808 Prof. Amici on a Property of Light, #c. 



when half the aperture is shut up by a semicircle of card, in 

 which case the image resembles that formed by a semi-lens. If 

 the card is turned round, so as always to cover one-half of 

 the mirror, the elongation, in the image of the star, will al- 

 ways take place in a direction perpendicular to the line which 

 separates the open from the covered half of the speculum. 



It is easy to ascertain that this effect does not depend on 

 the aberration of the light in the mirror, since, in this case, it 

 would take place in the direction of the diameter of the semi- 

 circle of card, and of the section of the semi-lens. 



In order to be still farther satisfied that the elongation of 

 the images did not proceed from the aberration of spherici- 

 ty, I placed, at the end of a refracting telescope, a rectan- 

 gular aperture, one of whose sides was quadruple of the 

 other, and I put it symmetrically around the axis of the tube. 

 Had any aberration been sensible, it would have shown it- 

 self by dilating the discs of the stars in the direction of the 

 greater side of the rectangle ; but this did not happen. The 

 image of the star was accompanied with two long luminous 

 tails, which, in turning round the card, kept always perpen- 

 dicular to the greatest side of the rectangle. 



This phenomenon, therefore, appears to me to be caused 

 by the light inflected by the sides of the diaphragm, and this 

 explanation is confirmed by another fact, which I have ob- 

 served in using Newtonian telescopes. If, when the telescope 

 is pointed to a star, the eye-glass is brought nearer the mirror 

 than distinct vision requires, we perceive, in the margin of the 

 luminous circle, which has the form of the mirror, a very 

 narrow band of brilliant light, which shows itself even round 

 the shadow of the small mirror, and of the arm which carries 

 it. The same thing takes place, if the eye-glass is drawn out 

 beyond the place of distinct vision. I cannot, therefore, at- 

 tribute this phenomenon to any other cause than the inflec- 

 tion of light by the sides of the small mirror, and its support, 

 and by the mounting of the large mirror. 



If we examine attentively the formation of the image of a 

 star, while bringing the eye-glass from the point of indistinct 

 to that of distinct vision, we shall see that the false disc of the 

 star proceeds, in a great measure, and almost entirely, from 



