334 THE SCIENCE OF MICROSCOPY, 



instrument can only be gathered by following the shadowed or 

 illuminated details with successive alterations of focus -, while the 

 details spreading out in length and breadth are followed by stage 

 or hand movements of the slide. But even then we only see one 

 aspect of each detail, except when the particles under observation 

 can turn or roll over, as when floating in fluid. When we try to 

 gain impressions of form — for example, of solid or semi-solid 

 elements (organic or inorganic) possessing different degrees of 

 transparency, molecular aggregation, refracting density -, or again, 

 structural outlines of a regular kind, as e.f/., the solid, hollow, or 

 flattened sphere, cylinder, spiral, 8cC. ; or again, lens-shaped 

 particles with concave, convex, or compound surfaces, whose 

 substance and transparency must vary with their thickness ; or 

 again, membranes which have parallel bounding surfaces or present 

 irregularities of surface, or smooth or waved face, or in whose 

 substance particles of unequal density or irregular shape are 

 imbedded, or which are laminated though structureless, or which 

 lie in contact with other membranes, and so forth j — in all these 

 and a thousand other circumstances the simple knowledge of 

 perspective effect does not suftice for the interpretation of the 

 characteristics of the optical images by which the aggregate of 

 structural and physical peculiarities is represented. And when we 

 add to these difficulties the observation of effects produced by 

 osmosis, by numerous re-agents, by heat or cold, or by growing 

 and moving particles of living matter, it must surely be admitted 

 that these various micro-physical studies render practical micros- 

 copy one of the most diflicult of the sciences of observation.* 

 But the practical microscopist who in his greatest strait is 

 singularly impatient of theoretical help (except where it seems to 



* Every histologist who bears in mind the voluminous literature of the 

 cell theory, and of such long-vexed questions as the existence of cell 

 membrane in connective tissue, blood, cartilage, bone, muscle, <S:c., -will 

 admit the obscurity of structural form and relation ; even where to the 

 partisans of each opposed doctrine the evidence of the microscope image 

 appears to them decisive in favor of their side of the question. 



