THE TWIN-OBJECTIVE BINOCULAR 45 



8. The instrument should usually be inclinable for comfortable 

 work; but, if not, it may be fastened on an inclined wooden base. 



9. Differences of focus in the two eyes are best cancelled by 

 wearing correcting glasses; differences in the two tubes can be 

 corrected by drawing out one eyepiece slightly. The special 

 correcting adjustment seems perhaps unnecessary, and is 

 neglected by many users. 



10. Often, correcting glasses for distance vision, in caps fitted 

 over the eyepieces, are required with the Greenough binocular 

 (Zeiss, Leitz). For, with converging tubes, the distance between 

 the centers of the eyelenses of the instrument may be 5 milli- 

 meters or so less than the interpupillary distance for distant 

 vision, because of the convergence of the eyes. 



11. Reading spectacles will not usually suit the Greenough 

 binocular (especially the lower powers), because the instrument is 

 often made for the focus of distant vision; and with a shorter 

 focus the fields will not coincide. 



