CHAPTER XXI 

 OUTPOSTS OF THE INTELLIGENCE SERVICE 



(r) DISTANCE RECEPTOR FOR LIGHT 



THE EYE 



" I had rather not describe it at all, so that neither the difficulty of the explana- 

 tion nor its length might cause me to be hated." Galen. 



The following points in connection with light are recalled to the 

 student's memory. 



(1) It can be shown in various ways that light travels in straight lines, e.g. 

 interception of light by opaque objects. 



(2) The path along wliich light travels from each point of a luminous object 

 is a ray. A collection of such rays converging so as to form a cone is termed 

 a pencil of rays. When the apex of the cone is at the source of light the pencil 

 is diven/oif : wlien the reverse is the case, one has a convergent pencil. When 

 a pencil of rays diverges from or converges to a point 6 metres or more away 

 (infinite distance) tlie rays are considered to be 'parallel. 



(3) The simplest instrument with which to obtain an image of an external 

 object is a pinhole camera, which consists of a rectangular box. One end of 

 the camera is made of ground glass, and in the centre of the opposite side is a 

 pinhole. Rays of light diverge from a luminous object, say an arrow [A — W) 

 placed in front of the pinhole (0). A divergent pencil of rays proceeds from 

 the arrowhead A. A certain very small collection of rays from A will pass 

 through 0, and reach the screen S. The result will be an image A' of A on 

 the ground glass. Similarly, every other point, RRO and W , of the arrow- 

 will produce divergent pencils of rays, some of which rays will pass through 

 to give a complete image, A'R'R'O'W . Now as all the rays entering the 

 camera have passed through the pinhole, it follows that they must liave 

 crossed at 0, and, therefore, the image will be inverted, viz. ,14' . . . A' . There 

 is a definite relationship between the relative sizes of object and image and the 

 relative distances of pinhole to screen 08 and pinhole to object OR. i.e. : 



size of image distance OS 

 size of object ~ distance OR 



By varying the position of the screen, the image may be made to vary in 

 size, and similarly, but in the reverse way, the size of the image depends on 

 the distance of the object from the camera. By increasing OS or reducing 

 OR within certain limits, the image nuiy be made larger. When OR is 

 reduced to a certain size, the pencils of rays coming from the points A and R, 

 and W miss the pinhole with the exception of the most divergent rays. 

 The image is, therefore, blurred. The further the object is from the camera 

 (within limits) the more will the less divergent rays from the extremities of the 



