330 THE SCIENCE OF MICROSCOPY. 



a reductio ad ahsurdum to deny a scientific microscopy, or to make 

 it equivalent with mere " expertness in securing the most perfect 

 performance of aji oljective, * seeing that this expertness is better 

 applied in the right interpretation of that performance. 



Of the causes of erroneous observation enumerated in the 

 preceding pages, great stress is usually laid upon the imperfection 

 of the instrument or unskilful use of it (technical manipulation). 

 Yet errors far more injurious to microscopy as a science of 

 observation arise from the other causes mentioned, those, namely, 

 affecting the individual qualification of the observer. 



When the same object is differently described by different 

 persons using different microscopes, the error of fact, which must 

 exist somewhere, may be attributed to (i) faulty lenses, (2) faulty 

 illumination, (3) faulty observation. 



I. Great and irreconcileable differences such as we find in 

 earlier micrography, can only be accounted for by bad lenses. 

 But with our present instruments there is but slight inequality of 

 performance, comparison being made with lenses possessing equal 

 magnifying power and angular aperture. Since, however, objects 

 (natural or prepared) differ widely in amount and complexity of 

 detail, the selection of the objective icr the kind of work required 

 of it, is of first importance as a saftguard against error of 

 observation. And here the question for the working microscopist 

 is not between good or bad lenses, but of their suitability to the 

 object under examination. In considering the grounds of 

 selection the necessity of a scientific determination of the function 

 of angular aperture becomes apparent. Shall then the guidance of 

 theory be accepted ? The microscopist has need of it in his daily 

 practice. He may not often require to exercise his judgment in 

 testing and choosing his apparatus ; but his work must be carried 

 out in habitual subjection to principle and method in applying his 

 apparatus according to special requirements. 



* Vide, Vol. I. page 365, Joxirnal of tlie K.M.S. London— '• Kote on an 

 article puWisbed in American Quarterly Microscopical Journal." 



