32 THE INTERFEROMETRY OF 



there is some variable element involved that escaped me, and it will hardly 

 be worth while to pursue the question further with the given end in view, 

 without a radical change of method. 



Screening middle parts of the grating (in relation to 15) did not lead 

 to noteworthy results here, but such experiments will become of critical 

 importance below. 



A word may be added in relation to Fresnellian interferences in the present 

 work. These would be liable to occur if the observations had been made outside 

 of the principal focus, with the sodium lines blurred. In all the experiments 

 on the excitation of a narrow ether gap, however, the D lines were clearly in 

 sight and sharp, so that the phenomena of non-reversed spectra and homogene- 

 ous light (in the next section) are not here in question. True, such interfer- 

 ences may often be found in the case of reversed spectra, when the sodium lines 

 are purposely blurred, by pushing the ocular toward the front or to the rear. 



12. Experiments continued. Homogeneous light. To turn to a second 

 class of experiments : very important results were obtained with homogeneous 

 light (sodium arc) on placing the DiD'i or D 2 D' Z lines in coincidence and then 

 broadening the slit indefinitely or even removing it altogether. A new type 

 of interferences was discovered, linear and parallel in character and inter- 

 secting the whole yellow field. These lines could (as above) be made to pass 

 from a grid of very fine, hair-like, nearly horizontal lines to relatively broad, 

 vertical lines, on changing the orientation of the grating G', figure 16. Small 

 changes of position of the grating produced a relatively large rotation and 

 enlargement of the lines of the interference pattern. The fringes, when verti- 

 cal and large, are specially interesting. The distances between successive 

 fringes obtained were about the same (accidentally) as the DiD 2 distance of 

 the sodium lines. They are quiet in the absence of tremor. If DiD'i or D 2 D' Z 

 were only present, the field would be an alternation of yellow and black 

 striations; but as both doublets are present, the interferences overlap the 

 flat (non-interfering) yellow field of the lines not in coincidence. The fringes 

 are nevertheless quite distinct. A single homogeneous line (like the green 

 mercury line) would give better results. It is necessary that the line selected 

 (say DiD'i} should coincide horizontally and vertically before the slit is 

 broadened. Otherwise no fringes appear in the yellow ground, or at least 

 not in the principal focal plane. On using a thin mica compensator, it is 

 easy to make these fringes move while the mica film is rotated ; and they pass 

 from right to left and then back again from left to right, as the mica vane 

 passes through the normal position of minimum effective thickness. Thus 

 this is a new form of interferometer with homogeneous light. The fringes 

 remain identical in size, from their inception till they vanish, while the microm- 

 eter M, figure 1 6, passes (as above) over about 15,000 wave-lengths. In this 

 respect the new interferometer differs from all other types, the two air-paths, 

 GMG' and GNG', alone being in question. The condition of occurrence will 

 be investigated in paragraph 13. 



