450 ON APEETTIRE AND DEFINITION OF MICEOSCOPE OBJECT GLASS 



different rays of the spectrum, as pointed out already in connection 

 with Mr. Sorby's table. It serves, moreover, to shew that the 

 focussing function by which the photographic picture is delineated 

 must be the really important instrument of definition, for the 

 amplification of the microscope-image by chemical rays acting 

 on a photographic plate is greater (as 3 : 2) than when seen by the 

 eye*. 



But the total inadequacy of the dictum that '' defining power 

 varies with the chord of aperture " should appear from the very 

 obvious fact that the definition of a modern camera lens or telescope 

 object-glass is more perfect with less angular aperture than belongs 

 to microscope objectives of even low power. Or, if this dictum be 

 intended to apply only to the microscope image, it remains to 

 be shewn how it may consist with the fact that the objective of say 

 120° of aperture, made fifteen years ago, does not equal in per- 

 formance the objective of same aperture made now, or with the fact 

 that an immersion lens of smaller angle sui-passes the definition of a 

 dry lens of greater aperture ; or again, with the fact that lenses 

 having the same angular aperture, but constructed by different 

 makers on different lines — nay, even when made by the same 

 optician, and on the same principles — vary in ''defining" power 

 according to the illumination and to the kind of object. 



Does not such a dictum (it were not worth while to disprove it 

 but for the prevalence of an error which might be traditionally 

 perpetuated amongst other myths appertaining to the history of 

 the microscope) ignore too much, and take too much for granted ? 

 Does it not ignore the defects which excessive aperture introduces ? 

 Does it not assume that difficulties of the optician are all solved 

 for him by angular aperture ? Has it not driven the optician 

 himself against his better knowledge and experience into a struggle 

 for a few more degrees of aperture, and filled the purchaser with a 

 delusion that an objective is cheap at any price which has these few 

 additional degrees of aperture ? 



* See page 245 of this vol., Abbe on the Theory of the[_]VIicroscope. 



