312 THE SCIENCE OF MICROSCOPY. 



it is only serviceable to those whose sight is inferior, or who have 

 no training. 



Looking back now to the distinction above indicated between 

 the instrument and its performance (which may be called the 

 science of the microscope), and the nature of the observation made 

 with it, (which may be called a science of microscopy), it is obvious 

 that while each continues to progress, it must reciprocally influence 

 the other, as well as the whole cycle of studies to which the 

 microscope ''is an invaluable adjunct." Thus every step gained in 

 power and accuracy of the microscope is followed by fresh 

 discoveries in natural science, while, conversely, this extension of 

 knowledge re-acts correctively upon received theory of the micro- 

 scope and accepted interpretation of the objective image. Again, 

 extended applications of the microscope lead to new and unexpected 

 results, and to problems yet unattempted, perhaps only to be solved 

 by fresh appeal to physical or physiological science. Throughout 

 all this action and re-action it is obvious that practical microscopy 

 makes corresponding advances with the improvement of the micro- 

 scope ; and until some ultima thule of discovery be reached, the 

 science of the instrument and the scientific use of it must go hand 

 in hand, mutually strengthening each other by theoretical reasonings 

 in advance of each position gained. 



Now it may be questioned whether practical microscopy can 

 afford to leave all theoretical reasoning to the optician, holding 

 itself, meanwhile, absolved from such interpretation of its work as 

 is demanded by optical law: for the studies of the optician cannot 

 stand in closer relation to the uses to which the microscope is 

 applied, than does the work of the practical microscopist to the 

 fallacies of observation against which he has to guard himself. Nor 

 is the practical optician the necessary representative (that is, solely 

 in virtue of his calling) of optical and physiological science applied 

 to the theory of the microscope. His first care is the construction 

 of the instrument upon sound technical principles : but it must, in 

 justice, be acknowledged that, in times of defection of theoretical 



