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 under 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 
© 
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- 
