OPTICAL PRINCIPLES OF THE MICROSCOPE. 9' 



equally serious arises from the unequal ref rang Utility of the several Col- 

 ored rays which together make up White or colorless light, 1 so that they 

 are not all brought to the same focus, even by a lens free from spherical 

 aberration. It is this difference in their refrangibility, which causes their 

 complete separation or ' dispersion ' by the Prism into a spectrum; and it 

 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 refrangible, 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 a 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 di- 

 vergent violet rays at E E. The 

 foci of the intermediate rays of 

 the spectrum (indigo, blue, green, 

 yellow, and orange) are interme- 

 diate between these two extremes. 

 The distance c D between the foci 

 of the violet and of the red rays 

 respectively, is termed Chromatic 



Aberration. If the image be re- Diagram illustrating Chromatic Aberration. 



ceived upon a screen placed at c 



the focus of the violet rays violet will predominate in its own color, 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 

 colored fringes in inverted order, formed by the other rays of the spec- 

 trum which have met and crossed. 3 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 colored fringes will be narrow- 

 est, the ' dispersion ' of the colored rays being the least. As the axial 

 ray A' B' undergoes no refraction, neither does it sustain any dispersion; 

 and the nearer the rays are to the axial ray, the less dispersion do they 

 suffer. Again, the more oblique the direction of the rays, whether they 

 pass through the central or the peripheral portion of the lens, the greater 

 will be the refraction they undergo, and the greater also will be their 

 dispersion; and thus it happens that when, by using only the central part 

 of a lens ( 13), the chromatic aberration is reduced to its minimum, the 

 central part of a picture may be tolerably free from false colors, whilst. 



1 It has been deemed better to adhere to the ordinary phraseology, when 

 speaking of this fact, as more generally intelligible than the language in which 

 it might be more scientifically described, and at the same time leading to no prac- 

 tical error. 



2 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 peri- 

 pheral ring; since, if its whole aperture be in use, the regular formation of the 

 fringes is interfered with by the spherical aberration, which gives a different 

 focus to the rays passing through each annular zone. 



