436 SPECIAL PHYSIOLOGY. 



The size of the image varies, of course, according to the distance 

 of the object, being smaller in proportion to the greater distance of 

 the object. The degree of convexity of the lens also affects the size 

 of the image ; for the greater the convexity of the lens, the shorter is 

 the focal distance, and the smaller the image produced. 



When the rays from a straight line, or from a plane surface, placed 

 parallel with the surface of a double convex lens, pass through it, the 

 image is always curved, or concave, towards the lens; and if the screen 

 for its reception be a plane surface, this image is defective, either at the 

 extremities or margins, or else in the centre. This, the error from 

 curvature, may be obviated by making the screen concave. 



The error called that of distortion, is due to the varying distances 

 of the parts of the same object; it therefore chiefly affects the extreme 

 marginal rays proceeding from very long straight objects. It is on 

 this account that the images of the parts of such objects, which lie 

 near to the margins of the lens, are proportionally somewhat smaller 

 than those of the parts lying opposite the centre of the lens. 



The errors of curvature and distortion may be diminished by limit- 

 ing the operation of the lens to its central part, by cutting off the 

 marginal rays with a perforated diaphragm. 



Another imperfection, called the error of confusion, is due to the 

 increasing irregularity of the refraction undergone by those rays 

 which fall with greater and greater obliquity on the lens. If the mar- 

 ginal rays are intercepted, this error may be diminished ; and if the 

 position of the lens be so changed that the rays fall on it directly, 

 instead of obliquely, it is entirely obviated. 



A camera obscura is a dark box or chamber, painted black in its in- 

 terior, and having, in its front, an aperture fitted with a double convex 

 lens, made to slide in and out, and, at the back, a screen of some 

 semi-opaque substance, such as ground glass, or tissue paper. When 

 an object is placed in front of the lens at a suitable distance, an in- 

 verted image of it is projected on to the screen. The distinctness of 

 this image may be diminished or increased, by changing the distance 

 of the lens from the screen ; and the introduction of a perforated dia- 

 phragm of blackened cardboard, or metal, between the lens and the 

 screen, by cutting off the aberrant marginal rays, will also improve 

 the distinctness of the image, and, at the same time, regulate the 

 quantity of light admitted into the camera. Such a chamber, filled 

 with water instead of air, having a concavo-convex lens fitted into the 

 aperture in its front, and provided, in its interior, with a double con- 

 vex lens, placed behind a perforated diaphragm, would closely resem- 

 ble, in its optical arrangements, the globe of the eye, and would form, 

 on the screen behind, inverted images of objects situated in its front. 



Sight. 



The eyeball, Fig. 83, is a natural camera obscura ; it is a dark 

 chamber, colored black, or brownish, within, by the choroid pigment ; 

 in front, it presents a convex, transparent, portion, the cornea, c, for 

 the admission of light into its interior, as well as for its partial refrac- 



