a - 
a 
of measuring the average size of fine particles. 105 
the same in all directions, we have the annexed construction. BD 
will represent the 
diameter ofa parti- 
cle of lamp-black, 
the ray AB is re- 
flected from its sur- 
face, the ray CB iss 
from thelayernext e D 
below; X is the angle made by the light with the plate, and the 
difference in path of the two rays will evidently be equal to CB 
— AB, a quantity readily found by calculation. 
ing the size of the lamp-black particles to be equal to 
000018 of an inch, the difference in path of the two rays for an 
angle of incidence of 18° is * 11, while the wave-length of 
the line C in the red space is nearly 000026 of an inch. This 
shows that the difference in the path is not far from half a wave- 
length of red light, if the larger of the two estimates of the size 
of the particles of lamp-black is employed. 
I then made a new set of experiments relative to the angle at 
which the red ray disappears, using as before lamp-black from 
parafiine. This was found to vary somewhat in different por- 
tions of the same plate, as is seen in the table below. : 
wep Oe I, 1G, 20", 20° = 20°14 
New microscopic measurements on the size of the lamp-black 
particles were made with a different microscope, the value of the 
micrometer not being known; it was estimated that the size of 
the smaller and more numerous particles varied from rise 
zsdvz Of an inch, but that there were more particles approaching 
the first number than the second, a circumstance of which I haye 
not taken any advantage in the following calculation. = 
Taking the mean of these determinations, and combining it 
with the mean of the first determination, we obtain for the 
_ ‘Size of the particles 55} ;='0000146 of an inch. 
Am. Jour. Sci.—Szconp Series, Vou. XLIII, No. 127.—Jam., 1867. 
14 
