Interference-Bands. 87 



to render achromatic, and therefore distinct, the first 20 or 

 30 bands. As the adjustment improves, a continually larger 

 number become visible, until at last the Avhole of the doubly 

 illuminated field is covered with fine lines. 



In these experiments the light is white, or at least becomes 

 coloured only towards the outer edge of the field. By means 

 of a fine slit in the plane of the spectrum we may isolate any 

 kind of light, and verify that the band-systems corresponding 

 to various wave-lengths are truly superposed. 



When the whole spectrum was allowed to pass, the white 

 and black bands presented so much the appearance of a grating 

 under the microscope that I was led to attempt to photograph 

 them, with the view of thus forming a diffraction-grating. 

 Gelatine plates are too coarse in their texture to be very 

 suitable for this purpose ; but I obtained impressions capable 

 of giving spectra. Comparison with spectra from standard 

 gratings showed that the lines were at the rate of 1200 to the 

 inch. A width of about half an inch (corresponding to 

 600 lines) was covered, but the definition deteriorated in the 

 outer half. A similar deterioration was evident on direct 

 inspection of the bands, and was due to some imperfection in 

 the conditions — perhaps to imperfect straightness of the slab. 

 On one occasion the bands were seen to lose their sharpness 

 towards the middle of the field, and to recover in the outer 

 portion. 



With respect to this construction of a grating by photo- 

 graphy of interference-bands, a question may be raised as to 

 whether we are not virtually copying the lines of the original 

 grating used to form the spectrum. More may be said in 

 favour of such a suggestion than may at first appear. For it 

 would seem that the case would not be essentially altered 

 if we replaced the real spectrum by a virtual one, abolishing 

 the focusing lens, and bringing Lloyd's mirror into the 

 neighbourhood of the grating. Bui then the mirror would be 

 unnecessary, since the symmetrical spectrum upon the other 

 side would answer the purpose as well as a reflexion of the 

 first spectrum. Indeed, there is no escape from the conclusion 

 that a grating capable of giving on the two sides similar 

 spectra of any one order, without spectra of other orders 

 or central image, must produce behind it, without other 

 appliances and at all distances, a system of achromatic inter- 

 ference-fringes, which could not fail to impress themselves 

 upon a sensitive photographic plate. But a grating so 

 obtained would naturally be regarded as merely a copy of 

 the first. 



Another apparent anomaly may be noticed. It is found in 



