I -jo 



\on-r< y< raed Spectrum Interferon) eiry. 



st rial ions, through sharp arrows, back to the hairlike forms. 

 The range has thus been increased by the dispersion. The 

 arrows are of the slender type, with re-entrant sides and part 

 of the outline accentuated. 



In the second order of spectra from G, the phenomena were 

 much the same, but far more brilliant. The arrows were now 

 evenly wedge-shaped and very slender. The fringes entered 

 as nearly vertical hairlike striations and after passing the 

 optimum vanished as inflated arrows. The range of visibility 



Fig. 14. 



was as before about 3*5 millimeters so that the change of order 

 has not had any further marked effect, such as might be 

 anticipated. As in the preceding paragraph, if the impinging 

 collimated beam is narrowed, the range of visibility decreases ; 

 in fact the arrows themselves are reduced to slightly oblique 

 lines. Within the limits given the fringes are well adapted 

 for interferometry. 



First order spectra are not available because of the large 

 value of i' in the case of the right-angled prism. 



Taking the results of the last two paragraphs together, the 

 increase of the range of displacement is due to the dispersion 

 of the prism. The breadth of the pencil, diffracted at the slit, 

 after leaving the collimator and prism, increases. It was 

 shown in the earlier report that inversion of spectra on a 

 longitudinal axis does not preclude the possibility of interfer- 

 ence. Taken as a whole therefore, the present results have a 

 direct bearing on Huyghen's principle. 



Brown University, 

 Providence, R. I. 



[To BE CONTINUED.] 



