PINHOLE FOCUS. 343 



merit. A sheet of thin semi-transparent paper being laid over the 

 ground glass plate, the outlines of the image formed may be easily 

 traced by a pencil. 



For the dark chamber camera, a tent, summer house, or other 

 convenient small building is used, at the top of which is arranged 

 an inclined mirror, so as to throw the light doivnwards on to a 

 horizontal table, painted dull white, or some form of screen capable 

 of being moved slightly up and down, so as to focus the image 

 according as near or distant objects 

 are in view ; a convex lens is placed 

 either in front of or under the 

 mirror, so as to form the image. 

 A block of glass of triangular section, 

 CDE, with a convex face on one 

 side, the other two being plane, 

 (fig. 189) maybe used instead of a 

 mirror and convex lens ; the rays 

 of light from a distant object, AB, 

 are made to converge on meeting 

 the curved front, and are then 

 totally internally reflected by the 

 inclined back surface, CD, so as Fig . 189 . Prism-lens for Camera, 

 finally to form an image at ab ; the 



glass prism-lens is fixed in a frame capable of swinging round a 

 vertical axis, so as to bring into view any part of the horizon. 



Expt 371. The Pinhole Focus. In the centre of one face of 

 a rectangular wooden or pasteboard box make a small hole with a 

 pin, and substitute a plate of ground glass or a sheet of greased 

 paper for the opposite side of the box. Hold the box with the 

 ground glass or paper screen in front of the eye, the pinhole being 

 directed to a brightly illuminated object or landscape ; an inverted 

 image will then be seen on the screen resembling that formed in a 

 photographic camera. The effect is heightened by throwing a 

 thick black cloth over the head and box (not over the pinhole) so 

 as to exclude all other light. With properly proportioned hole and 

 box and good external illumination, a tolerably clear definition of 

 the salient points of a landscape may be readily obtained ; but images 

 formed by a convex lens are generally much brighter and clearer. 

 With skill, however, it is quite possible to obtain fairly well-defined 

 photographs of scenery, &c., by means of a pinhole camera and 

 highly sensitive plates. 



Expt. 372. The Human Eye. The human eye, like that of 

 other animals, is essentially a kind of camera obscura, since the 

 " crystalline lens " in the front part refracts rays to a focus at the 



