ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



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in shadow, while the other half is illuminated as strongly or even 

 stronger than the field B, figs. 28 and 29. 



If depressions either with spherical surfaces or furrowed or bowl- 

 shaped (fig. 30) are found on the surface of a membrane, they produce 

 the same effect as concave lenses, and show their greatest brightness 

 when the tube is lowered. If, however, there are spherical, hemi- 

 spherical, or semi-cylindrical elevations, they act as convex lenses and 



Fig. 30. 



Fig. 31. 



L 



Serai-cylindrical elevations or depressions. Cylindrical elevations and depressions. 



show their greatest brilliancy when the tube is raised from a medium 

 focus. If furrow-like depressions alternate with semi-cylindrical eleva- 

 tions, the surface presenting a wavy appearance, the former appear bright 

 when the tube is lowered, the latter when it is raised, and when the 

 former show the highest degree of brilliancy the latter has a dull 

 appearance (fig. 31). 



With wave-like membranes the result is somewhat different, since 

 here both of the undulations, as well those which have their convex side 

 towards the observer, as those with the concave side so turned, act as 

 concave lenses. They therefore show their greatest brightness on 

 lowering the objective, and the same differences in the extent of the 

 lowering as in the case before mentioned of the semi-cylindrical tubes. 



From what has been said of glass threads and hollow cylinders filled 

 with fluid, it follows that more and less strongly refracting (i. e. dense 

 and less dense) parts of one and the same object, will act similarly to 

 the cylindrical elevations or depressions of a membrane. In observing, 

 therefore, in water the differences thus presented in the microscopical 

 image, it is necessary, in order to decide whether these depressions or 

 elevations are caused by variations in structure or in density, to change 

 the fluids, and particularly to use such substances as possess a greater 

 refractive power than the object under examination, whereby the image is 

 either (in the first case) changed according to the altered conditions, or 

 (in the latter case) is substantially unchanged. If the greatest brilliancy 

 appears when the tube is lowered, we have to do with an elevation, but if 

 when the tube is raised, it must be a depression. In order to facilitate 

 the determination of the position of the tube, we can either start with 

 a medium fucus, or the tube may be lowered from a point at which no 

 distinct image of the object is obtained. Depressions are then first 

 bright on a dark ground, elevations, on the contrary, dark on a bright 

 ground, till after further lowering of the tube the image is exactly 



