52 THE INTERFEROMETRY OF 



the character of coincidence; i.e., whether the rays are convergent or diver- 

 gent. Finally, a slight rotation of the slit around the axis of the collimator 

 rotates the fringes in the opposite direction to the sodium lines, and it is 

 rather surprising that so much rotation of slit (10 or 20) is permissible 

 without fatally blurring the image. The slightest rotation of one grating 

 relatively to the other destroys the fringes. 



Naturally, the colored fringes vanish when the slit is widened or when it is 

 removed. To give them sharpness, moreover, the beam passing through the 

 grating must be narrow laterally. It is possible to see these colored fringes 

 with the naked eye; but the transverse and longitudinal axes must in this 

 case be slightly thrown out of adjustment, so that the fringes are no longer 

 visible in the telescope. To the eye they form a somewhat fan-shaped set of 

 colored fringes; i.e., narrower below than above. Neither are the lines quite 

 straight. If the collimating lens is removed, a slit about o.i cm. wide across 

 a white flame will also show (to the telescope or to the eye) fine, strong lines 

 rotating in opposite direction to the slit, according as the transverse and longi- 

 tudinal axes are differently placed. As has been already stated, it is with 

 the latter condition that the focal plane in which the fringes He varies enor- 

 mously. 



Finally, when the sodium lines are superposed but the longitudinal axis of 

 the spectra not quite so, a second class of fringes appear, which, however, 

 are always more or less blurred. They rotate with great rapidity over 180 

 when one grating rotates over a small angle relatively to the other and the 

 angle between the longitudinal axes of spectra passes through zero. In the 

 latter position the regular fringes appear in full strength in the principal 

 focus. To see the secondary interferences, the ocular must be drawn inward 

 (toward the grating) , and these fringes increase in size with the displacement 

 of the ocular away from its position when regarding the principal focal plane. 

 This secondary set of fringes is always accompanied by another very faint 

 set, nearly normal to them and apparently quivering. The quiver may be 

 due to parallax and the motion of the eye. These are probably the vestiges 

 of the regular set of fringes, out of adjustment. 



21. Homogeneous light. Wide slit. Transverse axes coincident. If there 

 is no color-difference, fringes of the same kind will nevertheless be seen in the 

 telescope, on widening the slit indefinitely. Path-difference is here due to 

 differences of obliquity in the interfering rays. As in the preceding case, 

 accurate adjustments of the longitudinal and transverse axes (in case of 

 sodium, Di and D\ or D 2 and D' 2 coincide horizontally and vertically) of 

 the homogeneous color-field are essential if strong fringes are to appear in 

 the principal focus. These fringes are, as a rule, well marked, and widening 

 the slit merely increases the width of the channeled, homogeneous field of 

 view. If, owing to slight differences of grating space, the sodium lines are 

 not quite superposed automatically, this may be corrected by rotating either 

 grating, or else the apparatus as a whole, until the fringes are strongest. The 



