C. Barus — Use of Compensators. 307 



corresponding rays be drawn through the extremity of M and 

 JY, their fields of interference F and F' would begin in the 

 plane 11'. For axial rays it would be at i. Thus the locus as 

 a whole would not be a plane, and this seems to be the case. 

 If the telescope moves towards the grating, II' moves toward 

 the right in the figure, as though the virtual object beyond the 

 grating were fixed in position. At all events, the problem is 

 to find the interference diagram of two symmetrical, plane, 

 parallel spectra of different areas and placed at definite dis- 

 tance apart. 



The appearance of the fringes is indicated in fig. 7 where S 

 is the height of the spectrum, usually quite out of focus. 

 There are many more lines than could be drawn in the sketch. 

 The ends a and o seem to surround small ellipses, but these are 

 not quite closed on the outer edge. The center of symmetry 

 is at c. The demarcations are stronger and broader vertically, 

 if the distance apart of the doublet I, fig. 4, is small ; fainter 

 but nevertheless clear and narrower if this distance is large. 

 Horizontally the fine lines thread the spectrum. The best 

 results were obtained when the lenses I are less than l cm apart, 

 the middle band being about half as high as the spectrum. 

 Two contiguous lenses gave a design which nearly filled the 

 spectrum. For practical purposes the lens compensator L is 

 to be attached to the mirror, M, just in front of and moving 

 with it. It makes little difference, here, whether the concave 

 lens or the convex lens of the doublet L is foremost. 



If the micrometer M is moved, or if the telescope is slid to 

 the right or left, or forward, so as to take in other parts of the 

 spectrum, the nearly closed lines at a and b become finer and 

 finer crescent shaped lines, always open outward, till they pass 

 beyond the range of vision. The whole phenomenon remains 

 on the same level of the spectrum. On moving the telescope 

 towards G, fig. 4, the ocular has to be drawn outward (towards 

 the eye) till it is fully 2 cm beyond the position of the principal 

 focal plane. The whole spectrum is now seen with the interfer- 

 ences from red to violet (no ellipses), but having the same relative 

 position as before. The central horizontal band measures about 

 1/5 the height of the spectrum, while the fine parallel horizon- 

 tal lines extend to the upper and lower edges. The appearance 

 is now curiously like a blunt wedge ; the band is nearest the 

 eye and the lines running abreast extend towards the rear. 

 This impression is probably an illusion due to the shading. 

 The lines grow finer and are more crowded toward the bottom 

 and top of the spectrum. The illusion of a reentrant wedge is 

 thus not possible. 



To use this interference pattern for measurement, the cross 

 hair is supposed to pass through the region c, fig. 7, symmetric- 



