220 HERSCHEL. 



Herscliel fiiiisbed liis observatious of a comet tliat was visible in Janu- 

 ary, 1807, with tbe following remark: 



" Of the sixteen telescopic comets that I have examined, fonrtcen bad 

 no solid body visible at their center; the other two exhibited a central 

 light, very ill-dehued, that might be termed a nucleus, but a light that 

 certainly could not deserve the name of a disk." 



The beautiful comet of 1811 became the object of .his conscientious 

 labor. With large telescopes he saw, in the midst of the gaseous 

 head, a reddish body of planetary appearance, which bore strong 

 magnifying powers, and showed no sign of phase. Hence Herschel 

 concluded that it was self-luminous. Yet if we reflect that the planetary 

 body under consideration was not a second in diameter, the absence of 

 a phase does not appear a conclusive argument. 



The light of the head had a bluish-green tint. Was this a real tint, 

 or did the central reddish body, only through contrast, cause the sur- 

 rounding vapor to appear colored 1 Herschel did not examine the 

 question from this point of view. 



The head of the comet appeared to be enveloped at a certain distance, 

 on the side toward the sun, by a brilliant narrow zone, embracing 

 about a semicircle, and of a yellowish color. From the two extremities 

 of the semicircle there arose, toward the region away from the sun, two 

 long luminous streaks w^hich 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 appearances in all the 

 positions of the comet ; it was not then ijossible to attribute to it really the 

 annular form — the shape of Saturn's ring, for example. Herschel sought 

 whether a spherical semi-envelope of luminous matter, and yet diapha- 

 nous, would not lead to a natural explanation of the phenomenon. lu 

 this hypothesis, the visual rays, which on the Gtli of October, 1811, trav- 

 ersed a thickness of matter of about o99,(IOO kilometers, (218,000 English 

 miles,) while the visual rays near the head of the comet did not meet above 

 80,000 kilometers (50,000 miles) of matter. As the brightness must be 

 proportional to the quantity of matter traversed, there could not fail to 

 be an a})pearance around the comet of a semi-ring five times more lu- 

 minous 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 tail was not Hat, as it appeared 

 to be; it had the form of a conoid, with its sides of a certain thickness. 

 The visual lines which traversed those sides almost tangentiaily evi- 

 dently met much more matter than the visual lines passing across. 

 This maximum of matter could not fail of being represented by a max- 

 inuim of light. 



The luminous semi-ring appeared one day to be suspended in the 



