REVERSED AND NON-REVERSED SPECTRA. 



13 



The adjustment consists in first roughly placing all parts in symmetry with 

 sunlight, until the two spectra appear at E. The lens may be removed. 

 There should be a bright, narrow spectrum band on each side of and near the 

 edge of the prism mn; for it is clear that after passing the lens E, correspond- 

 ing rays from M and A T must both enter the pupil of the eye to be seen together. 

 To make the spectrum parallel, the mirror mn is rotated, as a whole, around 

 a vertical axis. The three screws on the mirrors M and N then assist in 

 completing the adjustment; the rotation around the horizontal axis brings 



the sodium lines in coincidence (both must be clearly seen and sharp and at 

 an appreciable distance apart); that around the oblique axis gives rise to 

 more or less overlapping, as required. The need of a sharp coincidence of 

 the sodium lines is very essential in all these experiments. 



After proper vertical position of mn has been found by slowly moving the 

 micrometer screw up and down, the fringes appear. They are usually very 

 fine lines, possibly indicating distant centers of the ellipses to which they 

 belong. The appearance is roughly suggested in figure 6 . They are thus totally 

 different from the preceding set, 3. They pass from the type a through b 



(contraction toward the violet end was not noticed) into the type c, when 

 the mirrors mn move in a given direction. The center of the ellipses is in 

 the vertical through the field of view for the adjustment b, in which case the 

 lines pass from end to end of the spectrum as a narrow band near the longi- 

 tudinal axis of actual coincidence of spectra, symmetrically. 



The height or breadth of the longitudinal interference band, d in figure 6, 

 is not greater than 1.5 to 2 times the distance apart of the sodium lines at 

 right angles to the band. From this the angular divergence of the breadth 

 of the band may be found, since X = D sin 6, where X is the wave-length of 

 light, D the grating space, and 6 the angle of diffraction. Hence for the two 



sodium lines 



= AX/>cos 6 



