THE ORGANS OF SENSE. 121 



THE FORMATION OF AN IMAGE IN THE RETINA. 



The eyeball may be compared with a photographic camera, the 

 cornea, aqueous humor, lens, and vitreous body forming a system of 

 lenses, the choroid coat being comparable with the dark lining of the 

 camera, and the retina acting as the sensitive plate. If a segment of 

 the sclera with the choroid be excised from the back of the eye of a 

 recently killed ox, and the eye be held in front of an electric lamp; 

 an inverted image of the lamp will be seen upon the retina, similar 

 to the image which may be observed on the ground-glass screen of 

 a camera. 



In the camera the image is usually, formed by means of a single 

 biconvex glass lens. The optical axis of such a lens is a line drawn 

 through its optical centre, and entering and leaving it in a direction 

 perpendicular to the plane of the lens. A ray of light passing along 



FIG. 34. Diagram of the course of parallel rays through a bicon- 

 vex lens (L), by which they are converged to the principal 

 focus (F). A, axial ray. (Starling's Principles of Physiology.} 



this axis enters and leaves the lens with its direction unchanged, but 

 rays falling upon the lens outside the axial ray and parallel with it are 

 refracted, both on entering and leaving the lens, towards the axial ray, 

 so that they meet it or come to a focus at a point which is known as 

 the principal focus of the lens (fig. 34). A pencil of divergent rays 

 falling upon such a lens will, if the lens have a sufficient degree of 

 convexity, be brought to a focus at a point behind the principal focus. 

 If an object, such as an arrow, be placed at some distance from a 

 biconvex lens, the pencil of rays from the tip of the arrow will be 

 brought to a focus behind the lens, and the same will hold for rays 

 from the butt and every other point on the arrow. The axial ray of 

 each pencil will pass through the optical centre (nodal point) of the 

 lens with its direction unchanged. The result will be the formation of 

 a real, inverted image of the arrow in the plane in which the rays are 

 focussed. 



The formation of an image in the eye is more complicated in that 

 the cornea, aqueous humor, lens, and vitreous body form, not a single 

 lens, but a system of lenses differing in refractive index, so that altera- 



