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 



