OPTICAL ILLUSION. 209 



explanation. That the microscope should shew you 

 things which you could not see without it, you ex- 

 pected before ; but that it should shew things which, 

 after all, are nothing, that you should see with your 

 eyes objects which do not exist, is marvellous indeed. 

 But it is true. 



It will take a good many words, I am afraid, to ex- 

 plain this ; and some precision of thought on your 

 part to understand it, when it is explained. It may 

 help 3^011, if you recall the familiar sight of a meadow 

 just ready for the mower under the summer breeze. 

 You have often admired the bands of silvery light, 

 alternating with gray shadows, that are ever flitting 

 across the field. What is it that passes over the sur- 

 face ? Nothing. You know that the charming ap- 

 pearance is caused by the alternate bending and rising 

 of the feathery heads of the grass-blossom in succes- 

 sion, as the breath of the breeze sweeps over them. If 

 we imagine all the stalks removed on both sides of 

 a single row of stalks, or, which is the same thing, 

 practically isolate our attention to this single file, the 

 matter will be simplified, and we shall have a very 

 close parallel to the chase in the microscopic live-box. 



The gill-thread, all along its two opposite sides, is 

 fringed with a row of minute hairs, so minutely at- 

 tenuated that the high powers of the microscope, we 

 have been supposing to be used, are insufficient to 

 reveal them in their individuality. These hairs, 

 which are known by the term cilia, are endowed 

 with the power of alternately bending and straighten- 



