THE SCIENCE OF MICROSCOPY. <^^^ 



whether of a direct or diffraction image (which need not here be 

 further particularised), give the force of law to the statement that 

 the magnifying power used should not exceed the necessary 

 amplification in each case, but must be exactly proportioned to the 

 requirement of the object, i.e., the nature of the constituent detail, 

 and the illumination to which it can be subjected. If this be a 

 general truth it is certainly a condemnation of general practice. 



On the other hand insufficient magnifying power can only be 

 considered a fault in so far as the difficulty of seeing details too 

 minute to be separated by it increases with the strain put upon the 

 objective by faulty illumination, deep eye-pieces, &c. AVhen used 

 by the microscopist to feel his way up to the higher resolution of 

 the object it is most serviceable ; and where its power fails, it 

 leaves correct indication for employment of greater amplification. 

 The ideal of a perfect arrangement of the whole optical apparatus 

 whether regarded from its scientific or practical point of view, 

 is the establishment of an exact relation between maximum 

 brightness of image, minimum necessary amplification, and sufficient 

 separating power for distinction of derail. And the regulation of 

 these points involves the application of scientific principle as well 

 as expertness in manipulation. 



In the next place, assuming the microscope arrangement to be 

 suitable, and the image to be as perfectly delineated as may be, 

 the observer has still before him a problem to solve rather than 

 a picture to read off. For the structure which occupies three 

 dimensions in the object is represented by effects of light and 

 shadow thrown on an optical plane, and the observer is forced to 

 compose a mental picture by putting together a number of images 

 corresponding with so many optical sections of the object. How- 

 ever thin the object may be, its amplification in the direction of its 

 thickness is a necessary consequence of the employment of magni- 

 fying power, without which indeed the separate distinctness of 

 parts necessary to be distinguished by the eye would not be 

 obtained. The configuration of parts in the line of the axis of the 



