12 OPTICAL PRINCIPLES OF THE MICROSCOPE. 



manifests itself, though in a less degree, in the image formed by a 

 convex Lens. For if parallel rays of white light fall upon a Convex 

 surface, the most refrangible of its component rays, namely, the 

 violet, will be brought to a focus at a point somewhat nearer to the 

 lens than the principal focus, which is the mean of the whole ; and 

 the converse will be true of the red rays, which are the least re- 

 frangible, and whose focus will therefore be more distant. Thus 

 in Fig. 9, the rays of white light, A b, a" b", which fall on the 

 peripheral portion of the lens, are so far decomposed, that the 

 violet rays are brought to a focus at c, and crossing there, diverge 

 again and pass on towards F F ; whilst the red rays are not brought 

 to a focus until D, crossing the divergent violet rays at E E. The 

 foci of the intermediate rays of the spectrum (indigo, blue, green, 

 yellow, and orange) are intermediate between these two extremes. 

 The distance c d between the foci of the violet and of the red rays 

 respectively is termed Spherical Aberration. If the image be 

 received upon a screen placed at c — the focus of the violet rays, — 



Fig. 9. 



Diagram illustrating Chromatic Aberration. 



violet will predominate in its own colour, and it will be surrounded 

 by a prismatic fringe in which blue, green, yellow, orange, and red 

 may be successively distinguished. If, on the other hand, the 

 screen be placed at d — the focus of the red rays, — the image will 

 have a predominantly red tint, and will be surrounded by a series 

 of coloured fringes in inverted order, formed by the other rays of 

 the spectrum which have met and crossed.* The line E E, which 

 joins the points of intersection between the red and the violet rays, 

 marks the 'mean focus,' that is, the situation in which the 

 coloured fringes will be narrowest, the ' dispersion ' of the coloured 



• This experiment is best tried with a Lens of long focus, of which the 

 central part is covered with an opaque stop, so that the light passes only 

 through a peripheral ring ; since, if its whole aperture be in use, the 

 regular formation of the fringes is interfered with by the spherical aber- 

 ration, which gives a different focus to the rays passing through each 

 annular zone. 



