290 A. ASHE ON OPTICAL TUBE LENGTH. 



of time and also obtaining results which correspond precisely 

 with those given by the other and more lengthy process. 



An example may be useful — 

 Magnifying power with the tube closed... ... ... = 100 



„ „ „ extended three inches = 150 



Increase ... ... ... ... ... ... = 50 



Therefore ... ... — — — = 6 (inches) tube length. 



Again, by the shorter way — 

 Number of lines that fill the field with the tube closed... = 30 

 Number of lines that fill the field with the tube ex- 

 tended three inches ... ... ... ... ... = 20 



Increase ... ... ... ... ... ... = 10 



20 X 3 

 Therefore inverting these — ' =: 6 (inches) tube length. 



