128 COSMOS. 



larly eiglit rays at angles of 45° in stars from tlie 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 he- 

 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°) ; 

 Eo 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 

 which would otherwise be more widely diffused. This obtains for the 

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

 actual disks, mahitains 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 necessai'ily im- 

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

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

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

 including its defects. As the stars have no sensible angular diameters, 

 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 the 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, Sur les rayons divergens des Etoiles in Delamotherie 

 Journal de Physique, torn. l*xix., 1809, p. 324. 



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

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

 assertion {Descr. de VEgypte, tom. vii., p. 423), that a star, as the nu 

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

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



