78 VISION. 
focus of his instrument by altering the position of 
the lens, screwing it nearer or further from the 
screen, so we adjust the focus of our eyes instinc- 
tively according to the distance of the object 
looked at, not indeed by changing the position of 
the lens but by altering its form so as to make it 
stronger or weaker as required. 
The sensitive curtain is called the retina; and 
in the early embryo, when the brain is as yet 
a hollow structure with delicate walls, the retina 
was continuous with the brain, forming in fact 
a portion of it, and lined on its deep surface 
with a layer of cells continuous with the epithelial 
cells lining the hollow of the brain. These cells 
at the back of the retina become developed into 
elongated structures called rods and cones, or 
included under the common name of bacillary 
elements ; and:the exceedingly minute bacillary 
elements are the organs on which the light of the 
inverted picture acts. They have a highly complex 
structure which need not engage our attention at 
present ; and the main part of the retina presents 
a remarkable modification of brain-structure, re- 
ceiving the fibres of the optic nerve and placed in 
structural continuity by means of other nerve-fibres 
with the bacillary elements. The rays of light 
pass through the retina proper without producing 
