

Some Spectroscopic Notes. 461 
field that are usually noticed. This is the interference 
effect. 
If a bright line upon a faint background is being examined, 
in addition to the above images we see a broad band in the 
centre of the field, and on each side of it a number of bands 
which are narrower and very much fainter. As many as 
eleven of these bands may be seen on both sides. This is the 
diffraction effect. The two effects are seen superimposed as 
: ° 
Fig. 1S. 

in fig. 13. If the echelon is now rotated the diffraction 
effect remains practically steady in the field, and the inter- 
ference images move across the diffraction-bands, becoming 
Fig. 1. very much brighter when they cross the 
centre of the chief diffraction-band. if we 
are examining a continuous spectrum we 
see only the diffraction effect. In the in- 
tensity-curve eR a &e 
Gee A er 
Let the figure represent two adjacent 
steps of an echelon. Let 7 be the width of 
astep (E F) and e the length of astep (DE). 
Let there be » complete steps. Let the 
angle of incidence of the light-rays be 7, and 
let us consider those rays that leave the 
echelon in the direction 6. Let us consider 
all the rays which cross the face CD. They wili all suffer a 
change of phase relative to the extreme ray through C. 
The path through D is longer than the path through C by 
7 (sin @—wpsin 7). 
If we denote the relative angular phase-difference by 2F, 
aps 4% 2 
sin 0—ys =)- 
J (sin @—yp sin 2) Dar 
Similarly, if we denote by 2E the angular phase-difference 
that the light which travels through E suffers relative to that 
