DIVERGENCE OF LIGHT. 



259 



gence of light takes place : he has further shown that in 

 a complete wave which is freely propagated, the rays are 



no theoretical considerations whatever, but mentions it only among 

 those unfinished inquiries which, as he says, he had left imperfect 

 and was unable to carry out. 



Both the fact, and all questions relating to it, seem to have been 

 overlooked until, in reference to a somewhat similar case, M. Babinet 

 supposed that iinder particular conditions the mutual interference of 

 the secondary waves might be interrupted by stopping one of the in- 

 terfering portions of light, and thus the other portion be rendered 

 effective, and consequently diverging rays made visible. The author 

 of this note, in relation to what appears a closely allied, if not iden- 

 tical phenomenon, the formation of a corona or ring of light round the 

 dark disk of the moon in a total eclipse of the sun, tried some analo- 

 gous experiments, and rendered the same kind of effect conspicuous 

 and easy to be studied by an arrangement of this kind : 



The rays of the sun are transmitted by reflexion from an inclined 

 mirror (m) through a small hole (h) in a shutter, and in the diverging 

 beam is placed an opaque circular disk (d) which intercepts the rays 

 at a point where they have an area considerably less than its own dia- 

 meter. From the edge of (d) rays are seen to diverge into its shadow 

 and cross at successive points along the axis ; they are thus rendered 



G 



visible by means of a small eye lens at (e) which presents the appear- 

 ance of the shadow of the circular disk, having a multitude of rays 

 converging inwards from its edge to its centre, where they form a 

 point or small circle of great relative brightness. If, on the other 

 hand, the disk (d) under the same conditions be viewed directly by 

 the eye, without the lens, its shadow is seen relatively and uniformly 

 dark, but surrounded by a bright luminous ring on its outside. The 

 same appearance of the ring is also presented if, instead of the solar 

 rays, we use the light of a flame placed at the principal focus of a lens 

 inserted in a screen so as to send out a beam of parallel pencils inter- 

 cepted in like manner by the disk. In this case, however, the con- 



