PHYSICS: C. BARUS 
577 
the white light impinging on the grating is previously dispersed, pref- 
erably by an Ives direct vision grating (with auxiliary prism). The 
rulings of both gratings (the Ives grating being between the collimator, 
at some distance, and the first grating of the interferometer) are to be 
parallel. If the grating constants Di are different (Z> = 167 X 10"^ cm. 
film, and D = 352 X 10"^ cm. ruled grating, were employed) the spectra in 
the telescope are naturally of different lengths: for the dispersion of the 
Ives grating is increased on one side and decreased on the other side, 
by the grating of the interferometer. Moreover this decrease from the 
larger dispersion of the first grating is beyond zero (achromatism) into 
negative values. Hence the corresponding duplicate spectrum in the 
telescope is a small spectrum with a large spectrum reversed relatively to 
it, while the inversion remains intact. In the experiment made, the 
larger doublet, D\ was somewhat more than twice as broad as the 
smaller Di A. 
It is now merely necessary to place any longitudinal axis (line of 
symmetry) of the two spectra seen in the telescope in contact; or it is 
but necessary that the spectra are longitudinally parallel and overlap. 
The phenomenon then appears at the intersection of the lines of longi- 
tudinal and of transverse symmetry. It is thus proportionately nearer 
the smaller Di A and farther from the larger D' D' doublets, but always 
between them. If the Di A He within the D'l D'2 lines, the fringes lie 
within the Di A pair. 
The phenomenon which should be observed with a powerful telescope, 
usually consists of three or fewer small elongated dots, lying within an 
elliptic locus, the locus usually having a transverse axis (parallel to the 
Fraunhofer lines) about two or three times as long as the longitudinal 
axis (parallel to lengths of spectra) . Usually the width was Di A and 
the length larger than D\ B'^^ but this ratio may be changed by screen- 
ing off the wavefront. The fringes were not more than one-half of 
Bx A apart. 
The interesting result is here again met, incidentally, that spectra 
from the same white source, though of different lengths, are neverthe- 
less quite capable of producing strong interferences on overlapping. 
In further experiments with the long collimator and very bright spec- 
tra, a variety of other forms was obtained. In most patterns the 
elliptic outline, sometimes circular, is always evident from the enhanced 
brightness of the bright fringes of the spot. As any adjustment of 
overlapping spectra suffices, the B lines may be quite out of the field, 
or the spectra may be slightly separated with the interference spot in 
the gap. 
