THE SCIENCE OF MICROSCOPY. 331 



II. Faulty illumination is undoubtedly a constant source of 

 error of observation. But imperfect as the general practice may 

 be at present, it is certain that the progress of microscopic research 

 cannot thus be permanently retarded. For a more correct 

 appreciation of the function of angular aperture necessitates the 

 reconsideration of appropriate methods of illumining each object, 

 and concurrently with this, a general revision of the theory as well 

 as practice of illumination. Sooner or later, therefore, discrep- 

 ancies of result, now traceable to want of fixed rule and method in 

 lighting the object, will disappear, as a more uniform practice is 

 observed. 



III. There remain yet to be considered, difficulties of various 

 kinds leading to error of observation which cannot be placed to the 

 score of bad lenses, or misuse of them, or even faulty illumination ; 

 difficulties which are permanent in so far as they seem inherent in 

 the optical conditions of the microscope image, or in the different 

 endowment and education of individual visual power. The same 

 difficulties, and the same struggle to comprehend and master them, 

 must be met by every one who makes any serious use of the 

 microscope, so long as our instruments and our visual power 

 remain as we now possess and use them. We cannot ignore this 

 fact and it is therefore well if we can also recognise the immense 

 advantage of being able to correct our impressions and guide them 

 by a reasoning experience that calls to its aid the various teaching 

 and interpreting power of optical and physical law. It is beyond 

 the scope of this article (which has already outgrown its intended 

 limits) to enter into any of the numerous questions of micro- 

 physics or to discuss any of the technical studies by which the 

 path of the student maybe smoothed. But we may in conclusion 

 cast a flying glance at some of the peculiarities of the microscope 

 image, and some of the difficulties met with in interpreting it, in 

 order, while noting the various influences which affect the 

 observer's judgment, to arrive at a nearer conception of the 

 subjects and objects of that science of microscopy for which we 

 contend. 



