M. Arago on the Light of Comets, 171 



This method had this peculiarity, that it was not requisite, in 

 order to apply it, that the volume of the comet should continue 

 ronstant ; it was sufficient that the changes should take place with 

 a certain degree of regularity. In this respect, however, Hal- 

 ley's comet proved an exception to the generality of cases ; the 

 variations in its intensity were so sudden, unexpected, and ec- 

 centric, that it would be rash to attempt to deduce any precise 

 inferences from observations which, in ordinary circumstances, 

 would certainly decide the point in question. On this occasion, 

 therefore, M. Arago abandoned the idea ; but, at the same 

 time, he tried to solve the problem by means of the properties 

 of polarised light. A few brief explanations will be sufficient 

 to make this latter mode of investigation understood. 



All direct light constantly divides itself into two pencils of 

 the same intensity, when it traverses a crystal possessing the 

 power of double refraction ; reflected light gives, on the con- 

 trary, in certain positions of the crystal through which it is 

 made to pass, two images of unequal intensity, provided, how- 

 ever, the angle of reflection is not 90°. Theoretically speaking, 

 nothing then can appear more simple than to distinguish be- 

 tween direct light and that which is reflected ; but in reality it 

 is not found to be so ; for in many bodies under certain angles, 

 and in others under every possible angle, the difference in the 

 intensity of the two images cannot be appreciated by our senses. 

 It is necessary to add, that the rays regularly reflected, change 

 their nature (became polarised) in the act of reflection ; while 

 those, on the contrary, which, after being absorbed, so to speak, 

 into the substance of bodies, rendering these bodies visible in all 

 directions, preserve with the direct light, the property of always 

 giving two images very nearly equal. Finally, it is to be observed, 

 that in the greater number of cases, and particularly in relation 

 to the heavenly bodies, the regularly reflected light which 

 reaches our eye, is such a small portion of the entire light, that 

 it is hardly to be expected that the dissimilarity will be per- 

 ceived between the two portions of the divided pencils. By 

 using various precautions, the detail of which in this place would 

 be without interest, M. Arago, however, always succeeded in 

 discerning a slight difference of intensity between the two images 

 of the brilliant comet of \S\% 



