306 THE SCIENCE OF MICROSCOPY. 



expect to — succeed in satisfying himself and others that his results 

 are the best which can be obtained. If he does not care to be 

 Well informed on the more obstruse points of the theory of the 

 microscope, he must at least study that part of the optical effect 

 depending upon the illumination, which is left in his own hands 

 to deal with as he best may. This he will scarcely conduct 

 successfully by following directions, the rationale of which he does 

 not understand. And for the same reason he will not find that he 

 can always repeat an accidentally successful manoeuvre. Thus 

 although the instrument, on whose perfection the optician has 

 spent such great labor and thought, cannot be fairly discredited by 

 any mismanagement, its best effects may be too often lost by faulty 

 illumination of the objects examined. But in his interpretation of the 

 results obtained, the microscopist is responsible to science for every 

 misstatement into which he is led by error of judgment, or by 

 optical effects which he may himself have unwittingly falsified. 

 How then is he to overcome this difficulty when the precise effect 

 required is not known before-hand, (as in examining fresh objects) 

 unless his management of the illuminating apparatus be guided by 

 a thorough appreciation of the characteristics of the microscope 

 image, and by a due insight into their optical causes, including the 

 effects resulting from the action of the object upon pencils of light 

 thrown upon it. This last point is, perhaps, of more direct 

 importance to the practical microscopist than any other, for the 

 reaction of the constituent particles of the object upon the pencils 

 of light by which it is illuminated, is the real initiation of the image- 

 forming process which is thus brought within the control of the 

 manipulator. And since the illumination cannot take care of itself 

 but must be intelligently directed, the knowledge and skill of the 

 microscopist who understands what depends on his management 

 and how he may effect the end he has in view, constitutes, so to 

 speak, part of the performance of the microscope. Even the 

 amateur, who though he may not care to devote himself to any 

 earnest problem in microscopy, yet looks a step or two beyond the 



