HOW ACHROMATISM MAY BE OBTAINED 17 



This will be fully understood by the aid of fig. 19. 



The white light, A A", falling on the peripheral portion of the lens, 

 is so far dispersed or 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 they reach 

 the point 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, limiting the violet and the red, is termed the longitu- 

 dinal chromatic aberration of the tens. 



If the image be received upon a screen placed at C, violet will 

 predominate, and will be surrounded by a prismatic fringe in which 

 blue, green, yellow, orange, and red may be distinguished. If, on 

 the other hand, the screen be placed at D, the image will have a 



FIG. 19. Chromatic aberration. 



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 joins the points of intersection between the red 

 and the violet rays which marks the mean focus, or the point where 

 the dispersion of the coloured rays will be least. 



The axial ray undergoes neither refraction nor dispersion, and 

 the nearer the rays are to the axial the less dispersion do they 

 undergo. Similarly, when the refraction of the rays is greatest at 

 the periphery of a lens, there the dispersion will be most. Hence 

 the peripheral portions of unconnected lenses are stopped out, and 

 the centre only often used that the chromatic aberration may be 

 reduced to a minimum. 



Manifestly, therefore, the correction or neutralisation of this 

 chromatic aberration, which is known in optics as achromatism, is a 

 matter of the first moment. Multiplied colour foci between C and 

 D (fig. 19) make a perfect optical image impossible. 



It is a question of interest and importance to the microscopist to 

 know how achromatism is obtained. 



In a prism the amount of dispersion or unequal bending of 

 E, and V (fig. 5) depends on two things : (1) the nature of the 

 glass of which the prism is composed, and (2) the refracting angle 

 B AC. 



If, for example, another prism were taken, made of a different 

 kind of glass, possessing only half the dispersive power of that in 

 the figure, but with the angle BAG 50, as in this case, the separa- 



c 



