124 Prof. 0. Barus on Elliptic and other 



fringes come from the edges of the crack S of the gratings,, 

 and that the remainder of the grating will not produce coarse 

 fringes. By moving the slit the fringes may be made to 

 appear in any other part of the spectrum. 



The same fact may be proved by putting a vertical slit 

 over the lens of the collimator and allowing white light 

 to fall on the edges of the grating at S. Coarse fringes 

 limited as to range and size are then seen throughout the 

 spectrum. 



Whenever the slit or vertical stop is used, the fringes are 

 exceptionally sharp and easily controlled for micrometry.. 

 It is not even necessary to adjust the two spectra horizontally 

 with the same care as when no slit is used ; but the vertical 

 coincidence of spectrum lines must be sharp. Naturally the 

 use of the slit has one drawback, as the resolving power of 

 the grating is decreased and the spectrum lines are only just 

 visible. The adjustment, however, may be made before the 

 slit is added. A few examples may be given : For a slit 

 1 mm. wide over the telescope or collimator, only the imme- 

 diate edges at the crack S, about 1/2 mm. each in breadth, 

 are active. A narrow range of large fringes is seen in the 

 field easily controlled by the micrometer screw. With a slit 

 3 mm. in width the lower limit is much increased, the upper 

 diminished, to a size of about 3' of arc per fringe. In the 

 absence of the slit the field is free from fringes. With a slit 

 6 mm. wide, the upper limit is again decreased, the lower 

 much increased ; nevertheless the finest fringes appear only 

 after the slit is removed. Using double slits over the colli- 

 mator, each 1 mm. wide and 3 mm. apart, fringes of medium 

 size limited at both ends appear ; 3 mm. slits 6 mm. apart 

 show only the very fine fringes, but both sizes are still 

 limited. Finally, when all but about 1/2 mm. of the edge of 

 the crack of the grating g' is screened off, whereas the whole 

 grating g (about 1/2 inch square) is without a screen, all the 

 fringes from the maximum size to complete evanescence 

 beyond the range of visibility are producible. Naturally if 

 the edge of g' is quite dark everything vanishes. 



It follows, therefore, that pairs of corresponding rays are 

 always in question, These corresponding rays are at a 

 definite distance, ND, apart, where D is the grating space 

 and N the number of lines per centim. of the grating in 

 question. This distance ND is greater as the fringes are 

 smaller, and may be of the order of a centimetre when the 

 fringes pass beyond the range of visibility. Again, ND is 

 equal to the width of the crack when the largest fringes 



