386 Preston—Radiation Phenomena in a Strong Magnetic Field. 
rectangular position it will cut off the two side lines, and so reduce the width of 
the modified line. If the resolution, however, has been sufficient to separate 
the inner edges of the two side lines by a distance greater than the width of 
the unmodified line than the modified line, according to theory, should appear to 
the eye as a distinct triplet, each component of which can be studied separately, 
but if the separation of the inner edges is less than this amount, but greater than 
zero, then, when the nicolis placed in position, so as to remove the middle 
compact, the side lines should be separated by a dark space, having the 
appearance of a doublet, and thus the existence of the triplet constitution of the 
modified line might be inferred, even though a field could not be obtained strong 
enough to resolve the line into a completely separated triplet. 
Although it is highly satisfactory to obtain evidence, in this way, of the 
existence of the triple constitution of the modified lines, it, no doubt, is still more 
eminently satisfactory to urge the resolution to such a pitch that the components 
shall be completely separated, and the triplet, if such exists, shall stand out as 
three distinctly separated lines. The character of each component can then be 
separately studied, and the laws which govern the resolution can be investigated 
under conditions which lend themselves with facility to accurate measurement. 
Further, the differences, if any, in the effects produced by the same magnetic 
field on the various spectral lines of the same substance, or different substances, 
can be examined in detail; and further information may be gleaned concerning 
the mutual relations of the spectral lines, in groups and series, and possibly a 
fuller insight may be acquired concerning those vibrations associated with 
matter which give rise to the spectrum itself. 
With the object of ultimately investigating these relations and with the imme- 
diate object of first verifying Dr. Zeeman’s experiments, I availed myself of the 
opportunity courteously afforded me of using the concave grating mounted in the 
Physical Laboratory of the Royal University of Ireland. This instrument has a 
radius of 21°5 feet, and is ruled with about 14,438 lines to the inch. The 
electro-magnet which I at first employed was kindly lent me by Prof. Barrett, 
of the Royal College of Science, Dublin: it is of the usual U-shaped type, 
and of very moderate power, having a core of about 2 inches diameter of 
soft iron. 
For the information of those who may wish to repeat these experiments, I may 
mention here that, at first, I had very great difficulty in obtaining any effect 
whatsoever. Indeed, it seemed as if the effect did not exist, for the spectral lines 
remained, as far as I could determine, absolutely unaltered when the magnet was 
excited. Often, indeed, the lines seemed to be altered in breadth or brightness 
when the field was excited, but the effect was so indefinite that it might easily 
have been attributed to the observer’s imagination. 
