A Micrometric Difficulty. By E. M. Nelson. 697 



ruled band may be accurately measured, but the number of lines 

 ruled may not be determined. 



Having found the cause, the next thing is to discover a 

 remedy. It is very simple when pointed out, for it is only 

 necessary to remember that when the lines are at a white -dot 

 focus, the white and not the black lines must be counted. Unless 

 the microscopist keeps a sharp look-out, a mistake may be very 

 easily made. The merest touch of a delicate focus will shift the 

 image from a white to a black dot, and it will depend upon the 

 point of correction of the object-glass (i.e. tube-length) whether 

 the white- or black-dot image is the stronger of the two, or 

 whether both are alike. The determination as to whether the 

 image is a white- or a black-dot one is not quite so easy as one 

 would suppose. The best advice is to use a large W.A., and be 

 on your guard. 



Dec. 21st, 1910 3 a 



