Prof. Amici on a Property of Light, Sfc. 307 



adapted a divided object-glass micrometer, I have remarked, 

 when the magnifying power was sufficient to make the phe- 

 nomenon perceptible, that, if I doubled the image, by sepa- 

 rating the semi-lenses, the luminous discs became elongated, 

 and assumed an oval form. The small diameter of the ellipse 

 thus formed is equal to the diameter of the primitive disc. 

 This elongation always takes place, provided that the tele- 

 scope is well centered in a direction perpendicular to the 

 section of the lens of the micrometer, and it is for this reason 

 that the distance between two stars is in no way diminished. 

 This elongation takes place only for the fixed stars, whose 

 diameter is inappreciable by the eye, and even when the mag- 

 nifying power is from 100 to 1000 times. With respect to 

 objects of an appreciable diameter, as the planets would be, 

 they are not subject to this luminous expansion, which alters 

 their form ; or, at least, I have not been able to observe it in 

 them. I have noticed several times, that the discs of the 

 satellites of Jupiter, though smaller in appearance than those 

 of the fixed stars, preserve themselves perfectly circular, and 

 have their contours well defined, even when their images are 

 doubled. This furnishes us with an easy criterion for dis- 

 tinguishing between a real disc and an apparent one, and, 

 consequently, for distinguishing, at first sight, a new planet 

 from a fixed star ; for, if the planet has not a disc extremely 

 small, if we separate the lenses of the micrometer, this disc 

 will preserve its form, while the image will lengthen itself, if 

 the star is a fixed one. * 



In seeking for the cause of this phenomenon, I found that 

 the elongation of the images could not arise from any proper- 

 ty of the semi-lenses, because the effect takes place even when 

 they are removed, and shows itself with Newtonian telescopes, 



" Sir William Herschel has published, in the Philosophical Transac- 

 tions for 1805, several experiments, for the purpose of establishing the 

 limits of the visibility of small objects in telescopes. He finds, that the 

 rays reflected by the central portion of the great mirror tend to augment 

 the false discs, while those reflected from the circumference tends to dimi- 

 nish them. The different effects, therefore, of the internal and external rays, 

 reflected at the surface of a mirror ten feet in focal length, is a criterion 

 for distinguishing a false from a real disc, provided that their diameter 

 exceeds one-j'ovrth of a second. — Amici. 



