90 REPORT—1845. 
their common centre of gravity in regular orbits, whilst this dynamic centre 
describes the cometary orbit round the sun. 
What we see at the head of the comet is the brightness formed by these 
numerous particles being lighted up by the sun, each one being too small to 
be distinguished separately. Thence the cause of the nebulous aspect of 
comets, resembling that of the accumulations of stars, which often from the 
same cause are seen as nebula. The form of each individual of these corpus- 
cles decides the fact of its having a rotatory movement or not. The form 
must be amorphous or crystalline, according to the matter and conditions at 
the moment of the first formation. This formation may be renewed as often 
as these atoms are put into a state of fusion, or subjected to a species of ce- 
mentation, which might very possibly occur when a comet passes very near 
thesun. Endowed with the facets of crystals, and obliged by their form always 
to preserve the sume direction towards the sun, these corpuscles may unite all 
the requisite conditions up to an entire reflexion of the solar rays. 
He who knows how much may be united in this phenomenon of entire re+ 
flexion, will understand the considerable illumination which it may spread to 
the greatest distance in space. 
We have only to admit that the atoms which form the zodiacal light, seen 
lighted up only by simple rays of the sun, are spread over far more distant 
spaces, to enable us to explain a dynamic origin for the tails, thus placing 
them amongst the phenomena of the zodiacal light, the parhelia, halos, the 
rainy bands of the Indian summer, and even the general world of atoms lighted 
up by the sun. 
Thus may comets be perhaps the grand reflectors of our solar system, 
sent us from time to time by the Creator of the world to throw light upon 
hitherto unknown parts of his Creation, too immense for our senses, and even 
for our minds! 
Report on the Actinograph. By Mr. Rospert Hun. 
Many circumstances have conspired to prevent the author from completing 
any observations with this instrument. A few rough experiments made with 
a view of testing the merits of it, comprise all that has as yet been done, 
The importance of a method of registering the amount of chemical in- 
fluence associated with the solar rays, is evident. When we consider the 
ever-varying conditions of these radiations, producing remarkable pheeno- 
mena, not merely during the changes of the year, but over the vegetable 
world, within the brief period of a day—when we find the practical photo- 
graphist stating that two hours before noon he can produce effects, which he 
cannot produce two hours after the sun has crossed the meridian, it will be 
clear to every one, that some accurate means of registering the relations be- 
tween the amount of light and actinic (chemical) power, which are evidently 
not in strict ratio to each other, is desirable. 
The instrument constructed for the Association, although not yet com- 
plete, answers this purpose remarkably well. It consists essentially of a 
fixed brass cylinder, about which is wound a piece of prepared photographic 
paper. This paper is so prepared with the bromide of silver, that it is 
equally sensitive to all the rays of the prismatic spectrum. Over this is 
placed another cylinder which is driven by clock-work, and it performs a 
revolution in twenty-four hours. In the moveable cylinder is a triangular — 
slit, the largest part being exactly one hundred times the size of the smallest, 
which is a mere point, and this opening is divided by bars into one hundred 
; 
