THE SCIENCE OF MICROSCOPY. 325 



It is not always safe then to accept certain appearances as a 

 definitive resolution of structure, and because they square with a 

 preconceived interpretation persistently look for and "bring them 

 out " by devices of illumination. The constant attempt to prove 

 *' structure '' by producing some known optical effect supposed to 

 be characteristic of it, tends to perpetuate error in practical 

 microscopy which optical science is not always present to rectify. 



Everyone knows that an object often exhibits different appear- 

 ances with a trifling turn of the mirror. But a change of phase 

 thus produced cannot mean change of structure. The image is 

 simply varied according as the particles in an object transmit, or dis- 

 perse the illuminating pencils thrown upon it when their direction of 

 incidence is changed. Particles become illuminated which were 

 not so before, or the pencil coming from the object may not enter 

 the lens. Yet in an optical sense each new phase produced by a 

 shift of light and shadow is an equally truthful expression of the 

 objective image, though neither may be more indicative of 

 " structure " than the other. If it be asked which of the various 

 effects that may turn up is to be taken as representing the actual 

 structure of an object, (or when the image is conformable with 

 the object) the only answer that can be given is that everij effect 

 helps in forming an opinion, but that the interpretation of each 

 image hangs upon some deduction from its optical characteristics 

 which shall indicate the physical constitution of the object. If for 

 instance, one image exhibit a geometrical delineation of the 

 principal outlines of the object, it remains to try if another image 

 obtained with different illumination or objective of wider angular 

 aperture will yield additional details. Or if an object examined 

 with a low power give indications of structure which require the 

 application of higher power to bring into view, then a series of 

 trials must be made with increasing amplification and suitable 

 regulation of the illuminating pencils, &c., &c. 



Thus in trying to learn the structure of an object by inference 

 from its microscope image, we cannot depend upon intuition of 



