128 COSMOS. 



iarly eight rays at angles of 45° iii stars from the first to the 

 third magnitude. As, according to Hassenfratz, these radi- 

 ations are caustics intersecting one another on the crystal- 

 line lens, they necessarily move according to the direction 

 in which the head is inclined.* Some of my astronomical 

 friends see three, or, at most, four rays above, and none be- 

 low the star. It has always appeared extraordinary to me 

 that the ancient Egyptians should invariably have given 

 only five rays to the stars (at distances, therefore, of 72°) ; 

 so that a star in hieroglyphics signifies, according to Hora- 

 pollo, the number five.f 



The rays of the stars disappear when the image of the 

 radiating star is seen through a very small aperture made 



ments of the head. The property of the telescope, in giving a definite 

 outline to images, causes it to concentrate in a small space the light 

 ^"hich would otherwise be more widely diffused. This obtains for the 

 6xed stars and for the disks of planets. The light of stars having no 

 actual disks, iTiaintains the same intensity, whatever may be the mag- 

 nifying power of the instrument. The aerial field from which the star 

 is projected in the telescope is rendered more black by the magnifying 

 property of the instrument, by which the molecules of air included in 

 the field of view are expanded. Planets having actual disks become 

 fainter from this effect of expansion. When the focal image is clearly 

 defined, and when the rays emanating from one point of the object are 

 concentrated into one point in the image, the ocular focus affords satis- 

 factoiy results. But if, on the contrary, the rays emanating from one 

 point do not reunite in the focus into one point, but form a small circle, 

 the images of two contiguous points of the object will necessarily im- 

 pinge upon each other, and their rays will be confused. This confusion 

 can not be removed by the ocular, since the only part it performs is 

 that of magnifying. It magnifies every thing comprised in the image, 

 includin°^ its defects. As the stars have no sensible aneular diameteie, 

 those which they present are principally owing to the imperfect con- 

 struction of the instrument (to the different curvatures of the two sides 

 of the object-glass), and to certain defects and aberrations pertaining 

 to the eye itself. The smaller the star appears, the more perfect is the 

 instrument, providing all relations are equal as to the diameter of the 

 object-glass, the magnifying power employed, and the brightness of the 

 star. Now the best means of judging whether the stars are very small, 

 and whether the points are represented in the focus by simple points, 

 is undoubtedly that of directing tlie instrument to stars situated very 

 near each other, and of observing whether the images of known double 

 stars are confused, and impinging on each other, or whether they can 

 be seen separate and distinct." (Arago, MS. of 1834 and 1847.) 



* Hassenfratz, Svr les rayons divergens des Etoiles in Delametherie, 

 Journal de Physique, tom. Ixix., 1809, p. 324. 



t Horapollinis Niloi Hieroglyphica, ed. Con. Leemans, 1835, cap. 13, 

 p. 20. The learned editor notices, however, in I'efutation of Jomard's 

 assertion (Descr. de VEgypte, tom. vli., p. 423), that a star, as the nu- 

 merical hieroglyphic for 5, has not yet been discovered on any monu- 

 ment or papyrus-rolT. (Horap., p. 194.) 



