(34 The Microscope. 



convex lens be applied to the aperture, all external 

 objects will be shown in the vivid brightness of 

 nature, so " that those who see it can never enough 

 admire it/' 



We can imagine the astonishment and delight of 

 the earlier spectators of Portals camera-obscnra ; but 

 what would have been their amazement to be told 

 that they themselves, and every bird, beast, and fish, 

 already possessed two optical instruments similar in 

 principle, and far superior in refinement of construc- 

 tion. 



The eye-ball is a little room, nearly spherical in 

 its form ; its outside wall is white, except at the front, 

 where it has a large, round, and slightly-projecting 

 window, called the cornea ; much like a watch-glass. 

 Behind this window is a permanent circular blind, 

 with a round aperture in its centre ; this is the Iris, 

 and the aperture is called the Pupil, and if you look 

 through it and try disregarding your own chubby 

 image on the cornea to get a glimpse of the little 

 room's interior, you will see that this is lined with 

 black, as every camera-obscura is or ought to be 

 to prevent confusion and indistinctness from double 

 reflections. The white outside circle is what I have 

 called the outside wall, and you can see at the left 

 where it is interrupted by the cornea, behind which 

 you can make out the section of the iris and its aper- 

 ture. The cornea is kept distended by a fluid like 

 water, and a similar purpose with regard to the rest 

 of the eye is served by a jelly-like transparent sub- 

 stance, called the vitreous humour. The line imme- 



