282 NATURE'S TEACHINGS. 



moon. This, in fact, is the meaning of the term " meniscus," 

 which is a Greek word, signifying a little moon. If the same 

 glass were solid, or even filled with water, it would form a 

 " plano-convex " lens. 



Of course the outer curve of the meniscus must be larger 

 than the inner curve, but in some cases the disproportion is 

 very strongly marked, the outer curve being very large, and 

 the inner curve very small. An example of such a meniscus 

 may be seen in the human eye. If the reader will refer to the 

 illustration on page 280, in which the structure of the eye is 

 shown, he will see the meniscus lens in combination with the 

 double convex. The former has already been explained, and 

 the latter is formed by the vitreous humour which fills nearly 

 the entire globe of the eye. Its larger curve is due to the form 

 of the eyeball, and the smaller to the convex lens. 



LONG AND SHORT SIGHT. 



IT has already been mentioned that the focus of a convex 

 lens is shorter in proportion to its convexity, and that in conse- 

 quence its magnifying power is increased. For example, the 

 large glasses through which pictures are viewed are compara- 

 tively thin in proportion to their diameter, while the lenses 



FRAME OF OWL'S EYE : LENS SHORT AXD LONG SIGHTED E?ES 



OF DITTO. (WITH SPECTACLES). 



employed for the highest powers of the microscope are scarcely 

 larger than small shot, and nearly as globular. It naturally 

 follows that any instrument to which a lens is adapted, whether 

 it be microscope or telescope, must depend for its focus on the 

 greater or less convexity of the lens in question. 



Again taking as our example the human eye, we find that 

 there are very few persons who from youth to age possess or 



