416 C. Barns — Interference of Reversed Spectra. 



been able to obtain results with these methods owing- to sub- 

 sidiary difficulties. 



2. Observations and experiments with a single grating. — 

 On considering tig. 1, it will be seen that the doubly reiiected, 

 doubly diffracted rays, are also in a condition to interfere. 

 Thus the rays GMGNG and GNGMG have identical path 

 length, or at least of path difference. It is improbable, how- 

 ever, that superimposed on the strong spectra, this 'effect could 

 be seen, and the inferences relatively to a phenomenon pro- 

 duced by double diffraction would not be modified. 



r jf- {-j 



Many experiments were made to ascertain the path differ- 

 ence within which the phenomenon is visible but led to no 

 new results ; i. e., the limiting distances of visibility along the 

 ray are 15,000 to 30,000 wave lengths apart, about as estimated 

 in the preceding paper. This is the characteristic feature of 

 the phenomenon. It is visible for a wide slit even after the 

 Fraunhofer lines vanish. It disappears by decreasing in width, 

 when the slit is closed. If the ocular of the telescope is drawn 

 out, the phenomenon may even be observed after the Fraun- 

 hofer lines have vanished, in the dark stringy spectrum of an 

 extremely fine slit. 



Since the phenomenon was originally produced with sunlight, 

 it might be supposed that the edges of the Fraunhofer line 

 under conditions of tremor would interfere with each other. 

 The crucial test was made by using an arc lamp spectrum, and 

 it was then found that the phenomenon appeared as well as 

 with sunlight. 



A further question at issue is the breadth of spectrum needed 



