MICROSCOPES. 197 



the image, and the field-lens in increasing the field. The 

 other eye-piece is called the negative, or Kelner's eye-piece, 

 and has no diaphragm, and the focus of the upper lens is on the 

 field- lens. The advantage of this is that we get a much larger 

 field, but the disadvantage is that the definition is not so good. 



Since we thus examine with the eye-piece an inverted image 

 of the object, it is in all cases seen as it were turned upside 

 down. This general principle of magnifying an inverted 

 image was brought into use at as early a period as 1590, by 

 Janssen, whose most interesting instrument is in the Exhi- 

 bition, though, in comparison with our modern microscopes, it 

 might be looked upon as little better than a toy. 



The magnifying power of compound microscopes depends 

 on both the object-glass and the eye-piece. The shorter the 

 focal length of the object-glass the larger is the image formed 

 in the upper part of the instrument. The final result also 

 depends on the magnifying power of the eye-piece itself. We 

 can thus increase the magnifying power of the instrument 

 by having the object-glass of shorter focal length ; and can 

 likewise increase the power by using eye-pieces of shorter 

 focal length. On the whole it is, however, better to use 

 an object-glass of short focal length, and an eye-piece of 

 moderate power, than to use an object-glass of long focal 

 length and enlarging the image with an eye-piece of too 

 great power, since we can thus utilize a greater beam of 

 light, and obtain better definition. 



The simplest form of achromatic object-glass that could be 

 used would be a combination of a double convex lens of crown 

 glass, with a plano-concave of flint ; but though one of this 

 kind may give a good result when the focal length is con- 

 siderable, it would be impossible to obtain good definition 

 when the focal length is small, and the aperture great. By 

 combining three such compound lenses of different sizes 

 somewhat differently corrected, the spherical aberration may 

 be very greatly reduced, and the general effort much improved, 

 since a short focus can be obtained with lenses of less curva- 

 ture than when only one compound lens is used. Until the 

 last few years most of the best object-glasses were constructed 

 on this principle. Some modern high- power object-glasses 

 are, however, much more complex, and consist of a combi- 

 nation of as many as eight lenses, nearly all of different 

 curvature or size. The magnifying power is in many 



