ACCOMMODATION 409 



also inverted ? The answer is simple, and is given in 

 Lesson X. 



3. The Mechanism of Accommodation.— In the 

 water camera, constructed according to the description 

 given above, there is the defect that no provision exists 

 for adjusting the focus to the varying distances of objects. 

 If the box were so made that its back, on which the 

 image is supposed to be thrown, received distinct images 

 of very distant objects, all near ones would be indistinct. 

 And if, on the other hand, it were fitted to receive the 

 image of near objects, at a given distance, those of either 

 still nearer, or more distant, bodies would be blurred and 

 indistinct. In the ordinarj' camera this difficulty is 

 overcome by sliding the lenses in and out, a process 

 which is not compatible with the construction of our 

 water camera. But there is clearly one way among 

 many in which this adjustment might be effected— namely, 

 by changing the glass lens ; putting in a less convex one 

 when more distant objects had to be pictured, and a more 

 convex one when the images of nearer objects were to be 

 thrown upon the back of the box. 



But it would come to the same thing, and be much 

 more convenient, if, without changing the lens, one and 

 the same lens could be made to alter its convexity. This 

 is what actually is done in the adjustment of the eye to 

 distances. 



The simplest way of experimenting on the adjustment 

 or accommodation of the eye is to stick two stout 

 needles upright into a straight piece of wood, not exactly, 

 but nearly in the same straight line, so that, on applying 

 the eye to one end of the piece of wood, one needle (a) 

 shall be seen about six inches off, and the other (b) just on 

 one side of it at twelve inches or more distance. 



If the observer look at the needle b, he will find that 

 he sees it very distinctly, and without the least sense of 

 effort ; but the image of a is blurred and more or less 

 double. Now let him try to make this blurred image of 



