34 
G. H. Parker 
Although the foregoing conclusions that each rhabdome receives 
a single impressi ou and that the retinal image is a single erect one, 
Supports, in all essential respects, Müller's theory of mosaic vision, 
it is possible, as Exner has recently shown, that in some instances 
the retinal Images are not formed in precisely the way that Müller 
«upposed. Müller believed that, of the rays of light entering a 
given ommatidium, only such as were approximately parallel to its 
longitudinal axis were condueted to the perceptive organ at its base, 
the rest being absorbed by its pigmented walls, and, further, that the 
light received by one ommatidium had no influence upon the per- 
ceptive Clements of adjacent ommatidia. In this way each omma- 
tidium produced independently one block in the upright mosaic image 
formed by the mutuai action of all ommatidia. Exner, likewise, 
believes that retinal Images can be formed essentially in this way 
and has called them apposition Images. In eyes in which a complete 
pigment sheath extends from the cone to the rhabdome, only appo- 
sition images would be possible, but there are also eyes in which the 
region between rhabdome and cone proper contains little or no pig- 
ment, and in such cases it is conceivable that the light entering the 
cone of one ommatidium might reach the rhabdome of another or 
possibly, several others, as Exner believes. This might seem at 
first sight to interfere with the clearness of vision, but, as Exner 
has shown in some instances at least, the dioptric organs of the eyes 
in question are so constituted that they each form a relatively large 
erect image near the level of the rhabdomes, and these images overlap 
one another so that the details in one coincide with tbe details in 
another, thus giving rise to a single erect image for the whole retina; 
in other words, the light that emanates from one small luminous 
object in the field of vision and enters tbe retina, even though it 
does so by means of many cones, is finally concentrated at one 
point in the sensory layer. The image thus formed has been appro- 
priately called by Exner a superposition image and differs from an 
apposition image, all other things being equal, only in its greater 
brightness ; i. e., one mosaic block in an apposition image is formed 
by the axial rays from one cone ; the corresponding block in a super- 
position image is formed by the corresponding rays plus many others 
that, after entering the retina through neigbboring cones, are turned 
by these so as to meet the given axial rays at the level of the per- 
ceptive layer. 
It is not my purpose to enter into a discussion of the optical means 
