77 



proceeding as carefully as at first, 

 point is again reached, look for the number and see 

 whether it agrees with the first ; very likely it does 

 not, which is owing to a want in the faculty of per- 

 ception, due to a too slight acquaintance with the 

 phenomena. These trials should be repeated until 

 the proper sensitiveness of feeling in making the 

 adjustments is acquired, and until they can be made 

 to correspond with a certainty to at least within two 

 divisions. 



Remove the eye-piece and attach one of higher 

 power. It must how, however, be remembered that 

 if there is a considerable difference in the powers 

 there will be a relative difference in their lengths, and 

 that this will cause a difference in the optical length 

 of the tube ; this not only will require another adjust- 

 ment for focus, but will partially destroy the correc- 

 tion as made with the low power. After some prac- 

 tice, the amount of variation may be fixed upon and 

 may be noted for the future ; but, to determine it, the 

 same plan as suggested with the low power eye- piece 

 should be followed. 



When it is fo^nd after repeated trials that sufficient 

 skill has been acquired to bring the collar to within 

 one division, the number and power of the eye-piece 

 should be scratched with a diamond upon the slide or 

 with pen and ink upon the label ; thus, if it is found 

 that with a 1J inch eye-piece the index shows 5, and 

 with a I inch eye-piece shows 5, it should be marked 

 U-5 and |-54. In eye-pieces of par-focal construe- 



