224 THE EYE 



component tending to rotation round the horizontal axis at right 

 angles to the visual axis, the second component tending to rota- 

 tion round the visual axis, clockwise. The two rotations round 

 the visual axis counteract each other, the remaining two act as a 

 couple, and reinforce each other. The result of the combined 

 action is to move the cornea vertically upwards. For movement 

 upwards and inwards, for instance, the co-operation of a third 

 muscle, the internal rectus, is required. The rectus inferior 

 and the obliquus superior combine in an exactly similar manner 

 to move the eyeball downwards. 



Ophthalmoscopy. 



When we look at a person's eye the pupil appears perfectly 

 black. Nothing can be seen of the interior because it is feebly 

 illuminated compared with the outside world. If we could light 

 up the interior it would become visible, just as we can see into 

 a lighted room at night if the window is not covered with a blind. 

 When we try to illuminate the interior of the eye, we find that 

 we must always interpose our head between it and the source of 

 light in our attempts to peer into it. This difficulty is overcome 

 by reflecting light from a mirror provided with a small central 

 aperture, through which the observer can look. This instrument 

 is called an ophthalmoscope. It may be used in two ways : 



A. In direct ophthalmoscopy, the mirror and the observer's 

 eye are brought close to the observed eye, and the refracting 

 media of the latter produce a virtual image, erect and magnified, 

 of the retina. The lens, etc., of the observed eye act in exactly 

 the same way as a magnifying glass, the object being just inside 

 the focus. 



B. In indirect ophthalmoscopy the observer holds a convex 

 lens in front of the observed eye, and places himself farther 

 away. The interposed lens brings the rays leaving the observed 

 eye to a focus between itself and the observer, who consequently 

 sees an inverted image of the retina. This is real and magnified, 

 the magnification depending on the lens used. 



Defects of the normal eye as an optical instrument. 



(i) The eye is not perfectly centred, i.e. the axis of the lens 

 does not coincide with the axis of the cornea. 



(ii) The optical axis passing through the centre of the cornea 

 and the centre of the lens does not coincide exactly with the 

 visual axis passing through the centre of the cornea and the 

 fovea centralis. 



