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 



