the Echelon Spectroscope. 403 



is, within limits of experimental error, double that of the 

 value of the same quantity for the components of the outer 

 lines, and the value of 8\/A, 2 H for both the blue and green 

 lines is the same, within limits of experimental error. 



Whether this connexion holds good for other compound 

 lines is a question which requires careful investigation, and 

 on which further experiments are being made. 



In working with this instrument, it was found that the 

 yellow and green lines were far easier to examine than the 

 blue, on account of the diminution in the dispersive power of 

 the instrument for the longer wave-lengths. Using this 

 grating for ultra-violet light, it would be extremely difficult 

 to try and separate the components due to the central order, 

 and those due to the other orders; even with the line (4358) 

 it was some time before the splitting of the outer components 

 became at all obvious ; and seeing that what has been called 

 the constant (ki) of the instrument diminishes approximately 

 as the square of the wave-length, it is quite likely that with 

 lines nearer the ultra-violet, components due to the fifth and 

 sixth orders away might appear in the field. The constant 

 (k 2 ) also diminishes with shorter wave-lengths. 



In order , therefore, to make this valuable instrument more 

 serviceable for this class of work, attention should be directed 

 to the increase of the constant without too much diminution 

 of the resolving power. As suggested by Michelson, this 

 might be done by surrounding the instrument, say, with 

 water. This, however, would reduce the resolving power to 

 only one third of its original value. A better suggestion 

 is that of using thinner plates for the instrument and more of 

 them. The plates might with advantage be made, say 4 mm. 

 thick with, say, thirty plates and *5 mm. steps, or even 

 •25 mm. steps. This would, we believe, make a more useful 

 instrument for this work than one consisting of thirty 8 mm. 

 plates, though theoretically the resolving-power is only half 

 what it would be in the latter case. There is, of course, 

 no doubt that this instrument forms a most valuable addition 

 to the apparatus available for work on the Zeeman effect. 



Blythswood Laboratory, 

 Renfrew, N.B. 



