134 ON COMETS. 



peculiarly interesting and instructive, it being only cm 

 very rare occasions that a comet can be closely inspected 

 at the very crisis of its fate, so as to witness the actual 

 effect of the sun's rays on it. In this instance, the pour- 

 ing forth of the cometic matter from the singularly bright 

 and highly condensed, almost planetary nucleus, took 

 place in a single compact stream, which after attaining a 

 short distance, equal to rather less than a diameter of 

 the nucleus itself, was so suddenly broken up and dis- 

 persed as to give, on the first inspection, the impression 

 of a double nucleus. The direction of this jet varied 

 considerably from day to day, but always declined more 

 or less in one direction from the exact direction from the 

 sun. So far as I am aware, the formation of an envelope 

 disjoined from the head was not witnessed in this comet. 

 (48.) And now, I daresay, all my hearers are ready to 

 ask After all what is the tail of a comet ? Is it material 

 substance in the first place ? To this I answer unhesi- 

 tatingly, Yes ! Donati's comet has given a decisive proof 

 on that point There is a criterion by which, when it is 

 observed, it can be positively asserted that the light by 

 which anything is seen has been reflected from a ma- 

 terial substance. The light reflected, when it exhibits 

 that peculiar property in which this criterion consists is 

 said to be polarized. The direct light of the sun or that 

 of a candle is not polarized, but when reflected at a par- 

 ticular angle on any surface but a metallic one, it is, and 

 if it is polarized, we may be sure that it is not direct 

 light thrown out by the object seen, but borrowed or in- 

 direct light No matter at present what this polarization 



