FORMATION OF IMAGE. 333 



great deal of light is absorbed by the pigment some reaches the 

 retinulae, and forms the erect mosaic or " apposition image " in 

 the manner indicated. But when the eyes of these animals are 

 fixed by reagents after being exposed to darkness, it is found 

 that the pigment blinds which separate the ommatidia from 

 one another are withdrawn. The pigment in the iris pigment 

 cells is drawn up towards the cornea, that in the retinulae has 

 retreated below the basement membrane towards the nerve 

 fibres.* It has been shown that under these conditions the 

 ommatidia no longer act separately, but that a combined image 

 is thrown on the retinular layer, the crystalline cones being so 

 disposed that the light from a given point falling on a consider- 

 able area of the eye, no longer obstructed in its course by the 

 blinds of pigment, is brought to a focus on that layer. In this 

 manner an erect " superposition image " is formed, the rays 

 refracted by a large number of crystalline cones being super- 

 posed at the focus on the retina, and a stimulus far stronger in 

 proportion to the intensity of the illumination than that of 

 the apposition image, though probably much less distinct in 

 details, is given to the retinulae. The eyes of insects such as 

 fireflies and many moths are permanently in this condition, and 

 are " day blind." On the other hand the eyes of butterflies 

 have the pigment permanently expanded, and are "night blind." 

 A very interesting confirmation of these results has been fur- 

 nished by the beautiful researches of Chun on the pelagic Schizo- 

 pods inhabiting the dark waters of the ocean at a depth of 300- 

 600 fathoms (Fig. 290). In these the retinular pigment has dis- 

 appeared altogether, while the distribution of that of the iris 

 pigment cells varies in different parts of the eye, according as 

 the ommatidia are directed sideways (lateral eye) towards 

 objects which may be illuminated by the phosphorescent organs 

 carried by the animal itself, or forwards (frontal eye) into the 

 dark region from which the rays of these organs are by their 

 position excluded. In the frontal eye moreover the number of 

 retinulae is far in excess of that of the crystalline cones, a con- 

 dition which is in harmony with the theory of the formation of 

 a superposition image, but unmet by the mosaic theory. 



* As in the pigment cells of the frog, the chromatophores of Deca- 

 pods, etc., the movement of the pigment takes place within the cell, and 

 the shape of the latter is unaffected by the movement. 



