I9II.] WITH REFLECTING GRATING. 131 



is diminished, the black band at G gradually vanishes and in its 

 place appear the coarsest fringes producible. When the slit F is 

 removed these coarse fringes disappear. The fringes visible 

 through the slit have however both an inferior and superior limit 

 of angular size. When e is diminished to zero they vanish and 

 when e is sufficiently increased they again vanish, though they now 

 appear when the slit is either removed or widened. From this it 

 follows that the coarsest fringes come from the edges of the crack 

 6^ of the gratings, and that the remainder of the grating will not pro- 

 duce 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 the vertical slit F over 

 the lens of the collimator and allowing the white light L to fall on 

 the edges of the grating at 5". Coarse fringes limited as to range 

 and size are then seen throughout the spectrum at g. 



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

 ceptionally 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 draw- 

 back, 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 i mm. wide over the telescope or collimator, only the 

 immediate edges at the crack S, about .5 mm. each in breadth, are 

 active. A narrow range of large fringes are 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 inches per fringe. In the absence of the slit the field is 

 free from fringes. \\'ith 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 

 collimator, each i 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 .5 mm. of the edge of the crack of the grating g' is 



