256 VISION. 



part of the retina, as follows: On blocking the right hand hole 

 in the card, the left hand image will disappear. 



The reversal by the brain is to be taken into account in 

 determining upon which side of the retina the disappearance 

 occurs, e.g., the side to which the image is referred in the field 

 of vision. 



Repeat the experiment, but on this occasion focus the near 

 image. Three images will again appear, but on blocking the 

 right hole in the card the image on that side disappears. This 

 proves that the eye is over-accommodated for the distant pin, 

 and that in consequence the rays must have crossed in front 

 of the retina. 



2o Listing's diagrammatic eye. 1 Make use of the rule of three 



formula given in the figure (Fig. 54) in the following calculations : 



Marriott's experiment and measurement of the Hind spot. 

 Seat yourself at arm's length in front of a sheet of paper 

 pinned to a wall (drawing board). Mark a spot straight 



1 Accommodation and refraction of the eye. (Bonders. New Sydenhain Society, 

 1864.) " For the ordinary eye we substitute one with a cornea, whose radius of 

 curvature is only 5 mm, while behind this is merely vitreous or aqueous humour, 

 without crystalline lens, and with a length of visual axis of 20mm. In such an 

 eye retinal images would have the same magnitude, the same distinctness, and 

 the same position which they exhibit in the emmetropic eye, with its cornea of 

 nearly 8mm radius of curvature, its crystalline lens of a little more than 43mm, 

 focus distance, and its visual axis of a little more than 22mm, and it can, there- 

 fore, really be substituted for this last." 



