BInndicstcr Moiioirs, Fo/. xliv. (1900), No. VI. 7 



apparatus sliowu at the mcctin<4", in their natural colours, 

 the spectra bein^t^ such that a sh'ght movement has practi- 

 cally no deleterious effect U[)on the colours of the pictures. 



With [gratings of say 30,000 to the inch, purer colours 

 still would result, for as is well known, the length of the 

 spectrum increases as the distance between the lines 

 dimininishes, and there is practically no limit to the 

 closeness of lines which this method of reproducing them 

 is capable of. 



It is quite evident, of course, that an)- colour combina- 

 tion besides the natural ones can be made by merely 

 altering the positions of the mirrors. The apparatus is 

 therefore particularly applicable to designs in colour, such 

 as stained glass, &c., for which purpose photography, 

 except as applied to the bicliromated gelatine film, is not 

 necessary. 



The requisite designs can be produced by hand, 

 and printed through the grating films, when colour combi- 

 nations of an almost infinite variet)' can be made up, by 

 altering the position of the mirrors. 



For producing merely natural colour effects by means 

 of diffraction-gratings. Professor Wood's method of using 

 three differently ruled gratings cannot well be improved 

 upon, or, at least, that would appear to be the case. 

 There is, however, the difficulty that parallel superposed 

 gratings give interference bands, but whether these would 

 be sufficiently obtrusive as to produce a visible effect can 

 only be determined by direct experiment. That there 

 are such bands on Professor Wood's own pictures is 

 plainly seen, but they are possibly due to the unequal 

 spacing of the lines. 



Up to the present time I have not succeeded in im- 

 pressing more than one grating film on to one gelatine 

 plate, but am in hopes that a metal matrix may yet be 



