a 

 quarter of an hour, the glycerine gradually diffusing into 

 the jelly, driving out the water, and the water gradually 

 replacing the glycerine. A jelly containing glycerine has a 

 higher refractive index than one containing water, con- 

 sequently the cylinders soaked in glycerine act as concave, 

 while those soaked in water act as convex lenses. 



The focal length will be found to be only 8 or 1.0 cms., 

 and very sharp images of the filament of an incandescent 

 lamp or a gas-flame can be obtained with them. 



Interesting refraction effects can be observed by nearly 

 closing the ends of a tin pipe o or 4 metres long and 10 or 

 15 cms. in diameter with plate glass, inclining the tube and 

 pouring in sufficient gasolene (petrolj to wet the entire 

 bottom of the tube. On tilting the tube back into the 

 horizontal position, the cross section of the circular end 

 appears deformed into an ellipse when viewed through the 

 opposite end with the eye near the bottom, and external 

 objects are seen much distorted. Proximity of a flame is to 

 be avoided. 



IjXXIX. On a Method of Counting the Rulings of a Dif- 

 fraction Grating. By G. W. G. Kaye, B.A.', B.Sc. The 

 National Physical Laboratory *. 



[Plate XV.] 



IT would appear from a review of the earlier determinations 

 of wave-lengths by the use of diffraction gratings, that 

 most of the results w r ere vitiated by an imperfect knowledge 

 of the value of the grating-space rather than by inaccurate 

 measurement of angular deviation. 



This was the case with the pioneer work of Fraunhofer 

 (1814-1823) with wire and later with glass gratings ; of 

 Ditscheiner (1864, 1866) o with one of Fraunhofer's gratings ; 

 and of Mascart (1864), Angstrom (1864) , and van der Wil- 

 ligen (1868), each of whom worked with Nobert's gratings f. 



* Communicated by Dr. R. T. Glazebrook, F.R.S. 



t For a bibliography see Bell, Phil, Mag. xxv. p. 250 (1888). 



