30 M. Arago on the Light qf Comets. 



to speak, in a few seconds, from the distance of Venus to that of 

 the Earth, of Mars, Ceres, and Jupiter ? If this, then, be true, 

 and seeing the comet with our telescope supplied with its weakest 

 magnifying powers when it crosses the orbit of Venus, we sub- 

 sequently examine it in succession with the help of a magnify- 

 ing power 3 times, 8, 17, 28 times stronger ; then, if it be always 

 seen, it ought to appear the same as when seen with the first and 

 feeblest magnifying power, at the epochs in which its proper 

 movement shall have transported it to distances from the sun, 

 equal to the radii of the orbits of the Earth, of Mars, Ceres, and 

 Jupiter ; and if it be not seen, for example, when it has reached 

 the orbit of Jupiter, it is not only because it has been subjected 

 to the enfeebling which might result from the scattering of the 

 matter of which it is formed ;— it is because it does not act as a 

 body which has an inherent light ; it is because it borrows its 

 brightness from the sun ! 



It is true that all comets are not equally eligible for these 

 kinds of experiments. We ought to select in preference those 

 comets without apparent nucleus, and without a tail, because 

 they appear to be less subject than others to sudden and irregu- 

 lar changes of figure, and because that, during the act of the 

 singular dilatation they undergo in removing from the sun, and 

 of which Mr Valz has given the law, it is probable that all the 

 parts, from the centre to the circumference, undergo analogous 

 changes. Without this condition, the natural dilatation of the 

 nebulosity cannot be assimilated to that which we obtain artifi- 

 cially in the foregoing proof by means of the eye-glass of the 

 telescope. The importance of this remark will be perceived, 

 when I notice that, in the comet of 1770, the nucleus and the 

 nebulosity properly so called, were very far indeed from expe- 

 riencing proportional changes *. 



* In proof of this assertion, I subjoin the measurements which Messier 

 has given both for the nucleus and nebulosity of the comet 1770 : — 



On the 17th June 1770, the nucleus measured 0.22, the nebulosity 5.23 



... 22d ... 0.33 ... 18.0 



... 23d ... 1.15 ... 27.0 



... 29th ... 1.22 ... 54.0 



... 2d July ... 1.26 ... 123.0 



... 3d August ... 0.64 ... 15.0 



... 12th ... 0.43 ... 3.36 



