REVERSED AND NON-REVERSED SPECTRA. 



87 



horizontally over the other. The forms, t, correspond to minimum path- 

 difference, remembering that in accordance with figure 59 all rays pass the 

 plate of the grating twice. 



Further experiments were made with sunlight to detect the changes which 

 befall the phenomena in different focal planes. The ocular of the telescope 

 was gradually drawn out from an inner extreme position to an outer extreme 

 position, through the normal position for principal focal plane. In this case 

 a variation of form corresponding closely to figure 62 was also observed. The 

 characteristic feature, however, was the prevalence of arrow-head or caret- 

 shaped lines, both in the case of the extremely fine striations and of the coarser 

 nodules. In the former case these roof -like designs were closely packed from 

 end to end of the phenomenon and usually pointed upward. They recall the 

 top edges of extremely eccentric ellipses in displacement interferometry, and 

 in view of their lateral motion with the micrometer M and the decreased dis- 

 persion due to double diffraction, their origin may be similar. 



39. The same. Inferences. When the pencils, Mm and Nn, figure 59, are 

 parallel and sodium light is used, the whole field is uniformly striated, whether 

 the striations are made fine or coarse. I have found it impossible, on placing 

 plate compensators (0.5 to 1.5 cm.) in both beams and rotating these to any 

 degree whatever, to produce any suggestion of a secondary periodicity in the 

 field. The fringes for a thick compensator, slightly wedge-shaped, merely 

 become a little finer. Films of mica are liable to blur the field. In general, 

 moreover, reflections would be relatively weak and thus inappreciable. They 

 would require a separate adjustment for coincidence and not appear with 

 the principal phenomenon. Hence the strands of interferences obtained in 

 case of crossed rays are in a measure unique. The second periodicity is not 

 stationary, but a part of the phenomenon. The glass plate of the grating 

 produces an effect in virtue of its thickness, precisely as in the case of the dis- 

 placement interferometry of my earlier papers. 



Experiments made with polarized light proved to be entirely negative. 

 The phenomenon appears between a polarizer and an analyzer so long as 



sufficient light is present to ex- 



i'Lf 'ru fy) /f )'7 fy} 

 hibit it. Observation with a 



nicol, in the absence of the polar- 

 izer, showed nothing but the ob- 

 vious effect of reflection. 



The occurrence of these par- 

 allel strands for crossed rays and 

 homogeneous light is thus diffi- 

 cult to explain. I have tried a great variety of methods of superposing 

 special interferences, etc., to produce the nodules with parallel rays, niM 

 and nN, or to break them with crossed rays, mN and nM, without avail. 

 There is no focal plane effect, nor any polarization effect. It is therefore 

 necessary to confront the case at its face value, as in figure 63. Here 5 



