Dr. G. J. Stoney on Microscopic Vision. 515 



lines occur where bright ones were before. At q all is again 

 in confusion, to be succeded on drawing the focus farther out 



/» / x /\ ^ '\ V / 



u ' \ ' \ / * / \ / 1 A 



\ > a / y V V 





h A X \ h "ip 



.'\;\/\/\/\/\s 



1 k m M m \ 



Image plane. 



by the reappearance of the ruling at r, with its bright and 

 dark lines in the same positions as on the image plane. 



This is what happens to beams u, u' , v, ?/, all of which 

 lie in one meridian plane. If, as usually happens, the corre- 

 sponding undulations in all the other meridian planes that 

 contribute to form the minute marking have their dotted 

 lines (the lines shown in fig. 2) about as much sloped, then at 

 about the height p they too will produce rulings in all of 

 which dark lines will now occur where bright ones did on the 

 image plane. Hence at the height p dark specks will be 

 produced, by the cooperation of all the beams, where bright 

 ones were produced on the image plane ; and light will be 

 distributed over the intervening spaces where on the image 

 plane the darker shades prevailed. The effect depends on the 

 inclinations of the dotted lines being sufficiently nearly the 

 same in different longitudes. Usually they approximate 

 sufficiently for at least one of the alternations from bright to 

 dark spots to be well seen, and not unfrequently a second or 

 third may be imperfectly traced. 



If, as in fig. 2, v' and v are at the same inclinations as u 

 and u', then the dotted lines are equally sloped to the right 

 and left, and the dark specks at level p are directly over the 

 bright ones on the image plane. But this adjustment seldom 

 happens to be accurately secured, and the dotted lines in 



