407 



Note on the Imitation of Polarised Light Effects by 

 Diffraction ; an Optical Curiosity, with one or two 

 USEFUL Applications. 



By J. Rheinberg, F.R.M.S. 



{Read Ajjril 20th, 1900.) 



Under my microscope this evening is placed a small piece of 

 wire gauze,* appearing resplendent in colours, which change as 

 the gauze is rotated. In certain positions of the gauze all the 

 horizontal or vertical wires assume the same colour throughout ; 

 in other positions brilliant colours of varied hues are developed on 

 each wire. 



The colours are not the ordinary spectral or rainbow colours, 

 and the whole is strongly suggestive of a polarised light effect, 

 for which it would be readily mistaken by an observer, before 

 inspection revealed that there was no polarising apparatus, and 

 that the object was quite opaque. 



What we have in reality to do with, is a diffraction effect. All 

 that has been done is to place a straight-lined diffraction grating 

 of 100 lines per millimeter (about 2,500 per inch) just above the 

 objective. 



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 image of the 

 interspaces, they produce no observable effect, being, in fact, 

 flooded out. W^here, however, the bright colours fall upon the 

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



* The gauze used had wires ^ mm. apart. 



