DEVICES WHICH OBVIATE ACCOMMODATION 



255 



corresponding level on the sides of the chorioidal mountains. Presumably 

 the bat sees the visual field at a given distance as a (relatively!) sharp 

 reticulum, the lacunae of the lace-work being much more badly blurred. 

 The small bats (Microchiroptera) do not have this device, but they are 

 not at all dependent upon their vision, which is very poor; and their eyes 

 are so tiny anyway as to need no accommodation or substitute therefor. 

 2. A tilted attitude of the retinal surface relative to the visual axis of 

 the eye. This is the equivalent of slanting the plate of a camera so as to 

 have, in simultaneous sharp focus, objects at different distances — as is 

 done for example in photographing tall buildings from the ground. In 

 some invertebrate eyes the retina is built like a flight of steps. Among 

 vertebrates the rays and the horse (and probably many other, unstudied 



Fig. 102 — Two devices which make accommodation unnecessary. 



a, retina and chorioid of a fruit-bat, Pteropus medius, showing chorioidal mammillation 

 which places the visual cells at many diflFerent levels, x 48. After Kolmer. x, x- visual-cell 

 layer, b, eye of a ray, Raja balls, in vertical seaion, showing how retina forms a 'ramp' — 

 the axial length of the eyeball changes continuously in the vertical meridian. Based on a 

 drawing of Franz, c, eye of horse in vertical seaion, showing ramp retina. Redrawn from 

 Nicolas. 



ungulates) appear to have produced a similar device, using a ramp rather 

 than a stairway. The retina is progressively farther from the lens super- 

 iorly than it is inferiorly (Fig. 102b, c). The horse, which has no power 

 of accommodation, apparently has only to tilt the eye slightly up or 

 down to have a sharp retinal image of any object over a considerable 

 range of distance. This is however only a suspicion which awaits ex- 

 perimental justification. 



3. The use of a stenopaic aperture. This, which is simply a single or 

 multiple pin-hole pupil or a device which gives the effect of one (as in 

 the seals — see p. 447), has the virtue of producing a pretty sharp image 

 regardless of the distances from it to the object and the retina. In fact, 

 we may fairly say that the vertebrates brought the need of accommoda- 

 tion upon themselves, in the first place, when they adopted the lens as a 



