the Echelon Spectroscope. 



393 



e thickness of glass traversed 

 in 10,000, or *01 per cent. 



plate, for example one degree, th 

 is only increased by one part 



Since the retardation is increased by one part in 10,000 and 

 the original retardation is 5000 wave-lengths, the increase in 

 retardation is approximately half a wave-length ; this amount 

 of tilt, therefore, is that required to obtain the double-order 

 condition from the single order. If we increase the tilt to 

 1*5 degree, the retardation is increased approximately to one 

 part in 5000; and we should obtain, therefore, the single- 

 order condition again, but with the 5001st order. 



§ 5. The Magnet, 



Before proceeding to a consideration of the results obtained 

 of the Zeeman effect, some mention may be made of the 

 magnet used in these experiments. A photograph of it is 



given in fig. 6 



Fis-. 6. 



The pole-pieces were 5 inches square, and were turned down 

 to a cone whose semi-angle was 60°, with a flat *5 inch in 

 diameter at the tips. When viewing along the lines of force, 

 one of the pole-pieces used was bored out, the hole being 

 •5 inch in diameter at the tip and *75 inch towards the outer 

 face. 



Phil. Mag. S. 5. Vol. 49. No. 299. April 1900. 2 E 



