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A FuRTHEu Note on " Optical Tube Lexgth." 



By a. Ashe. 



(Bead May ]9th, 1S93.J 



In the last number of our Journal there is a brief account of 

 a new method of estimating the optical tube length of a micro- 

 scope. 



You may recollect that the method described is based upon 

 the increase in power obtained by withdrawing the eye-piece 

 through some known distance. 



It is, however, quite possible to dispense with the measure- 

 ment of the power in two positions, and thus to simplify the 

 determination, as well as to economize time, which to many 

 persons is a matter of considerable practical importance. The 

 following details will perhaps render the matter clear : — 



Instead of measuring the power of the microscope twice over 

 it is sufficient to place a micrometer or other divided scale on 

 the stage and count the number of lines that fill the field of 

 view from side to side, then to pull out the draw-tube some 

 inches and repeat the counting. 



Of course the greater the increase in power the fewer will be 

 the number of lines seen. In other words, the number of lines 

 and the magnifying power are in inverse proportion to each 

 other. 



Now, for the purpose in view, it does not matter one iota what 

 the actual powers of the instrument may be, with its draw- tube 

 in various positions, so long as we know the proportion those 

 powers bear to each other, and this proportion we shall find in 

 the relative number of lines which fill the field of view at the 

 same points. 



Hence (bearing in mind the inversion of the ratio) we may 

 look upon the number of lines counted as though they were the 

 actual powers of the microscope, and proceed at once to apply 

 the formula mentioned in the other paper, thus saving a deal 



