438 
THE PHYSICS OF A SUNBEAM. 
BY ROBERT HUNT, F.R.S. 
T O reduce our inquiry into the most simple form, it appears 
necessary that the reader should be invited to accompany 
the writer into the dark room employed inliis illustration of “Light 
and Colour;” — a room, mdeed, so dark that even after the eye 
has accustomed itself to the absence of any illuminating power, 
it shall not be enabled to distinguish either the forms or colours 
of surrounding objects. Under such conditions the eye is a 
useless organ ; the sense of sight is not excited, we have lost 
the power of distinguishing bodies, which we cannot touch. 
Some agent external to ourselves is required to complete the 
connection between us and distant objects — to excite, indeed, 
the sense of sight. 
Let us perforate with a steel point the window- shutter ; a sun- 
beam enters and falls on the floor, forming a disc of bright 
light. From this circle radiation takes place; and, although 
it may be faintly, still the reposed eye begins to see. The 
radiations have diffused themselves, they have fallen on the 
surfaces of the articles around us, and then again the weak rays 
have been radiated to the eye. The myriad delicate nervous 
fingers, which spread over the retina, are touched and they 
tremble at the touch. The varying sensations are conveyed to 
the brain and the phenomenon of vision is established. 
If the hand be placed in the path of the sunbeam the sensa- 
tion of warmth is communicated; we feel there is heat in the 
ray ; or, if a thermometer is so placed that the beam falls 
upon its bulb, the mercury will expand and rise in the tube. 
Thus we have the evidence of two distinct classes of pheno- 
mena ; a third awaits our investigation. 
It is well known that photographic pictures are produced by 
a peculiar chemical change effected upon some salt of silver by 
the agency of the solar rays. If a piece of paper be covered 
with the chloride of silver, — which is purely white, — and if this 
be placed so that our luminous pencil falls upon it, the chemical 
surface will be rapidly darkened over the space covered by the 
sunbeam. The change which has taken place, and which is 
indicated by the alteration of colour, merits some attention. 
