O. N. Rood on the tint from mizing blue and yellow. 3869 
collected, and also by-one on Physical Geography. In addition, 
the plan of the Survey, as we learn from the Preface, contemplates 
a second volume on Paleontology by Mr. Gabb, Dr. Leidy, Dr. 
Newberry and others; one on the Botany of the State, by 
Prof. W. H. Brewer; three or four on Zoology by Dr. Cooper, 
Mr. T. Gill, etc.; and one of Maps, Sections, ete. 
This first volume on the Geology is beautiful as a work of 
art, both in its printing and in its numerous landscape and 
other illustrations, and well worthy of the rich State from 
which it emanates. 
p to the commencement of the current year the aggregate 
sum appropriated by the Legislature of the State for carrying 
on the work has been $95,000, In addition to this, $9,000 has 
€en appropriated for publishing the results. 
is sum has been most economically used, the volumes 
bear ample testimony. This is less than $16,000 per year for 
field-work, salaries, and all expenses whatsoever, except those of 
publishing, With this sum a reconnoissance has been made of a 
territory larger than the combined areas of the three largest 
States east of the Mississippi river, portions of it over most in- 
ospitable regions; considerable areas have been examined in 
Some detail; the main feature of the geology of a region em- 
bracing at least 200,000 square miles has been demonstrated, 
and the key furnished for future work over the immense region 
west of the Rocky Mountains; and, aside from the practical 
questions of the greatest moment, most important contributions 
have been made to many departments of science. : 
. We not only earnestly hope but trust that the Survey with 
Its publications may be continued to a satisfactory completion. 
It surely will be if the Legislators of the State view the work 
with the interest that is felt by the world outside. J. D. D. 
Art. XLIV.—On the Green tint produced by mixing blue and yel- 
low powders; by O. N. Roop, Prof. of Physics in Columbia 
College. 
It is known to every one, that the mechanical mixture of 
yellow and blue powders, (‘water or oil colors,”) produces a 
more or less lively green. This fact, with others similar in 
nature, was formerly used by some physicists in support of the 
view that there are only three primary colors—red, yellow and 
blue,—and that the other tints of the spectrum are formed by a 
mixture of these three in certain proportions. Helmholtz has, ' 
however, shown that the mixture of the blue and yellow of the 
Spectrum produces not green, but white light. Similarly it is 
Am, Jour. Sor.—Szconp Serres, Vou. XLI, No. 123.—Mar, 1866. 
47 
