i) es = i 
LABOURS RELATIVE TO THE SOLAR SYSTEM. 299 
through contrast, make the surrounding vapour appear 
to be coloured? Herschel did not examine the question 
in this point of view. 
The head of the comet appeared to be enveloped at a 
certain distance, on the side towards the sun, by a bril- 
liant narrow zone, embracing about a semicircle, and of 
a yellowish colour. From the two extremities of the 
semicircle there arose, towards the region away from 
the sun, two long luminous streaks which limited the 
tail. Between the brilliant circular semi-ring and the 
head, the cometary substance seemed dark, very rare, 
and very diaphanous. 
The luminous semi-ring always presented similar ap- 
pearances in all the positions of the comet; it was not 
then possible to attribute to it really the annular form, 
the shape of Saturn’s ring, for example. Herschel 
sought whether a spherical demi-envelop of luminous 
matter, and yet diaphanous, would not lead to a natural 
explanation of the phenomenon. In this hypothesis, the 
visual rays, which on the 6th of October, 1811, made a 
section of the envelop, or bore almost tangentially, 
traversed a thickness of matter of about 399,000 kilo- 
metres, (248,000 English miles,) whilst the visual rays 
near the head of the comet did not meet above 80,000 
kilometres (50,000 miles) of it. As the brightness 
must be proportional to the quantity of matter traversed, 
there could not fail to be an appearance around the 
comet, of a semi-ring five times more luminous than the 
central regions. This semi-ring, then, was an effect of 
projection, and it has revealed a circumstance to us truly 
remarkable in the physical constitution of comets. 
The two luminous streaks that outlined the tail at its 
two limits, may be explained in a similar manner ; the 
