Notices respecthig New BooJcs. 69 



form part of a disk very perceptibly larger than that to which the 

 remainder of the moon belongs, 



" This apparent encroachment of the edge of a luminous object 

 upon the obscure space which surrounds it, carries with it an illu- 

 sion of an opposite kind, with regard to an obscure object pro- 

 jected upon a luminous field. The dimensions of this object then 

 appear to be diminished : for the irradiation produced along its out- 

 line by the surrounding luminous field extends within this outline. 



"It is needless to insist upon the importance of irradiation in 

 astronomy. An illusion which tends to increase the apparent di- 

 mensions of luminous objects projected upon an obscure ground, and 

 to diminish those of obscure objects projected upon a luminous field, 

 must, it should seem, have an effect more or less decided upon all 

 observations the object of Avhich is the measurement of the apparent 

 diameters of celestial bodies, eclipses, the transit of planets over the 

 sun, &c. The phsenomenon has accordingly, in an especial manner, 

 exercised the sagacity of astronomers. But what is singular is, that 

 the observations present in this respect the greatest divergence ; 

 some seem to show a marked influence of irradiation, others appear 

 completely exempt from the errors which accompany it. Hence 

 also a divergence of opinion among astronomers relative to the very 

 existence of the phsenomenon, some of them admitting this existence, 

 and others calling it in question. It is therefore important to seek 

 out the truth amidst all these doubts, and to determine the causes 

 which must have given them birth. I think I establish beyond a 

 doubt that irradiation really exists, that it is one of the phsenomena 

 of vision most easily proved, and that if, in the observations made 

 through astronomical instruments, it has sometimes ceased to mani- 

 fest its influence, it is from causes that may be accounted for. 



" On the other hand, several theories have been proposed in suc- 

 cession to explain the cause of irradiation. One of these theories, 

 although very ancient, is generally adopted. It consists in ad- 

 mitting that the impression produced at the bottom of the eye by a 

 luminous object, spreads upon the retina to a little distance all 

 around the space directly excited by the light, so that the total sen- 

 sation corresponds at that time to an image rather larger than 

 the true one. This simple theory has however met with opponents, 

 and a different explanation has very recently been put forward. I 

 examine the different hypotheses proposed, and I endeavour to sup- 

 port by new proofs the one whose princijDle I have just mentioned. 



" The phsenomenon is indeed governed by remarkable laws, which 

 may lead to processes adapted for guaranteeing the observations of 

 its influence. Besides the laws already known, experience has led 

 me to admit some new ones. I examine all of them theoretically, 

 and I point out simple processes for verifying them. 



" The principal results of my inquiries may be summed up in the 

 following manner : 



" 1. Ocular irradiation is perceptible at all distances, from any se- 

 paration whatever to the shortest distance of distinct vision. 



"2. Ocular irradiation increases with the continued contemplation 

 of the object. 



