THE EYE AS AN OPTICAL INSTRUMENT. 187 



it, and hold a sheet of white paper at the proper distance behind 

 it, we may notice two effects. In the first place (and this is 

 often disregarded) the burning lens, although made of trans- 

 parent glass, throws a shadow like any opaque body ; and next 

 we see in the middle of this shadow a spot of dazzling brilliance, 

 the image of the sun. The rays which, if the lens had not 

 been there, would have illuminated the whole space occupied by 

 the shadow, are concentrated by the refracting power of the 

 burning glass upon the bright spot in the middle, and so both 

 light and heat are more intense there than where the unrefracted 

 solar rays fall. If, instead of the disc of the sun, we choose a 

 star or any other point as the source of light, its light will be 

 united into a point at the focus of the lens, and the image of 

 the star will appear as such upon the white paper. If there is 

 another fixed star near the one first chosen, its light will be 

 collected at a second illuminated point on the paper ; and if the 

 star 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 surface, a similar line or sur- 

 face 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 corresponds to it in strength and colour of illumi- 

 nation, 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 prepared 

 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 



