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Note on Fluorite in Apochromatic Objectives. 

 By E. M. ISTelson, F.R.M.S. 



As fluorite is becoming scarce, an important question arises 

 as to whether fluorite is or is not present in any given lens. 

 This can readily be determined by means of a polariscope. 



The ISTicols are crossed, a two-inch objective is placed on 

 the nose-piece, and a low eye-piece employed. The various 

 portions of the objective to be tested are unscrewed, and each 

 combination is separately placed on a glass slip on the stage 

 and examined in the dark polarized field. If the combination 

 contains fluorite, there will be a luminous white silky appear- 

 ance, but if there is no fluorite, then the field will remain dark. 



The following are examples : — The Zeiss, 24 m.m. apochro- 

 matic, has three elements, of which the middle contains fluorite. 

 The apochromatic 12 m.m. has four elements, of which the 

 second and back contain fluorite. The apochromatic 6 m.m. 

 has three elements, of which the middle and back contain 

 fluorite. The apochromatic 3 m.m. has five elenients, and the 

 last but one contains fluorite. ^ 



But with i-egard to this last example it should be noted that 

 all 3 and 2 m.m. objectives are not alike. 



