ELEMENTS OF 



VEGETABLE HISTOLOGY 



THE MICROSCOPE 



CHAPTER I. 



PRELIMINARY CONSIDERATIONS. 



Before describing the mechanical and optical parts of a com- 

 pound microscope, it is essential to know something about the 

 action of the transparent bodies, as prisms, lenses, etc., on light 

 rays how a lens forms an image of an object, and how the 

 image is magnified, etc.; in other words, a little of the ele- 

 mentary physics of light. 



Light travels through homogeneous, transparent media, as 

 air, water, glass, in straight lines, and a very narrow cylinder 

 of light is called a ray, or better, a pencil of light. Bays are 

 represented in geometric illustrations by straight lines. The 

 fact that light travels in straight paths may easily be shown by 

 admitting a small beam through a hole in a shutter into a dark- 

 ened room in which dust particles are floating around. The 

 illuminated particles will be seen to lie in a straight line. The 

 formation of sharp shadows by obstacles in the path of light 

 is another evidence that light travels in straight lines. When 

 the rays come from a distant source, as the sun, moon, stars, a 

 distant flame, they are practically parallel, and a beam of such 

 light is spoken of as parallel light or beam. 



A convergent beam of light is one in which the rays come 

 together in a point or focus. 



A divergent beam of light is one in which the rays emanate 

 from a point or focus. 



MIRRORS. When light falls upon bodies, it is in general 

 reflected. If the surface be rough, the rays will be reflected in 

 every conceivable direction, each point of it becoming, as it 

 were, a new source of light. It is for this reason that rough 

 bodies are seen, and also from any position. When the surface 

 of a body is smooth, the light rays falling upon it are reflected 

 in a definite direction according to fixed laws, and such surfaces 

 are called mirrors. "Smoothness" is a relative term, but with 

 reference to light rays, it means that there are no unevennesses 

 which are at all comparable in size with the wave-length of the 

 waves of light, which is very small. When the surface is plane, 



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