TEST OBJECTS. 117 



has prepared glass plates, on which are ruled, with a diamond, 

 systems of a hundred lines, which, 10 by 10, approach closer to- 

 gether, according to a certain standard. Ordinary instruments 

 make out the 6th and 7th systems as separate lines. Superior 

 instruments reach the 8th and 9th. No instrument has yet 

 unfolded the 10th system of lines. This is certainly a great 

 triumph of art. 



