TAKING CAKE OF LENSES. 19 



" So that, if you are going to study optics," continued 

 Lawrence, " and are to have any nice lenses and prisms to 

 make experiments with, I advise you to be very careful 

 how you rub them with dusty fingers or dusty cloths." 



"Yes, I will," said John. 



" A good lens," said Lawrence, " is a -very delicate thing, 

 and sometimes a very costly thing. It requires, in the first 

 place, a very nice preparation and mixture of the materials 

 out of which the glass is made, and great care in the mak- 

 ing of it, to secure its being uniform and homogeneous 

 throughout, so as to act upon the light in the same way in 

 every part. Then it is a very nice operation to grind it 

 precisely to the true form, and to polish it perfectly. So 

 that, when you get a good lens, if you ever do get one, you 

 can't be too careful of it." 



While Lawrence had been saying these things, John had 

 been attentively examining the eye-glass, without, how- 

 ever, touching the glass at all. 



" Yes," said he, " yours is a concave lens ; it is thinner 

 in the middle than at the edges. I want one which is 

 thicker in the middle than at the edges." 



"Perhaps the landlady will lend you her spectacles," 

 said Lawrence. 



Lawrence and John were at this time in lodgings in Lon- 

 don. The keeper of the lodging-house where they had 

 taken their rooms was quite an elderly woman, and very 

 soon after he had given Lawrence's eye-glass back to him, 

 John heard her footsteps in his bedroom, which was a 

 small room adjoining their sitting-room. John went in 

 immediately, and asked her if she used spectacles. She 

 said she did sometimes. John asked her if she was willing 

 to lend him her spectacles a few minutes; he wished to 

 make an experiment on light with them. 



" Certainly," she said. He could have them as well as 



