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J. RHEINBERG ON THE IMITATION OF 



It will be seen at once that when the wires are parallel to the 

 lines of the diffraction grating (Fig. 1 a), then, if they are spaced 

 regularly, the colours developed upon them must be the same in 

 each case, but so soon as the wires are rotated, then, instead 

 of having equally wide spaces lying transverse to the grating, 

 the width of the spaces varies in a regular manner (Fig. 1 b), 

 and the spectra formed vary accordingly, so that we get the 

 different colours showing themselves on the same wire. 



It should be noted that for the production of brilliant effects, 

 the magnified image of the wire gauze, or object grating, as we 

 may call it, must bear some sort of relation to the diffraction 

 grating over the objective. The finer the latter the coarser the 

 former may be, and vice versa, so long as we employ the same 

 objective. 



Fig. I. 

 Grating 



There are one or two useful purposes to which the principle 

 involved in the use of a diffraction grating over the objective 

 adapts itself. 



For instance, I have lately been making a number of experi- 

 ments with wire and other gratings, and it is rather a tedious 

 matter to measure whether the wires and interstices are evenly 

 spaced. But if these are placed on the microscope stage, with a 

 suitable grating over the objective, any irregularity in the spacing 

 or ruling of the object grating reveals itself immediately to the 

 eye in a forcible manner by reason of the different coloration of 

 the particular wire or set of wires (or rulings) to the others. It 

 is therefore an aid to the calibration of such gratings. 



Another purpose for which the arrangement described may be 

 used, is in investigations on colour sensation, as we can readily 



