260 ON THE THEORY OF THE MICROSCOPE. 



to be solved, namely, tlie final re-union, at the eye-point above 

 the ocular, of all these rays in a focal plane, which exhibits the 

 outspread picture of the object from which they came. Or, 

 remembering that light is motion, we might say that focal plane 

 in which are assembled, in equal phase of motion, the whole 

 sum of undulations which started from the object beneath the 

 microscope. 



All that takes place through the combined function of objective 

 and ocular, i.e.^ the microscope as a whole, is the presentment to 

 the eye, in amplified delineation, of an image of that which is 

 actually or partially existing in the prepared object. The particu- 

 lar calculations for, and corrections of, the lenses of the objective- 

 system concern only those causes of error which are inherent in 

 its own construction, and do not apply in correction of any faulty 

 illumination of the object, i.e., do not counteract any excess or 

 deficiency or misdirection of the illuminating rays. Of course, 

 an objective which is most perfectly corrected, and takes in the 

 large?t possible number of illuminated paits of the object without 

 loss of defining power, " performs " better than one less skilfully 

 constructed, but it adds nothing to the actual detail m the object, 

 as also, if a really good lens, it takes nothing away. Its best per- 

 formance depends on suitable illumination ; on the other hand, 

 it may fail to shew what actually exists in an object, not because 

 it is an ill-constructed objective, but simply because of 

 inefficient or improper illumination, or because the test is 

 disproportionately severe. 



A theory of the microscope is, therefore, not complete until 

 the principles are clearly laid down by which the intensity 

 of illumination of the magnified image may be regulated 

 and controlled, and by which, moreover, the mode of action of 

 illuminating apparatus of all kinds may be analysed and estimated 

 at their true worth- Professor Abbe has briefly enunciated these 

 principles in Hhe essay from which the foregoing pages are 

 translated. But for reasons already given this statement has been 

 omitted in the present communication. It had been my inten- 



