jg RADIATION. 



it was attached, and handed it to John, saying at the same 



time,* 



" There it is ; but find out whether it is convex or con- 

 cave with your eyes, if you can, and not with your fingers." 



" Why not with my fingers ?" asked John. 



" You can feel of it if you find it necessary," said Law- 

 rence, "but the less we touch polished glass with our hands 

 the better. There are always particles of dust floating in 

 the air, and these alight on our fingers and on the glass, 

 and when we rub our fingers over the glass we rub the 

 surface with these." 



"And does that do any harm?" asked John. 



" It depends upon what the particles of dust are com- 

 posed of," replied Lawrence. " Some of them are minute 

 fragments of cotton or woolen fibres worn off" from clothes. 

 They would not do much harm. Some are minute spores 

 of plants." 



" What are spores ?" asked John. 



" A kind of seeds," said Lawrence. " They are from such 

 plants as form mould and mildew ; and some smaller still 

 so small, indeed, that the plants themselves can not be 

 seen except with a microscope ; and you can judge how 

 small the seeds must be. These would not do much harm 

 any more than the woolen and cotton abrasions. But there 

 is another kind of dust which comes from the road, and 

 which consists of minute scales of iron, from the shoes of 

 the horses, and the tires of the wheels, or, what is still 

 worse, of fragments of stone from the pavements, some of 

 which are siliceous that is, of the nature of flint, and are 

 exceedingly hard. When you rub these over the glass 

 with your fingers, or with a cloth, or a piece of leather, al- 

 though no one rubbing produces any perceptible effect, 

 after a time the fine polish begins to be dimmed. 

 * See Frontispiece. 



