MICROSCOPES. 195 



since, in order to get very high magnifying powers, the focal 

 length must be very short, and then the objects are so very 

 close to the lens that it is almost impossible to perform any 

 necessary operations under them. 



Independently of the so-called spherical aberration, we 

 have to contend with what is called chromatic aberration. 

 As I have said, the spherical aberration might be overcome if 

 it were possible to grind elliptical or hyperbolic lenses ; but 

 even then we should still have to contend with what is called 

 chromatic dispersion. When light is passed through a prism 

 with inclined faces, it is more or less bent out of its original 

 course ; but the different rays are thus refracted to a different 

 extent, the blue rays being much more bent than the red rays 

 and the green to an intermediate extent. The result is that 

 there is a different focal point for the different rays of the 

 spectrum. When white light passes through any ordinary 

 simple magnifying lens the focus for the blue is thus shorter 

 than for the red. Hence, on looking at an object, you see it 

 fringed with colours, and get an indistinct image, and you 

 see colours that ought not to exist. At one time it was 

 thought that the evil could not possibly be remedied ; and 

 even so great an authority as Sir Isaac Newton concluded that 

 it could not be overcome. Fortunately, however, we have 

 now the power of overcoming it to a very great extent. The 

 principle on which this depends is, that the extent to which 

 the different rays are separated by different kinds of glass 

 their so-called dispersive power does not vary directly as 

 their refractive power, and it is thus possible to combine 

 together two different kinds of glass, so that the dispersive 

 power of one may counteract the dispersive power of the 

 other, and yet only partially counteract its refractive power. 

 This is best illustrated by what is seen in compound prisms 

 made of crown and flint glass, the relative dispersive power 

 of which latter is much greater than that of the former. 

 According to the angles of the two kinds of glass, prisms may 

 be so combined that we may obtain dispersion without refrac- 

 tion, or refraction without dispersion, the former being a 

 so-called direct-vision prism, through which the light passes 

 in a direct line, but much dispersed, and the latter an 

 achromatic prism, bending the light but not dispersing it, and 

 thus showing objects out of the direct line, but free from 

 coloured fringes. This is what we want in constructing 



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