th 
ra 
E. Emerson on Improvements in the Photographic Camera. 227 
4th. The masses of the last two groups are complementary to 
. each other. Ten times the mass of the group situated at the distance 
of the earth, plus three times the total mass of the small planets between 
Mars and Jupiter, form a sum equal to the mass of the earth. 
The last conclusion depends upon the determination of the 
distance of the earth from the sun by observations of the tran- 
sits of Venus, a determination which astronomers agree in con- 
sidering as very accurate. 
Art. XXX.—Upon some Improvements, proposed by Sir David 
Brewster, in the Photographic Camera; by EpwINn EMERSON, 
of Troy Uniy., N. Y. 
instrument of the photographer ;” in the mean time, however, he 
suggests that “the use of a lens of rock-erystal, which has a low 
wi ec ; 
an inch, would make an approximatively perfect camera,” (pp. 
137, 138). 
Being convinced, by the excellent results obtained by cao 
on is a very excel- 
rtrait combination 
The 
Ph : upon the gro 
otographers Bet pe aad ol course, in a majority of 
