THE EYE AS AN OPTICAL INSTRUMENT. 211 



happen to send out red rays, its image on the paper will 

 also appear red. The same will be true of any number 

 of neighbouring stars, the image of each corresponding 

 to it in brilliance, colour, and relative position. And if, 

 instead of a multitude of separate luminous points, we 

 have a continuous series of them in a bright line or sur- 

 face, a similar line or surface will be produced upon the 

 paper. But here also, if the piece of paper be put to the 

 proper distance, all the light that proceeds from any one 

 point will be brought to a focus at a point which corre- 

 sponds to it in strength and colour of illumination, and 

 (as a corollary) no point of the paper receives light from 

 more than a single point of the object. 



If now we replace our sheet of white paper by a pre- 

 pared photographic plate, each point of its surface will be 

 altered by the light which is concentrated on it. This 

 light is all derived from the corresponding point in the 

 object, and answers to it in intensity. Hence the changes 

 which take place on the plate will correspond in amount 

 to the chemical intensity of the rays which fall upon it. 



This is exactly what takes place in the eye. Instead 

 of the burning glass we have the cornea and crystalline 

 lens ; and instead of the piece of paper, the retina. Accord- 

 ingly, if an optically accurate image is thrown upon the 

 retina, each of its cones will be reached by exactly so 

 much light as proceeds from the corresponding point in 

 the field of vision ; and also the nerve fibre which arises 

 from each cone will be excited only by the light proceeding 

 from the corresponding point in the field, while other 

 nerve fibres will be excited by the light proceeding from 

 other points of the field. Fig. 30 illustrates this effect. 

 The rays which come from the point A in the object of 

 vision are so broken that they all unite at a on the retina, 

 while those from B unite at b. Thus it results that the 

 light of each separate bright point of the field of vision 



