ON COMETS. 135 



is, all I wish to convey is, that there is a simple enough 

 experiment which everybody who understands optics 

 knows how to make, which if the result be of a certain kind, 

 the reflection of the light is demonstrated (the con- 

 verse, be it observed, does not hold good) in an instant, 

 by merely looking through a small instrument contrived 

 on purpose. Now, Mr Airy, the present astronomer- 

 royal, a person who is not only an excellent astronomer, 

 but who stands very high as an authority on this especial 

 branch of optics, applied this test to the light of the 

 comet's tail on the 2yth September, and found it polar- 

 ized. The tail then shone by reflected light, and there 

 was also another particular indication or character of the 

 polarization impressed, which the same trial afforded, and 

 which enabled him to say positively that the light had 

 been reflected from some source of light agreeing in 

 situation with the sun. 



(49.) The tail of the comet then was material substance.* 

 But now, only conceive what must be the thinness, the 

 almost spiritual lightness of a vapour or fog, which, oc- 

 cupying such an enormous space, would not extinguish 



* I applied the same test to the comet of 1862. There are various 

 modes of making the trial. Mine was by looking at the comet 

 through an achromatized doubly refracting prism, and turning the 

 prism round in its own plane. I could perceive no alternate maxima 

 and minima of brightness in the images. But in this case it is the 

 positive result which is conclusive. Everything depends in the first 

 instance on the relative situations of the objects and the eye. And, 

 moreover, the light of the comet of 1862 was far inferior to that of 

 Donati's, rendering the experiment pro'tanto more delicate and it 

 is very possible that to septuagenarian eyes, indications of partial 

 polarization might escape observation. 



