THE AID OF THE ACHROMATIC FRINGES. 



93 



This, curiously enough, is about the same value as was obtained in case of 

 equation (n), so that the same deductions apply. The conditions are some- 

 what more favorable for larger values of IJL. Thus in the limiting case M = \/3> 

 the^path-difference would be about doubled. 



68. Experiments. To carry out these experiments at the present time is of 

 course out of the question ; but a number of contributory observations may be 

 made with advantage. The case of figure 88 is in a measure similar to figure 

 92, where the dispersion of the cylinder G in the former case is simulated by 

 the prism P and the auxiliary mirrors m, m', of the latter. If the slit of the 

 collimator at L is not too coarse, two reversed spectra will be seen in the 

 telescope at T, which on being superposed by rotating m or N on a vertical 

 axis will show a vivid linear phenomenon in the lines of symmetry of the two 

 superposed spectra. On rotating m or A^ on a horizontal axis, the distance 

 apart of the fringe dots along this line may be given any reasonable value at 



pleasure. With simple means, therefore, the experiment can be made quite 

 sensitive. These displacements are at once referred to the definite wave-length 

 in which the linear phenomenon is put. The dispersion of the prism has no 

 bearing on the clearness of the phenomenon; 30 and 60 prism were tested 

 with like results. 



To obtain the achromatics and increased luminosity in the spectrum fringes 

 (now to be horizontal bands throughout from red to blue), the rays of the 

 spectrum will have to be reassembled, and that may be done as in figure 93, 

 by inserting a second prism P' in a way to counteract the effect of the first. 

 If the achromatics are to be obtained, the glass-paths of the two rays in P and 

 P', respectively, must be coincident. Hence, the axis of the collimator at L 

 must be inclined to accommodate the angle of minimum deviation of the iden- 

 tical prisms P,P'; and while N and m are parallel, N' and m f normal to each 

 other, L and T have their axes symmetric to N. The adjustments are not 

 difficult, as they need not be perfect to secure good achromatics; but if they 

 are not made, the fringes are numerous, colored, and unsatisfactory. 



The experiments, figures 92 and 93, differ from the case figure 88, because 

 the rays are parallel in the former case and condensed to a caustic by the 

 eccentric refraction of the cylinder in the latter. Hence with these a short- 

 range telescope with strong objective is necessary; but, as has been stated, the 

 lines of the solar spectrum nevertheless come out surprisingly clearly. Experi- 



