THE AID OF THE ACHROMATIC FRINGES. 



99 



225, though the results are less smooth here from deficiencies in the orienta- 

 tion (45) of the achromatics. There was no fault to be found with the clear- 

 ness of fringes or with their abrupt evanescence. 



If the spectro-telescope Tg (fig. 95) with a very fine slit is rotated, the fringes, 

 as in figure 99, remain parallel to the length of the spectrum passing through 

 the forms a, b, c, d, e, where at c the spectrum is reduced to a single colored 

 line parallel to the slit. The fringes remain parallel to the edge of the prism. 



2W 260 



Hence if the form b, for instance, coincides with the achromatic or mono- 

 chromatic fringes, it will be retained sharply on opening the slit wide, whereas 

 a, c, d, e, requiring a fine slit, will vanish with the Fraunhofer lines. In the 

 absence of a slit the whole colored field bursts into sharp fringes whenever 

 the proper angle A<p of the telescope is reached. If the slit is a little too broad 

 to show the solar lines distinctly, the monochromatic fringes may often be 

 detected cross-hatching the vague Fraunhofer lines, even when the spectrum 

 fringes are still strong. 



If the fringes of a fine slit are at say 45 to the axis of the spectrum, their 

 inclination will change to 135 on passing the stage c. However, there is, in 

 such cases, a considerable change of angle relative to the spectrum as well as 

 of size, so that the conditions of compensation are complicated. 



74. Summary. It has been shown in the experiments that the fringes 

 (monochromatic) due to differences of inclination of rays, and the fringes 

 (dispersion) resulting from differences in wave-length of rays may be made of 

 nearly equal size by displacing any mirror of the rectangular interferometer 

 normal to itself (AAf). The fringes will not, how r ever, generally have the same 

 inclination. This may be imparted to the spectrum fringes by rotating the 

 spectro-telescope (prism edge) on its axis, until the inclinations also coincide. 

 In reality the phenomenon is more complicated as the spectrum fringes change 

 both size and inclination on rotation of the spectrum. In case of the comple- 

 tion of this twofold adjustment the slit of the collimator may be made indefin- 

 itely wide or removed altogether (undesirable light is to be screened off) . The 

 spectrum fringes may thus be given any intensity of illumination at pleasure, 

 while the wave-length corresponding to any fringe may be found by narrowing 

 the slit until the Fraunhofer lines reappear. When the fringes are small the 



