OF THE MICBOSCOPE. 17 



organs of vision. Now, the eye, considered merely as an optical 

 instrument, is in reality a small camera obscura in which the 

 cornea, crystalline lens, and other transparent portions, combine 

 to throw upon the retina an image of external objects. That 

 the transparent portions of the eye do in fact act as a lens, and 

 throw a real image upon the retina or posterior portion of the 

 eye, is easily shown by taking the fresh eye of an ox and grad- 

 ually shaving off the coating at the back until it becomes 

 transparent. If the eye, so prepared, be then held towards 

 a window or any very bright object, a distinct but inverted 

 image of the window or other object will be seen on the coat 

 of the eye. 



The action of the eye in this case is the same as that of a 

 lens, and the general mode of action of lenses under such cir- 

 cumstances may be easily illustrated by means of a common 

 hand magnifier or even a spectacle glass. If the reader will 

 hold before a window, at a distance of, say, six feet, a sheet of 

 white paper, and will place a magnifier in front of the paper, 

 then by properly adjusting the distance between the magnifier 

 and the paper, a picture of the window will be thrown on the 

 latter. If the magnifier and paper be now removed to a dis- 

 tance of twelve feet from the window, the picture of the latter 

 will be only half as large as it was in the first place, and it will 

 also be found that the distance of the lens from the paper will 

 have to be readjusted and made less. 



That the eye possesses this power of adjustment we are all 

 conscious, for we feel that if, when the eye is adjusted for the 

 distinct vision of distant objects, we suddenly look at those 

 which are near, the condition of the eye requires to be changed 

 before a distinct view can be had, and to make this change 

 requires an effort of which we are perfectly conscious. 



When a lens is held in front of a sheet of paper, so as to 

 throw on the latter a distinct image of the objects in front of 

 it, the distance between the paper and the lens is called the 

 focal distance or focal length of the latter. This, as we have 

 just seen, varies with the distance of the object which gives the 

 image. In order, therefore, to secure a standard in this 

 respect the object selected is always one whose distance is so 

 great that it may be practically regarded as infinite. 



