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BACTERIOLOGY. 



image, inverted and reversed, lies inside of tlie principal 

 focus of the eye-piece, and the rays of light leave it as if 

 they came from a real object. If a screen was placed at 

 this point, it would show an image. Since this image lies 

 inside of the principal focus, it becomes magnified by the 

 eye-piece (E), and the virtual image (C D) is produced. It 

 has been, not inaptly, said that " a compound microscope is 

 a simple microscope applied, not to the object, but to its 

 image already magnified by the first lens." 



Fig. i6. Real image (Carpenter). 



The single lens naturally represent the earliest form of 

 the microscope. The English monk, Roger Bacon, in 1276, 

 appears to have been the first to recognize the peculiar 

 properties of a lens. It was he who applied the new knowl- 

 edge to the construction of spectacles. It was not, how- 

 ever, till the beginning of the 17th century, that the micro- 

 scope was sufficiently perfected to bring about the discov- 

 ery of the existence of microscopic life. The discovery of 

 the compound microscope may be said to have been made 

 by Galileo in 1610, although it is probable that others may 

 have antedated him by a few years. The early compound 

 microscope consisted of a single lens for an objective, and 

 of another single lens for an eye-piece. Such an instru- 

 ment, necessarily gave very imperfect results. 



The image produced, by a single lens, is not a perfect 

 reproduction of the original object. All the rays of light 

 do not meet in the same plane, and hence, the image has a 

 spherical appearance. This fault of a lens is known as. 

 spherical aberration. Again, the lens, acting as a prism, de- 

 composes some of the rays of light which pass through it. 



