ON THE STUDY OF PHYSICS 305 



in a liquid of the same refractive index as itself, it im- 

 mediately disappears. I remember once dropping the 

 eyeball of an ox into water; it vanished as if by magic, 

 with the exception of the crystalline lens, and the sur- 

 prise was so great as to cause a bystander to suppose that 

 the vitreous humor had been instantly dissolved. This, 

 however, was not the case, and a comparison of the re- 

 fractive index of the humcr with that of water cleared 

 up the whole matter. The indices were identical, and 

 hence the light pursued its way through both as if they 

 formed one continuous mass. 



In the case of snow, powdered quartz, or salt, we have 

 a transparent solid mixed with air. At every transition 

 from solid to air, or from air to solid, a portion of light 

 is reflected, and this takes place so often that the light is 

 wholly intercepted. Thus from the mixture of two trans- 

 parent bodies we obtain an opaque one. Now, the case 

 of the towel is precisely similar. The tissue is composed 

 of semi-transparent vegetable fibres, with the interstices 

 between them filled with air; repeated reflection takes 

 place at the limiting surfaces of air and fibre, and hence 

 the towel becomes opaque like snow or salt. But if we 

 fill the interstices with water, we diminish the reflection; 

 a portion of the light is transmitted, and the darkness of 

 the towel is due to its increased transparency. Thus the 

 deportment of various minerals, such as hydrophane and 

 tabasheer, the transparency of tracing paper used by engi- 

 neers, and many other considerations of the highest scien- 

 tific interest, are involved in the simple inquiry of this 

 unsuspecting little boy. 



Again, take the question regarding the rising or falling 

 of the dew a question long agitated, and finally set at 



