67 



thick covers and under-correction with thin covers, 

 the amount of correction varying in a different ratio 

 to the amount of variations from the normal thick- 

 ness. The chromatic correction will also lose corres- 

 pondingly, but to not so high a degree. While a devi- 



Fig. 18. 



ation of a few hundredth millimeters in either direc- 

 tion will, perhaps, not signify, that which occurs in 

 covers classified in price-list under one number is 



