CHAPTER IX. 



SHARP SPECTRUM FRINGES WITH AN INDEFINITELY WIDE SLIT, INCLUDING 

 THE SUPERPOSITION OF FRINGES DUE TO THE COLOR AND TO THE 



OBLIQUITY OF RAYS 



70. Introductory. To obtain sharp spectrum fringes it is necessary, as a 

 rule, to use a slit narrow enough to show the Fraunhofer lines. Hence there 

 is sometimes a deficiency of light from this reason alone. It occurred to me, 

 on producing identical fringes of inclination (achromatics or monochromatics) 

 and of color (dispersion), that by their superposition a slit of any width (or 

 an entire absence of slit) would be admissible, without destroying the fringes 

 in the impure spectrum resulting. 



Furthermore, if the edge of the prism is rotated 180 around the axis of the 

 spectro-telescope, the inclination of all spectrum fringes must be symmetric- 

 ally reversed, i.e., inclination up toward the right (positive) will become incli- 

 nation down on the right (negative) to the same amount. The identical result 

 may also be reached, independently, by displacing one of the mirrors of the 

 interferometer parallel to itself (path-difference) until the fringes passing 

 through their maximum size reach the opposed inclination and size. Hence 

 there must be a relation of a periodic kind between the displacement of 

 mirror AN and rotation of the spectro-telescope A<p, by which sharpness of 

 fringes in the absence of a slit is conditioned. 



This device of locating an angle of rotation of the telescope by sharpness of 

 fringes may possibly be used for other purposes, somewhat after the manner 

 of the halfshade or the sensitive tint; for, if small, they jump suddenly out of 

 an intensely brilliant unbroken spectrum band when a definite Ap is reached. 



Finally, the fringes, being examples of interference of intense non-reversed 

 spectra, should be available in such experiments as described in the last 

 paper, Chapter VIII, for instance. No deficiency of light need therefore be 

 apprehended. 



71. Apparatus. To fix the ideas it will be necessary to give a diagram of 

 the apparatus (fig. 95) employed. It is the self-adjusting interferometer, very 

 serviceable here because of the large number 



of separate adjustments to be made, each of 

 which might otherwise require long searching 

 for fringes. White light L from a collimator 

 takes the paths 123457 and idySjT, N being ,_ ,_ ^ 

 a half-silver. The telescope T is provided 

 with the direct-vision grating g, capable of 

 rotating around the axis 5 T (angle A<p) . T and 



g are preferably rotated together, as a rigid system. The mirror MM' consists 

 of two independent, nearly coplanar parts as shown, one of which, M for 

 instance, may be displaced parallel to itself by the micrometer-screw along the 

 96 



Jlf/ 



