162 REPORT—1868. 
respect to the majority of comets, which are subjected to a less fierce glare of 
solar heat, it may be suggested that this supposed difficulty is one of degree 
only; for we do not know of any conditions under which even a gas permanent 
at the temperature of the earth could maintain sufficient heat to emit light 
—a state of things which appears to exist permanently in the case of the © 
gaseous nebule. 
The important difference which exists between the spectrum of Brorsen’s 
comet and that of Comet II., 1868, appears to show that the constitution of 
comets may possibly not be in all cases the same. In a paper presented to 
the Royal Society, I have ventured on some speculations with reference to the 
bearing of these observations on certain phenomena which are usually exhi- 
bited by comets. 
§ IV. Ozsmrvattons or THE Sun. 
_ I have observed the sun with the spectroscope with three distinct objects _ 
in view :— 
1. To discover if the light from the less luminous part of the sun near 
the limb gives a spectrum which differs in any respect from that which is 
