REVERSED AND NON-REVERSED SPECTRA. 99 



the case of highly diffracting grating (D=io" 6 Xi75), the results appear best 

 in the second order. 



The same beautifully duplicated fringes were obtained with a transmitting 

 film grating of about the same dispersion, particularly well in the first order. 



The sodium flame gives too little light for the present purposes, but the 

 phenomenon is seen. 



Believing that some irregularity might be introduced by the double-sodium 

 line, I installed a mercury lamp for comparison. In the first experiment a 

 film grating (D= 173X1 o" 6 ) was used, the ocular traveling outward from the 

 principal focus. Both the green and the double yellow mercury lines enlarged 

 and showed fringes of increasing size and number together. The green field 

 had a darker band, the yellow a bright band in the middle. As the fringes 

 enlarged, each split up into secondary fringes, 4 or 5 eventually, and this 

 again occurred for both green and yellow fields. 



Rotating the grating around a vertical axis seemed to shift the primary 

 fringes laterally over the stationary secondary fringes. A concave lens for 

 positions anterior to the principal focus and a convex 

 lens for posterior positions (toward the eye) were 

 successively added to increase the range of observa- 

 tion. On both sides of the principal focal plane 

 (fig. 71) fringes occur, which enlarge with the dis- 

 tance x from that plane. As they enlarge, each fringe 

 splits up into secondary fringes, which in turn enlarge. 

 Sometimes the arrangement is irregular. Green and yellow fields may 

 overlap, but they do not do so conformably. 



The undeviated ray, however fine the slit may be, merely shows a stringy 

 field, sometimes suggesting structure, but never showing clear-cut fringes. 



The same kind of results were obtained with a reflecting grating of about 

 the same dispersive power. In the second order the fringes were particularly 

 clear and regular. Primary fringes, finally, carried three to four secondary 

 fringes each. 



Next, a ruled transmitting grating of less dispersive power (grating constant 

 352Xio~ 6 cm.) was adjusted for mercury light. Here in the undeviated ray 

 and in the first order no clearly separated fringes were obtained. In the second 

 and third orders, however, they were very perfect, and followed the above 

 rules, showing sharp secondary fringes. 



It follows, therefore, that a certain degree of dispersion is needed to resolve 

 the fringes, which is inadequate in amount in the order zero, in this case, 

 and scarcely so in the first order. In the higher orders the conditions are met. 

 Using a very fine slit, however, I later just succeeded in separating the fringes 

 in the first order. 



Finally, I returned to the endeavor of detecting diffraction fringes in the 

 undeviated image, using a micrometer slit, a good achromatic lens (or no lens), 

 and a distant (2 meters), moderately strong telescope. In this case separated 

 and distinct diffraction fringes, white throughout, were undoubtedly obtained. 

 They moved with the eye so as rarely to be stationary and in the same direc- 



