78 JOURNAL OF THE [April, 



visual appearances and make theories to account for them in 

 accordance with facts already learned. We try to vary the con- 

 ditions as much as we can ; we change our objectives ; we try 

 central light and oblique light ; we examine one specimen dry 

 and another in a dense medium ; one by transmitted and another 

 by reflected light ; but when we approach the limit ci minuteness 

 of object or detail which our instruments will define, we are in 

 the same situation as when using our natural eyes across a chasm, 

 neither better nor worse ; we have to account for what we see by 

 a reasonable hypothesis which will make it take an intelligible 

 place amongst natural objects. 



Our skill as microscopists, apart from the technical dexterity 

 in the use of our tools, consists largely in devising varied experi- 

 ments and changes of condition, so as to enlarge the body of 

 evidence from which we draw our inductive conclusions. To 

 assist ourselves in this, we also catalogue such facts and methods, 

 and such cautions and warnings, as our experience (or that of oth- 

 ers) has taught us. Let us look for a moment at some examples. 



We know very well that we are liable to illusions of sight, so 

 natural and so powerful that even the intellectual certainty that 

 they are illusions will not destroy them. If we are looking 

 through the Abbe binocular eyepiece, using the caps with semi- 

 circular openings, we see a hemispherical object as if it were a 

 hollow bowl, and, visually, it refuses to be anything else. But 

 this is not peculiar to microscopical vision, for we do an ana- 

 logous thing with the stereoscope, and by wrongly placing the 

 pictures may make an equally startling pscudo-perspective. 



We find that what we call transparent bodies are full of Tines 

 as dark as if made with opaque paint, and throw far-reaching 

 shadows. But I see similar ones in the cubical glass paper- 

 weight on the table before me, and know that by the laws of 

 refraction the surface of a transparent body is always dark when 

 its angle to the eye is such as to cause total reflection of the light 

 in the opposite direction. By the same law we know that if the 

 angle of total reflection in the same transparent cube were dif- 

 ferently placed with regard to the eye, the now dark surface 

 would become a mirror, reflecting the sky and distant objects as 

 brilliantly as if silvered. Our diatom-shells give us constant 

 experience in these phenomena. A prismatically fractured edge 



