26 M. Arago on the Light of Comets. 



the visibility of a comet does not depend, or depends very little, 

 on the angle which it subtends. 



When, with the help of opaque screens, we reduce the surface 

 of the object-glass of a telescope to a third, a fourth, a tenth, 

 &c., of its original extent, we diminish in the same proportion 

 the number of the rays which go to the formation of the images 

 which this telescope supplies, or, in other words, we diminish 

 their intensity ; and when we substitute for the other glass of the 

 telescope, viz. for that small lens, placed near the eye, and which 

 is called the eye-glass, a lens of the same kind, but of higher 

 powers, the magnifying power — the size^— is increased. We can 

 thus give to the objects at which we look twice, thrice, four 

 times, ten times, &c., the dimensions in some observations that 

 we do in others. 



The object-glass of the telescope being considered as a fixed 

 opening, if by a change of the eye-glass the magnifying power 

 should be increased, the intensity of the image would go on di- 

 minishing, since the[same quantity of light, — that which the end 

 of the glass admits, — would then be spread over a greater sur- 

 face. And it may be perceived, that, by proportioning in a 

 suitable manner the portion of the object-glass which the opaque 

 screens leave uncovered, with the change in the eye-glass, it may 

 be so arranged that the enfeebling which results from the en- 

 largement of the image may always be compensated by the ad- 

 mission of a greater number of rays ; so that we may gradually 

 give to the images of the moon, of a planet, of a comet, dimen- 

 sions twice, thrice, four times, ten times greater, than in a first, 

 observation, preserving for them throughout all their modifica- 

 tions the same constant intensities. 



If we subject to these various proceedings a comet whose dia- 

 meter equals, we shall say, a minute, and successively enlarge 

 it, without varying its intensity twice, thrice, four times, . . . , 

 ten times, it may be perceived that, as to equality of brightness, 

 the image of the dimensions of a minute may be very easily in- 

 creased to an image of two, three, four, . . . , ten minutes *. 



• This experiment, and the consequence which follows from it, cannot give 

 rise to any uncertainty, when the natural intensity of the observed comet is 

 such that it can be perceived only with difficulty, — when an additional degree 

 of enfeebling would render it completely invisible. 



