Barrett—On Entoptic Vision. 53 
shadow of the object, and not an inverted, lenticular image, will 
be thrown upon the retina. This shadow will be erect, correspond- 
ing to the position of the object, but will be projected from the 
retina as an inverted image; thus if a pin, with the head upwards, 
be held between the screen and the eye, an enlarged image of the 
pin with the head downwards will be seen. In like manner any 
obscurity within the eyeball, when revealed by pin-hole vision, 
is seen completely inverted laterally as well as vertically, though, 
as with the pin, the shadow of the obscurity on the retina is not 
inverted. 
§ 6. 
The fact that in ordinary vision we see things erect, though the 
retinal image is inverted, presents no real difficulty. The mind 
does not contemplate the retinal image as a photographer views 
the inverted image of a landscape when focussing his camera. 
In fact, we are wholly unconscious of the existence of the retina, 
or of the size or position of the retinal image, except from 
experiments on eyes other than our own.' Visual perceptions are 
always referred to a position in space external to our eye, and are 
projected outwardly by our mind along the “‘ line of visible direc- 
tion,’ as Brewster termed it. Now, this line is almost exactly 
coincident with a straight line joining any point in a visible object 
external to the eye, if seen by direct vision, with the corresponding 
image of that point on the retina. These lines necessarily cross 
each other at 4, fig. 5, which is the centre of visible direction in the 
eye. ‘The inverted image, therefore, gives rise to the perception 
of an erect object and vice versa. 
The centre of visible direction /, fig. 5, may be taken as 
coinciding with the optical centre of the eye. The optical centre 
of a thin lens is such that all rays not remote from the axis passing 
through it are undeviated by the lens; but every lens, or system 
of lenses, of which the two faces are not in the same medium, as is 
the case with the eye, has two geometrical points which replace 
the optical centre of an infinitely thin lens. These two points are © 
termed nodai points ; and these in the eye lie so close together that 
1 Or by means of a mirror and ophthalmoscope a skilled observer can use his own 
eyes as if they belonged to some one else. 
