OF ARTS AND SCIENCES. 101 



whicli is not the same as change of direction, but which is produced 

 whenever a ray changes its dh-ection by reflection, or refraction, or 

 double refraction ; in consequence of which change the susceptibiHty 

 of the ray to a sec6nd reflection or refraction is altered. The first 

 reflector or refractor, by which the change is produced, is called the 

 polarizer ; and the second reflector or refractor, by which the change is 

 tested, is called the analyzer. 



" To test polarized light is, therefore, to settle the question, whether 

 the light under consideration has been already once before reflected or 

 refracted. In this way, the polarization of light has been or may be 

 used to discover whether the light of comets, of the rainbow, of halos, 

 of coronce, of sheet-lightning (or summer lightning), of the aurora, or 

 the zodiacal light, is reflected, as that of the planets and satellites is 

 reflected ; or whether it is self-generated on the spot. 



" What I have further to say relates to the polarization of the light 

 of comets. 



" Arago undertook, in 1811, to apply to the remarkable comet of that 

 year the no less remarkable discovery of Malus in regard to the polar- 

 ization of light. Arago used the doubly refracting analyzer, and ex- 

 pected to discover whether or not there were sensible traces of polar- 

 ization in the light of the comet, by observing whether the relative 

 brilliancy of the two images, produced by double refraction, changed 

 when the analyzer was revolved. He decided in the affirmative, 

 though he felt, as he admits, less confidence in his decision, on account 

 of the uncertainty which always adheres to comparisons of relative 

 brightness by the eye simply. The comet of 1819 afforded another 

 opportunity of repeating the experiment. Still Arago was not so con- 

 fident in the accuracy of his results as not to desire a third trial, and 

 in a novel way, on Halley's comet, at its last return in 1835. On this 

 latter occasion, Arago used a polarizer in which a doubly refracting 

 prism is combined with a thin plate of quartz, and the two images are 

 colored with complementary tints when polarized hght is transmitted. 

 More delicate traces of polarization, he thought, would show them- 

 selves by difference of contrasted tints in this polariscope, than by the 

 corresponding changes of brightness in the old instrument. This last 

 attempt left no doubt in Arago's mind that the light of the comets was 

 polarized, and consequently that the comets shone principally by re- 

 flected light, coming of course from the sun. This conclusion does not 



