﻿586 Prof. E. "\V. Wood on the Intensification 



used for the purpose of reproducing objects in their natural 

 colours must be of uniform intensity over their whole extent ; 

 that is, they must appear uniformly illuminated when the 

 pupil of the eye is brought into coincidence with the f ocussed 

 spectrum. Very often a point would rule a millimetre or 

 two of great brilliancy and then break down, ruling the 

 remainder of the surface (10 cms.) with perfect uniformity, 

 but with lines so fine that the spectra were scarcely visible 

 when it was held up to a gas-flame. Cross-sections of these 

 gratings, examined under the microscope, showed that the 

 lines were usually deep enough, but that they lacked sufficient 

 breadth. 



"Remembering Lord B.ayleigh/s interesting work on the 

 development of latent scratches on glass by etching with 

 hydrofluoric acid, it occurred to me that treatment with 

 dilute acid might improve the gratings. On referring to 

 Lord Rayleigh's paper I found that he had thrown out this 

 same suggestion, but without trying it apparently. An old 

 grating, which, when placed on the vievving-stand for the 

 diffraction colour photographs, showed scarcely any luminosity 

 at all, was partially submerged in the acid (about 1 part to 

 100 of water) for half a minute, washed, dried, and put back 

 on the viewing- stand. The portion treated fairly blazed 

 with red, green, or blue light, according to the position of 

 the eye. Further experimenting showed that nearly all 

 of the bad gratings could be made first class by a bath of a 

 minute or two in the acid. The back surface was painted 

 with paraffine to prevent the development of latent scratches 

 by the acid. 



Lord Rayleigh has shown that the action of the acid is 

 remarkably uniform, it being an easy matter to remove a 

 layer of glass one or more wave-lengths in thickness.. 

 Furthermore the surface remains optical, even after long- 

 immersion, which makes it appear probable that the per- 

 formance of a glass grating would be quite as good after 

 etching as before. In some cases I have succeeded, by 

 starting with a fairly brilliant grating, in producing a grating 

 which almost completely suppressed the central image, did 

 so in fact for a limited region of the spectrum. When 

 held up to a distant window, backed by the sky, the ruled 

 surface appears like heavily smoked glass. The gratings 

 are i( blacker " in fact than any that I have ever been able 

 to produce on bichromated gelatine, or by converting " silver 

 on glass 5 '' gratings into silver iodide, the method used by 

 Quincke in his stud)* of gratings giving coloured central 

 images. It is important to determine exactly how much. 



