USB OF THE MICROSCOPE. 53 



The refractive power of the object, or of the medium in 

 which it lies, is sometimes a source of error. Thus a 

 human hair was long thought to be tubular, because of 

 the convergence of the rays of light on its cylindrical con- 

 vexity. A glass cylinder in balsam appears like a flat 

 band, because of the nearly equal refractive powers of 

 object and medium. The lacunae and canaliculse of bone 

 were long considered solid, because of the dark appear- 

 ance presented on account of the divergence of the rays 

 passing through them. Their penetration with Canada 

 balsam, however, proves them to be cavities. Air-bubbles, 

 from refraction, present dark rings, and, if present in a 

 specimen, seldom fail to attract the first attention of an 

 inexperienced observer. The difference between oil-globules 

 in water and water in oil, or air-bubbles, should be early 

 learned, as in some organized structures oil-particles and 

 vacuoles (or void spaces) are often interspersed. A globule 

 of oil in water becomes darker as the object-glass^ is de- 

 pressed, and lighter when raised ; while the reverse is the 

 case with water in oil, since the difference of refraction 

 causes the oil particles to act as convex lenses, and those 

 of water like concave lenses. 



Other errors arise from the phenomena of motion visible 

 under the microscope. A dry filament of cotton, or other 

 fabric absorbing moisture, will often oscillate and twist 

 in a curious way. 



If alcohol and water are mixed^ the particles suspended 

 acquire a rapid motion from the currents set up, which 

 continues till the fluids are thoroughly blended. Nearly 

 all substances in a state of minute division exhibit, when 

 suspended in fluid, a movement called the " Brownonian 

 motion," from Dr. Robert Brown, who first investigated 

 it. It is a peculiar, uninterrupted, dancing movement, the 

 cause of which is still unexplained. These movements, 

 as all others, appear more energetic w T hen greatly magni- 

 fied by strong objectives. It requires care to discriminate 



