: 
: 
L 
4 
: 

Bright Spectrum Lines. 489 
Similarly for the second bright fringe 
2D cos @=(m—2)r 
Ty/ 4m ey 
m— 2 

hence io 
and so forth for R;, R,, &e. 
_The following table gives the values of Rj//, R,/f, &c. for 
different values of m :— 






| Mm. Ry/f. BUF ty Bele R,/f- | R,/f. | 
a en 
eee. 1°732 ae ee = = 
oa 1-118 2-828 0 —- | = | 
eae: 82 | 1-732 | 3873 & hes 
BOOP... 143 22 (sa ge 59 -292 329 

From this table we see that when m=1, 7. e., the difference 
of path is ene wave-length, there is only one intertference- 
band, and its radius is infinite, thus the field would be 
uniformly illuminated. When the difference of path is two 
waye-lengths there are only two fringes, the first whose radius 
is 1:732f; the radius of the second being infinite. For m=3 
there are three fringes. The entire system of bands could 
only be observed by means of infinite glass plates. We also 
see that as m gets large, which in practice is generally the 
ease, the lower row in the table shows us that the distance 
between the first and second rings is much larger than that 
between the second and third, and so on moving out in the 
system. Thus the separation of the fringes gradually 
diminishes as we go out from the centre, and hence the 
advantage of making the observations on the central fringes. 
This is clearly shown by the figures on the Plates, which are 
reproduced from photographs*. 
This interference method, besides being applied for the 
analysis of spectrum lines, can be used in the study of the 
changes in the wave-length of any radiation under the dif- 
ferent conditions as indicated above. Any small change will 
be shown by an increase or decrease in the diameters of 
these rings, and since very clear photographs can be taken, 
very accurate measurements on the changes produced can be 
obtained. 
* We regret the reproduction of the photographs of the plates in 
half-tone does not do justice to the figures sent; in particular the 
“screen” used has caused “ghost” circles to appear on fig. 16 which 
have not been successfully got rid of.—Ebs. 
