Note. 703 



it is not always possible to correctly count the number of lines 

 actually ruled.* 



In practice it is important so to adjust the object-glass, either 

 by collar or tube-length, that the strongest image is formed before 

 the upper diffraction image has begun to roll. 



Together with these photographs, Grayson has sent some ad- 

 ditional examples of his beautiful rulings. They consist of a whole 

 inch divided into hundredths, and they are ruled upon glass, quartz, 

 silver film, and speculum metal. 



The material which contains the ruling is in the first three 

 attached to an ordinary 3 by 1 glass slip, but that upon speculum 

 metal is attached to a 3 by 1 brass slip. 



PL XIV. fig. 4, drawn to a scale of two-thirds, shows the manner 

 in which they are mounted. 



The rulings upon glass are similar to those upon ordinary micro- 

 meters, but the lines are much finer in that ruled upon cpaartz. 

 The lines on the metallic plates, when a strong light is thrown 

 upon them, look like very fine incandescent wires. 



These new rulings will be found to be useful for a variety of 

 purposes ; for example, shortly after their arrival a particular 

 divided lens-micrometer was calibrated, which would not have been 

 so easily done without their aid. 



* Upon numerous occasions I have counted one more line with oblique than 

 with axial large cone illumination, even when the adjustments were as perfect as 

 possible. It would seem that to have one more line than actually exists is a 

 normal condition of oblique light resolutions. 



