106 



that it is most easily noticeable. It is a desirable 

 quality and indicates to a considerable degree the 

 quality of objectives. It is impossible to obtain 

 absolute flatness of field, in objectives of sufficient 

 angular aperture to meet the requirements of the 

 present day. 



Working Distance. This term, strictly con- 

 sidered, is an invariable quality of the objective 

 and is the distance between the front lens in the 

 objective and the object, when the objective is in 

 focus and is corrected for that object. All objec- 

 tives require a certain amount of projecting metal 

 to protect the front lens and this, with a certain 

 thickness of cover glass, lessens the working dis- 

 tance. In objectives with fixed mountings this 

 may be 2 and with thick cover glasses is, consider- 

 able. It is comparatively unimportant however, 

 for the working microscopist to know the working 

 distance .per se of his objectives, but of considerable 

 moment to know what is the actual space between 

 the objective and cover glass. 



In objectives of low and medium power, it is of 

 little consideration ; but where it must be ex- 

 pressed in y^-Q or yo^-Q mcn > it becomes a matter of 

 importance. 



Working distance is spoken of as being long or 

 -short and varies with the power and angular aper- 

 ture. Generally the working distance decreases 

 with the increase in numerical aperture and be- 



