THE STEREOSCOPE. 285 



ring ; while the latter causes one part to slide over the other, 

 and is analogous to the comb. 



The movements of this organ are believed to be as involuntary 

 as the dilatation and contraction of the iris ; but, whatever 

 may be the case, it is one of the most beautiful examples of 

 natural mechanics, and far surpasses the most delicate machine 

 that can be made by man. 



In the illustration of the microscope, which is to be found 

 on page 286, both these movements are given, the double 

 vertical wheel being the coarse movement, and the fine move- 

 ment being supplied by the single vertical wheel just above 

 them. 



WHILE we are on this subject, we may see how Art uninten- 

 tionally copies Nature, even in trivial details. Every one who 

 is in the habit of using optical instruments, more especially 

 those who are forced to wear spectacles, are aware of the neces- 

 sity of keeping the glasses as clean as possible, and, where the 

 instruments are delicate, always have by them a piece of clean 

 wash-leather for the express purpose of wiping the glasses. 



Here, again, Nature has anticipated Art. In our own case, 

 we have in the human eye a good example of such natural 

 mechanism, the eyelids being formed quite as much for the 

 purpose of washing the surface of the eyeball as of excluding 

 light. 



Many animals are provided with a special apparatus for the 

 purpose, called the "nictitating membrane." It is, in fact, a 

 sort of inner or supplementary eyelid, which can be drawn over 

 the eye while the external lids remain comparatively unmoved. 

 It is very conspicuous in the owls, and gives to those birds 

 that almost comical look of perpetual blinking with which we 

 are so familiar. 



THE STEREOSCOPE AND PSEUDOSCOPE. 



MANY persons have wondered how it happens that, as we 

 have two eyes, we do not see two images instead of one. 

 Practically, this is always the case, for the eyes, especially when 

 they look on solid bodies, see two different images, because they 

 contemplate the object from different points of sight. 



