4.60 Mr. R. A. Houstoun : 
three blue lines (4812, 4721, 4681) are all doublets, the 
components being of equal intensity. For 4812 dnd is 0:09, 
for 4721 0:08, and for the third line something similar. 
Hixamining Hg vapour under the same conditions, the green 
line appeared with two satellites, one corresponding to A 
(fig. 5) and the other to the doublet (BC), while the yellow 
line appeared as a double line with a weak component 
towards the red. For the green line dW=—0:24, —0°08, 
and for the yellow —0-06, +0°12. 
The nickel line (5476) and the Na D lines were single. 
The red C line of hydrogen was double, dX being =0°065, 
or about one-half Michelson’s value. This explains why the 
line does not appear double in the first-order spectrum of a 
Rowland’s grating. 
It seems on the whole tolerably certain that the satellites 
of the green line of Hg vary in number and position with 
the conditions of the source. What these determinative 
conditions are it is difficult to say. 
We can look on the multiple reversals of the lines as being 
made up of simple reversals of the components of the complex 
spectral lines. This seems more probable than alternate 
layers of radiating vapours at different temperatures, as it is 
difficult to see how these alternate layers can arise. The 
above measurements were made with a micrometer eyepiece 
and not on a photographic plate. This explains probabiy 
why such reversal effects are not more frequently seen. 
§ 3. The echelon used for the above observations was made 
and mounted by A. Hilger. The telescope and collimator 
were both fixed and mounted on the one stand. ‘The echelon 
rested on a piatform between them ; by moving this about a 
vertical axis, the different images could be brought into the 
field of view. The rays of light from the collimator do not 
fall normally on the echelon. In the discussion usually given 
it is assumed that they do so, that the collimator and echelon 
remain fixed and that the telescope moves. The formule 
are the same for both cases, though the discussion is slightly 
different. I have derived the formule for the case of the 
fixed telescope, using the graphical method given in Preston’s 
‘Light, and as the echelon spectroscope illustrates this method 
very neatly the discussion is given here. 
It will be well to describe the appearances seen in the 
field. First of all, if the instrument is focussed on a well- 
defined line we see several images of it in the field. As the 
echelon is rotated these images cross the field, and as each 
crosses the centre of the field it gets very much brighter. 
In fact it is only the two or three in the centre of the 
