THE EYEPIECE 149 



3. For all apocliroinatic objectives, use coiiipen.saiiiifj; 

 eyepieces (Zeiss, Bausch, Sp(uicer, Watson), or holoscopic 

 eyepieces (Watson), or periplane eyepieces (Leitz). 



4. For optimum photography use either projection 

 eyepieces (Zeiss) or homals (Zeiss). 



Since the writer regularly uses no eyepiece lower than 

 12.5 times, he finds the compensating oculars of 12.5, 15, 

 and 20, with high eyepoints, best; and to avoid time spent 

 in changing, he employs them throughout, with yellow- 

 green glass, even for low and medium achromatic objectives. 



Changing Eyepieces. — Eyepieces are probably in some 

 cases changed too frequently. W^ith three or four objec- 

 tives on the nosepiece, there should be little necessity for 

 frequent changing of eyepieces; if a large cone of light is 

 used, and eyepieces not too much below the maximum 

 useful magnification are selected. If the 10 (or 20) apochro- 

 matic objective is procured, the excellent compensating 

 eyepieces, with high eyepoints, can be used with all the 

 higher objectives, whether achromatic, fluorite, or apochro- 

 matic. Where eyepieces are ''sprung in," as in Zeiss's 

 excellent binocular, an occasional wiping of the bearing sur- 

 faces with alcohol-moistened filter paper, or the application 

 of vaseline, will make their withdrawal easy. The right- 

 hand eyepiece may require to be pulled out, to examine 

 the back of the objective, at each change of objectives. 



Magnification. — The standard magnification of the eye- 

 piece is now reckoned as the magnification given by the 

 eyepiece to the image which is, or would be, formed at 

 180 millimeters (or less) from the objective. The virtual 

 image is supposed to be located at 250 millimeters from 

 the eye. The old method of reckoning gave the magni- 

 fication of the objective as 250 millimeters, divided by the 

 focal length of the objective in millimeters; instead of about 

 180 millimeters, divided by the focal length, as at present. 

 On the other hand, the old eyepiece magnification was 

 180 millimeters (or so) divided by the focal length of the 

 eyepiece; instead of 250 millimeters divided by the focal 

 length, as at present. Since the focal length of the achro- 



