DIOPTEIC MECHANISM OF THE EYE 309 



independently of the drop of the lens during forced accommodation. 

 But in any case the defect is very small and produces no 

 perceptible visual disturbance. 



To demonstrate the existence of astigmatism not due to 

 asymmetrical curvature of the cornea, it is necessary to eliminate 

 the influence of the latter by looking through water. Under 

 water the human eye becomes extremely hypermetropic, because 

 no reflection takes place at the surface of the cornea. In fishes 

 the hypermetropia is compensated by the marked curvature of 

 the lens, which becomes spherical. For man to see plainly under 

 water a biconvex lens of about 28 D. is required, or (as Dudgeon 

 proposed) a concave lens of air, made of a watch-glass applied to 

 the eye in a water-tight tube. If vision under water through this 

 correction - lens does not eliminate the astigmatism, it probably 

 depends on defective centring or on an obliquity in the lens. 



(e) Another common defect of the dioptric apparatus (present, 

 according to Johannes Muller, in most individuals) is the so-called 

 monocular polyopia (monocular diplopia or triplopia). This con- 

 sists in seeing double or even triple images of an object with one 

 eye, under certain conditions. 



Monocular polyopia is independent of ametropia of the dioptric 

 apparatus, since it can be observed both in myopes and in hyper- 

 metropes. 



In the author's own case monocular diplopia and triplopia 

 occur as follows. When looking with either eye without corrective 

 lenses for presbyopia at black lines arranged as a cross, or as radii 

 of a circle, traced on a white card (Fig. 142), double, treble, or 

 even quadruple images of each line are seen, especially of those 

 in certain planes, according to the degree of illumination, the 

 distance of the card, and the rested or fatigued state of the eye. 

 The multiple images are approximately parallel, and vary con- 

 siderably in distinctness, one being sharper, the next less distinct, 

 and so on. Their apparent distances are not equal, as the trans- 

 verse or oblique lines seem more distant than the vertical. On 

 looking at the figure with either eye through a correction -lens 

 and at the proper distance, all the radii appear single, though 

 more or less sharp or blurred according to the physiological 

 astigmatism ; only when the card is brought nearer, or moved 

 beyond the limits of accommodation, do the images of all the 

 lines appear double. Finally, on looking at the figure, with 

 or without correction -lenses, through a pin-hole in the card 

 (stenopaic diaphragm) the lines all appear single and equally 

 distinct, whether the figure is brought close to, or moved away 

 from, the eye. 



These observations prove that monocular polyopia is due to 

 diffusion-circles, since it entirely disappears when the objects are 

 sharply focussed. To account for the phenomenon it is necessary 



