Interference with Reflecting Gratings. 129 



for n = to equation (17), as was actually observed (figs. 2 

 and 3). 



Finally, equation (14) might be used for observation in the 

 incremented form 



A(de/cW)= J ^- l Ae; .... (17) 



but I did not succeed with it. One loses track of the run of 

 a fringe with de. 



11. Coloured Slit Images and Disk Colours of Coronas. — 

 In the above experiment the fringes were but a few minutes 

 apart. It is obvious, however, that if S is sufficiently small 

 the fringes will grow with decreasing n, in angular magnitude, 

 until there are but a few black bands in the spectrum. Under 

 these circumstances the undeviated image of the superimposed 

 slits must appear coloured, particularly so, if an effect equivalent 

 to N is present throughout the grating. This phenomenon of 

 coloured slits is apparently of interest in its bearing on the 

 theory of coronas, where there is also an interference pheno- 

 menon superimposed upon a diffraction phenomenon, as is 

 evidenced by the brilliant disk colours. For instance, suppose 

 a corona were produced by a sufficient number of fog par- 

 ticles distributed throughout a plane normal to the undeviated 

 rays. Now let the alternate particles be moved in the same 

 way slightly to the rear of their original position, and let the 

 distance between the two planes be small relatively to the 

 wave-leno-th of lioht. In such a case there should be two 

 identical coronas superimposed in all their parts, and they 

 should therefore interfere. Inasmuch, however, as even small 

 fog particles are of the order of size of "0001 cm. and their 

 mean distance apart 50 times larger, i. e. '005 cm., it remains to 

 be proved whether such an effect can be looked to as an 

 explanation of the disk colours of coronas. 



The phenomena of § 4 must be producible with a trans- 

 mission grating, separated parallel to the rulings, with one 

 component grating movable micrometrically, though I have 

 not yet tested this case. Obviously it will only be interesting 

 if obtainable from film gratings. 



In this connexion I may state, in conclusion, that I have since 

 succeeded in producing the elliptic interferences discussed in 

 my last paper (I.e.), with such gratings, almost as brilliantly 

 as with gratings ruled on glass. The film for this purpose 

 is cemented, under pressure, with chloroformed Canada 

 balsam, between unequal thicknesses of plate glass. 



My thanks are due to Prof. A. A. Michelson, of the 

 University of Chicago, for the loan of one of his reflexion 

 gratings, with which the above results were tested. 



Phil Mag. S. 6. Yol. 22. No. 127. July 1911. K 



