314 THE SCIENCE OF MICROSCOPY. 



in which the specific effects of large angular aperture, and the 

 influence of diffraction, (within and outside of the microscope,) 

 have been fully explained. And as this theory is still too little 

 known to be generally accepted, its influence upon current opinion 

 and practice hardly comes into present estimation. Meanwhile 

 explanations of the ''*' powers " of the microscope (objective) have 

 not been wanting. But coming ex post facto, they have had no 

 significance for the optician : while, on account of their incom- 

 petence to explain the true function of the objective, as well as 

 from their being per se unproven propositions, they tended rather 

 to mislead than enlighten the microscopist, whose work lay in the 

 study of a variety of complex structures rather than a few prescribed 

 test performances. And thus it has happened that practical micro- 

 scopy, feeling much interest in verifying the powers of the 

 microscope, yet taking little heed of theoretical considerations, 

 gradually receded from the stand point of the theorist. But in 

 rejecting the guidance of optical law in his interpretation of purely 

 optical effects, the microscopist deprived himself of the surest 

 means of estimating correctly the phenomena presented in the 

 microscope image, and also of controlling his own manipulation 

 of the instrument, the object, and the methods of illuminating it. 

 It must be admitted therefore that a science (as distinguished from 

 an art and practice) of microscopy, has neither been explicitly 

 propounded in our micrographic literature, nor adequately developed 

 in the ordinary course of daily work. 



Every one indeed expects and insists that the performance of the 

 microscope shall be extended until it reaches the extreme limit 

 of what is attainable. And further, that this performance shall 

 rest upon the attestation of optical science rather than upon hap- 

 hazard empiricism. Everybody, moreover, agrees that the require- 

 ments of practical microscopy can only be satisfied by such further 

 addition to our visual power as is thus vaguely assumed to be within 

 reach, though no precise intimation is given of the particular 

 direction in which this aid is to be extended, — whether, for instance. 



