78 



identical in refraction and dispersion with those of 

 crown glass and it is for this reason often termed 

 homogeneous immersion fluid, but for brevity ob- 

 jectives 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 will 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 pro- 

 gressive firms and it is hoped that this will become 

 a universal custom in time. When an objective is 

 not marked the purchaser should require to know 

 the tube length with which it is to be used. 



Nomenclature, or Rating of Objectives.- 



While for many years objectives, or rather the 

 mountings, were marked arbitrarily by the makers 

 and differently by each maker, it is now customary 

 to mark the objectives so that the figures shall 

 indicate the true optical value ; on the continent 

 of Europe in millimeters and in England and this 

 country in inches. The objectives are rated accord- 

 ing to a single lens, which the combined value or 

 equivalent focus of the lenses contained in the 

 objective, shall equal. This is also true of eye- 

 pieces. So, if two combinations equal in magnify- 

 ing power a single lens of 1 inch focus they are 



