88 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



As is well known, such a grating forms an uncoloured central image 

 of a bright line, flanked on each side by a number of spectra, violet 

 side inwards, red outwards. In the wire gauze each space between the 

 wires takes the place of the bright line, and forms its own white central 

 image in the proper place, with several broadened-out spectra on each 

 side. The spectra produced by a number of the spaces overlap and 

 produce composite colours. When these colours fall upon the bright 

 white imago of the interspaces, they produce no observable effect, being, 

 in fact, flooded out. Where, however, the bright colours fall upon the 

 dark image of the opaque wires, they readily manifest themselves. When 

 the wires are parallel to the lines of the diffraction grating (fig. 9, 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 gratings 

 the width of the spaces varies in a regular manner (Fig. 9, B), and the 



Fig. 9. 



Grating 



spectra formed vary accordingly, so that we get the different colours 

 showing themselves on the same wire. 



There are one or two useful purposes to which the above principle 

 may be applied. Thus, in experimenting with wire and other gratings 

 it is tedious to measure whether the wires and interstices are evenly 

 spaced ; but in this diffraction method any irregularity in the spacing 

 or ruling of the object grating reveals itself immediately to the eye by 

 reason of the different coloration of the particular wire or set of wires (or 

 rulings) to the others. 



Again, the arrangement may be used in investigations on colour sen- 

 sation, as it is easy to obtain an admixture of two or more pure spectral 

 colours in the natural proportions present in white light. If a screen bo 

 taken with two adjustable slots, any part of the spectrum of the one may 

 be made to overlap the spectrum of the other, and, by having the slots 

 A and B arranged tho one a little above the other (tig. 10), the top or 

 bottom of the held of view shows the two colours separately, and in the 

 central part of the field we have the admixture. 



This arrangement of slots, one above the other, seems to have been 

 employed many years ago by Helmholtz (see his Handbnch der Physio- 

 logischen Optik, p. 353). It is also referred to by Ogden N. Bood in- 

 his Modem Chromatics. But apparently tho employment of gratings 



