168 THE MICROSCOPE. 



structure of human hair. "When we view this object 

 by transmitted light, it presents the appearance of a 

 flattened band with a darkish centre ; this, however, 

 is entirely due to the convergence of the rays of light 

 produced by the convexity of the surface of the hair. 

 That it is a solid structure is proved by making a 

 transverse section of the hair-shaft, when it is seen 

 quite filled by medullary substance, with the centre 

 somewhat darker than the other part. It is, in fact, a 

 spiral outgrowth of epithelial scales, overlapping each 

 other like tiles on a house-top, which impart a striated 

 appearance to the surface. A cylindrical thread of 

 glass in balsam appears as a flattened, band-like streak, 

 of little brilliancy. Another instance of fallacy arising 

 from diversity in the refractive power of the internal 

 parts of an object, is furnished by the mistakes for- 

 merly made with regard to the true character of the 

 lacunae and canaliculi of bone structure, which were 

 long supposed to be solid corpuscles, with radiating 

 opaque filaments proceeding from a dense centre ; on 

 the contrary, they are minute chambers, with diverging 

 passages, excavations in the solid osseous substance. 

 That such is the case, is shown by the effects of Canada 

 balsam, which infiltrates the osseous substance. 



The molecular movements of finely divided particles, 

 seen in nearly all cases when certain objects are first 

 suspended in water, or other fluids, is another source of 

 embarrassment to beginners. If a minute portion of 

 indigo or carmine be rubbed up with a little water, 

 and a drop placed on a glass slide under the micro- 

 scope, it will at once exhibit a peculiar perpetual motion 

 appearance. This movement was first observed in 

 the granular particles seen among pollen grains of 

 plants, known as fovilla, and which are set free when 

 the pollen is crushed. Important vital endowments 

 were formerly attributed to these particles, but Dr. 

 Robert Brown showed that such granules were com- 

 mon enough both in organic and inorganic substances, 

 and were in no way " indicative of life." 



Accessory Apparatus. In the more perfectly furnished 

 instruments, a number of accessory pieces of apparatus 



