18 
ME. J. E. H. GOEDON ON THE DETEEMIN ATION OF 
nearly as X -2 . A sodium flame was first tried but found not to be sufficiently steady ; 
so it was decided to use a white light and a bisulphide-of-carbon prism. A very 
powerful paraffin-lamp with a duplex burner was finally chosen as the source of light. 
It was used with a lens and a slit, and threw a brilliant spectrum about 8 centims. long 
on a card. In the card was a vertical slit which admitted the light into the Nicol. By 
moving the prism, or lens, the spectrum could be moved along the card, and any desired 
colour admitted through the slit into the Nicol. In order to localize the light the lamp 
was removed from the lantern, and a spirit-lamp put in its place. The flame was 
coloured with thallium, and the green line projected on the card so as to fall upon the 
slit in it at minimum deviation. The lantern, lens, and prism being then clamped, the 
lamp was replaced, and the green light which fell upon the slit was of the same wave- 
length as that giving the thallium line. It was intended to make also observations at 
the sodium and lithium lines, but it was found impossible to isolate the sodium light, 
while to isolate the lithium the slits had to be made so narrow that the intensity was 
not sufficient to work by. 
In employing the Jellett analyzer, if the light used was not perfectly monochromatic, 
the effect of the magnetic force was to colour the two halves of the field different colours, 
rendering all comparison of intensities impossible. After a great number of preliminary 
observations (continued for about seven months), all of which were rejected as I found 
means to improve the methods used, the following sets were made. 
In these sets no readings were rejected, except on one occasion, when, finding the first 
few readings did not agree well with one another, I rejected the whole of them, and 
after a short rest began them again from the beginning. 
In all the observations the Nicol was so placed that the division line of the “ Jellett” 
was about horizontal, so that if there were any difference in the light admitted at the 
right and left of the field it should affect both halves equally. 
Thus the light on leaving the lamp passed through the apparatus in the following 
order: — slit, lens, prism, slit, Nicol, collimator, tube, Jellett and its mountings. 
“ Current direct ” is that direction of current which causes the north-pointing end of 
the suspended magnet to move toward the east. 
After each reading, the circle was slightly displaced at random. 
