78 PHOTOMICROGRAPHY 



made to counteract cover-glass thickness, will introduce 

 spherical errors. With low powers the error intro- 

 duced in this way is insignificant, and the tube length 

 may be altered to obtain a final adjustment of magni- 

 fication to plate size, etc., but the method should only 

 be resorted to when no other is available. This factor 

 is consequently not often used, unless the tube length is 

 altered to correct for cover-glass thickness. 



Initial Power of the Objective and Power of the Eye- 

 piece. When making up a battery of lenses it is necessary 

 to know their powers. The magnification an objective 

 will produce in a photomicrograph at 10" image distance 

 can always be roughly determined by dividing the image 



10 250 



distance by its focal length P ~. ; = or 



f . in inches f. in mm. 



And most makers follow Abbe's suggestion, and call this 

 its "initial magnification." But at least one, Leitz, 

 uses the term " initial magnification " in the sense 



A 



defined by 7 r~i re ' 



J focal length 



Whichever method is adopted, the table of magnifica- 

 tions for objectives and eyepieces given in the maker's 

 catalogue will give the magnifications produced in an 

 image at 10" from the eyepiece ; but in order that this 

 may be so, the power of the eyepieces must be expressed 

 in accordance by 



A 



focal length of eyepiece, 

 or if A occurs in objective factor by 



250 

 focal length of eyepiece. 



Consequently the values given in the catalogues 

 for the powers of the eyepieces are of no use unless 

 used with that maker's objectives, or with those of 

 another maker who expresses the initial magnifying 



