615 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
These gray molecules, which persist in their primitive state cnly 
in the rods, upon disassociation furnish us with the gray sensa- 
tions. In the cones the gray molecules have undergone a develop- 
ment such that a certain portion only of the molecule becomes dis- 
associated by the action of light of a given color. 
The differentiation of the primitive gray molecule is supposed 
to have taken place in three stages. (Fig. 2.) The first stage is 
represented by the simple, primitive gray molecule, so constructed 
that it is disintegrated by light 
of any color, thus producing a 
gray or white sensation. In the 
second stage the molecule is 
more complex and contains two 
Stage 1 groupings, the disassociation of 
one of which gives the sensation 
of yellow and the disassociation 
of the other gives blue. The 
simultaneous disassociation of 
both gives white. This stage 
persists in the peripheral por- 
tion of the retina where neither 
green nor red can be perceived 
as such. In the third stage the 
yellow grouping is divided to 
form two new combinations, the 
disassociation of one of which 
produces the sensation of green 
and the other the sensation of 
red. If the’ red) and) ereen 
eroupings are disassociated to- 
gether the resulting sensation is 
yellow; whereas the simulta- 
Fic. 2.—Diagram to illustrate the Franklin JI€OUS disassociation of the red, 
theory. The blue, green, and red sroup- oreen, and blue groupings pro- 
ings are represented by an outer, middle, 4 ; z - 
and inner circle of dots, respectively. Dis- duces the white sensation. 
associated groupings are omitted. Schenck ® has somewhat ex- 
tended this theory by describ- 
ing the development of the primitive gray molecule. Since in 
twilight vision the red end of the spectrum is lost, and the green- 
blue portion is its brightest part, he considers that the photo- 
chemical substance of the rods is attuned only to the green-blue 
light, which is perceived as colorless. Later this photo-chemical 
substance becomes sensitized in two stages, first to include the 
@ Schenck, F. Uber die physiologischen Grundlagen des Farbensinns. Sitz.- 
ber. d. Gessell. d. ges. Naturw. z. Marburg, 1907, jahrg. 1906, pp. 133-164. 
