power can not be immersion. The purpose of immer- 

 sion is to obtain higher optical results ; is in fact a 

 necessary condition, and an objective which is con- 

 structed as a dry one cannot be used as an immersion, 

 and vice versa. Although there have been objectives 

 constructed which can be used both as immersion and 

 dry, they have gone into disuse as they must suffer 

 when used in one or the other direction or in both. 



Water was for many years used as immersion fluid, 

 but specially prepared cedar oil was found to give 

 better results and has almost entirely taken its place. 

 The refractive and dispersive properties of cedar oil 

 are almost identical with those of crown glass, and as 

 cover glass has very nearly the same properties as 

 crown glass, the term homogeneous immersion fluid is 

 often applied to the oil, but for brevity, objectives 

 which are constructed to be used with it are called 

 oil immersion. 



Tube Length. Objectives are constructed and 

 their aberrations corrected for the length of tube with 

 which they are to be used, and as has been shown in 

 a previous chapter there are now two generally accepted 

 standards. They are corrected for either the long or 

 short tube and specially marked by progressive firms, 

 and it is hoped that in time this will become a universal 

 custom. When an objective is not marked the 

 purchaser should require to know the tube length with 

 which it is to be used. 



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