106 O. N. Rood on Fresnel’s Theory, etc. 
The average angle of disappearance of the red ray being 20°, 
there results a difference of path =-0000998; that is, the differ- 
ence of path is to the wave-length of red light nearly as 10 to 26. 
When the difficulty of obtaining an approximate measure- 
ment of the size of the particles of lamp-black is considered, it 
is surprising to see how nearly the calculated difference in path 
approximates to half a wave baat of the light in set 
wave-length of C 000026 
Biving a still nearer approximation to a difference of 4 a wave 
ngth 
These experiments then seem to point out the correctness of : 
Fresnel’s theory, and we should I think be justified in reversing — 
the process, and sige the angle of disappearance of the red ray, 
in connexion with the known wave-length of this ray, for the — 
purpose of calculating the average size be small particles or the 
ae depth of fine scratches or furrow 
I give below the calculated values of the average size of the 
oatilclcn of lamp-black and magnesia. 
Lamp-black from paraffine, 70000188 calculated. 
“ce “ os asu * 
Size of particles of magnesia, "0000338 calculated. 
- = « 000361 measured. 
Some experiments were made on the angle of disappearance 
of the red ray with lamp-black produeed by the burning of 
ferent substances; where the figures are connected by a bracket — 
it is intended to indicate ot the two angles were obtained from — 
e same portion of the plate | 
Lamp-black isa pamak Lamp-black from a solution of 
from stearine. from camphor. spirits of wore tine in alcohol. 
18°25 16° 
18 “75 15 ‘9 21 
16 151 20 
16 15 °4 t 
17°25 15°°6 Sic 
It would appear from these last a Segoe that the a < 
re of the particles of Pe So k from ietiges camphor 18 
mewhat greater than fi ee cs while in the case of 
* *beethings -fluid” the particles are smaller. 
New York, Dec, 4th, 1866. 
