G Bar us —Interferences of Grossed Spectra. 491 



or of proceeding, in other words, on the plan of Rowland's con- 

 cave reflecting grating. In such a case the light would enter in 

 the direction TG, fig. 1, b, be diffracted along GM, back along 

 MG and then return along GT at a slightly higher or lower level 

 than on entering. The equation just given would still apply 

 and many interesting modifications are suggested. Experi- 

 ments of this kind are in progress. Moreover, in case of the 

 plane transmitting grating and plane mirror, as above shown, 

 the same simplification is possible, if the lens is replaced by 

 the telescope at T. But in this case the spectra are intersected 

 by strong stationary interferences due to reflections from front 

 and rear faces and consequently not conveniently available. A 

 reflecting grating and telescope would not encounter this 

 annoyance. In general, however, as in the disposition adopted 

 fig. 1, b, the light enters opposite the observer and the light 

 directly transmitted can be screened off; this is a practical 

 convenience in favor of the transparent grating. The reflected 

 spectra used may be placed at any level by rotating the mirror 

 mona horizontal axis. 



On further repeating the work by the use of the concave 

 mirror in, and a strong eyepiece at T, fig. 1, 6, and using a 

 compensator, I eventually succeeded in erecting the interfer- 

 ence design, G, fig. 1, a. It then took the form given at D 

 and this seems to furnish the final clue to the subject. In 

 other words, the design consists of a new type of extremely 

 eccentric ellipses, with their long axes parallel to the Fraun- 

 hofer lines, each end having the outline of a needle point, 

 possibly even concave outward. Only one end of a closed curve 

 is obtainable. These jet black lines dance on the highly colored 

 background of less than half the width between the two 

 sodium lines. The interference design, therefore, would be the 

 same (apart from color) as that which would be obtained, if the 

 spectrum containing ordinary elliptic interferences were to 

 shrink longitudinally from red to violet, till it occupied less 

 than half the space between the two D lines. In fact I have 

 at other times obtained just such patterns, with all the colors 

 present, but not in the pure yellow, as in the present case. 

 Vertically, the path difference is always due to more or less 

 obliquity of the rays passing through the plate of the grating. 

 Horizontally, however, the equivalent path difference results, 

 in the present case, from the fact that one wave-length of a 

 pair has increased, whereas the other has diminished, while 

 both may pass through the same thickness of glass and air. 



I. Case of coincident spectra with one reversed on a given 

 longitudinal axis. — For this experiment it is necessary to 

 reflect the first order spectra issuing at the grating G, figure 2, a, 

 from the ruled face g (a narrow preferable horizontal blade of 



