Remarks on the Tails of Halley^s Comet. 3527 



observation of M. de la Nux in 1759, when its naaximum lengths 

 were 19°, 25°, and 47°. Whenever it presented such lengths, it 

 was very narrow, (" elle s' amincissait beaucoup :" — and " I'amin- 

 cissement etait devenu extreme.")* It is an interesting coincidence, 

 that an unusual narrowness through the greater part of its extent, 

 characterized it at its recent appearance when at its greatest lengths, 

 i. e. 25°, 35°, and 45°. From this circumstance we may, in the 

 absence of any positive information, presume that M. de la Nux 

 saw it indirectly. Before the perihelion passage a tail was scarcely 

 if at all detected.f The mean of the above lengths observed on the 

 16th, 17th, and 11th of October, 1835, appears to be greater than 

 that given by any three observations since 1456, either before or 

 after the perihelion passage. This is not to affirm, that, including 

 all its dimensions, its magnitude is greater; for, except in 1759, the 

 narrow portion may have been wanting. It is, moreover, not im- 

 probable, that most observers have given the length as seen with a 

 moderate obliquity of the optic axis, a length intermediate between 

 the real minimum and maximum for any one instant, or in other 

 words, intermediate between the length of the whole and that of the 

 part possessing nearly a maximum intensity. 



The greatest length alluded to in the Annuaire as seen at Paris at 

 the late return, is 20°. It was with the naked eye, but whether 

 with the axis directed toward the object, is not stated. The sky here 

 was cloudy and foggy ; length less than 2°. At the observatory of 

 Paris on that evening, it appeared but half as long with the finder 

 as to the naked eye. This M. Arago pronounces " a result truly 

 singular." I frequently observed the same thing with the five feet 

 telescope and attributed it to the moderate aperture of the instru- 

 ment. Could not this have been the cause in both cases ? This 

 effect was not produced by our 2^ ft. telescope having a larger aper- 

 ture compared with its magnifying power, and was more remarkable 

 with the higher than with the lower magnifying powers of the larger 

 telescope. 



Although the Annuaire contains numerous observations of the 

 length of the tail at the former returns of the comet, it gives but 

 three for 1835. The last was by M. Schwabe at Dessau, who found 

 it to be 7° on October 26th, when clouds here obstructed the view. 

 The new moon and the opacity of " Indian summer" were for a while 



* Annuaire, p. 227. t p. 229. 



