; THE SCIENCE OF MICROSCOPY. 32f 



objective and ocular lenses. We have here, namely, to do with the 

 media air, water, oil, &c., traversed by the pencils of light entering 

 and leaving the object. The enormous improvement of the 

 microscope image thus successively gained for ""practical" micro- 

 scopy, is the sole and undivided achievement of scientific theory, 

 which thus justifies its claim by effecting results which it has 

 predicted, and towards which the microscopist, who sees onlv 

 '' a tool '' in his instrument, has contributed nothing. But the path 

 thus opened for farther research is conditional on his working in 

 harmony with theoretical principles which assert their sway in 

 practice. For, again, theory in showing the true relation between 

 the physical constitution of an object and the optical characteristics 

 of its image, has completed what was long wanting in the explan. 

 ation of angular aperture. The importance of this exposition of the 

 function of angular aperture, hitherto overlooked by all '' practical " 

 microscopists and opticians, has already been proved by the 

 astonishing performance of immersion lenses, constructed for 

 admission of the widest possible illumination pencils on the 

 principle of '^homogeneous immersion." 



And recurring for a moment to the subject of illumination, it is 

 simply impossible to name any point of practical microscopy in 

 which theoretical guidance can be more conspicuously useful, than 

 in the study of various effects of illumination depending upon the 

 management of various apparatus, the selection of central or oblique 

 light, the use of wide or narrow illuminating pencils, by combination 

 of diaphragm with mirrors, condensers, &c. No practical micros- 

 copy can be trusted which does not fulfil the conditions Imposed 

 by theory in applying the several modes of illumination, or which 

 does not avail itself of every theoretical indication of the means by 

 which the best possible performance of an objective may be ensured, 

 and the proper intensity of light to be thrown on each object 

 regulated. 



Hence it is not enough that a theory of the microscope should 

 be accepted in the abstract, yet be neglected in practice where the 



