Chemistry and Physics. 297 
another. Their diameter is several times the length of a wave of 
light, and hence we may treat the question as a case of simple 
geometrical reflection. Computing by Fresnel’s formula the amount 
of light reflected by the collodion and glass at various angles, we 
find the maximum amount of light would fall at a distance 2¢ cot v, 
in which ¢ is the thickness of the glass, and v the critical angle. 
On development, therefore, a ring might be formed around the 
image with this distance as a radius, and shading off more abruptly 
inside than outside. To test these results an image of the sun was 
formed by a lens of 6” focus on a plate 12” thick. A well de- 
fined annulus was formed shaded as described above. With a 
plate of double thickness, the mean diameter of the annulus was 
doubled. Two plates were then employed separated by a thin 
A ring was formed on the first plate and a diffused image 
on the second. A measurement of the intensities agreed closely 
with the result of theory. Holes and slits were cut in platinum 
foil, placed on a dry plate and exposed directly to sunlight. Rings . 
were obtained in one case, and parallel lines joined by semi-circles 
in the other, showing that the effect is not due to the lens of the 
camera. 
Three methods suggest themselves for diminishing irradiation : 
first, to coat the back of the plate with some irerey color ; 
p] 
out, and is more effective than the first. With albumen preserva- 
tive particularly, the amount of irradiation is reduced materially, 
— though the film appears nearly transparent.—PAil. Wag., |, 
; E. ©. P. 
ll. Conical Refruction.—M. Novor has endeavored to find a 
ec 
indices of numerous biaxial crystals, arragonite will be 
the most favorable mineral ; but chemical subst 
_ three substances appear capable of replacing arragonite, sugar, 
bichromate of potassium and tartaric acid. The method of age 
> Seige ae face of the bichromate are normal to one of the optical 
or tartaric acid it is not the same, and the direction of 
