Ill 
of Edinburgh , Session 1875-76. 
however, to mention one feature with which I was much struck, 
and which I have never seen remarked on — namely, that, on 
watching the western sky from sunset onwards, it is impossible to 
tell when the diffused sunlight ends and the zodiacal light begins 
till it becomes so dark that the form of the latter can be traced to 
a considerable altitude, when it is seen to be longer and narrower, 
and inclined to the vertical at a considerable angle. I am strongly 
inclined to believe that near the sun it is very much wider than at 
some distance from it, for I have very good reason to think that 
what by people generally would be taken as simply the last of the 
sunset glow is really due to the zodiacal light. This part, of 
course, can only be seen in places where the twilight is very short. 
The best time for making observations I found to be about two 
hours after sunset, when all traces of twilight had certainly dis- 
appeared, and consequently all risk of confusion with it was gone. 
On January 31st, in lat. 8°N., long. 56° W., the light was very 
bright, and I made some spectroscopic observations. At two hours 
after sunset the light was visible for 90° from the horizon, and so 
bright was it towards the horizon that I was able to get a distinct 
spectrum. I first opened the slit very wide, when I observed a 
broad strip of light, nebulous at the extremities, with a distinct 
reddish tinge at the one end ; then, by gradually closing the slit, 
I obtained a narrower but tolerably pure continuous spectrum, in 
which I could distinctly see reddish, orange, and greenish-blue, and 
on making comparison with the spectrum of a lamp (placed at the 
far end of the ship so as not to dazzle my eyes), I estimated that 
the spectrum extended from the red to past the position occupied 
by the F line in the solar spectrum. A large number of observa- 
tions taken on other nights, whenever the circumstances were 
favourable, entirely confirmed these first observations. On several 
nights, and especially on February 27th, when off Ponce, in the 
Island of Puerto Eico, I observed the spectrum from a short time 
after sunset till long after the last traces of twilight had dis- 
appeared, hut no change was noticeable after the spectrum had 
become so faint that the Fraunhofer lines could not be distin- 
guished, except in the brightness of the spectrum, as I was still 
able to see colour distinctly, but no traces of any bright lines. 
On February 26th and 27th I took a number of sextant measure- 
