134 ON COMETS. 



peculiarly interesting and instructive, it being only on 

 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 



