XIX, 1. Rheinberg: Common Basis ofthe Theories ofMicroscopic Vision. 13 



quently we get superposed on the orange Image at its maxinumi 

 brightuess, a larger illuminated surface of red, and a still lai'ger 

 one of bhie and violet. We liave tlierefore an Image with a bhüsh 

 fringe. 



It would not serve the ends of tliis paper to enter further into 

 the subject of achromatism , suffice it that all the combiuations of 

 lenses we have to deal with, are either imdercorrected, over cor- 

 rected, achromatic or apochromatic. In tlie first the Image shows a 

 blue fringe as above, in the second the blue rays converge to a 

 focus further off than the red and the Image has a red fringe. In 

 the achromatic lenses two colours of considerable ditference of wave 

 lengtli are brought to the same focus , and the fringe due to the 

 other colours — the so-called secondary spectrum — is confined witliin 

 very uarrow limits and is very faint , whilst in the apochromatic 

 lenses, three colours in ditferent portions of the spectrum are united 



White 



\ Red Curve 



\ Green " 



\ Vioiet " 



\ 



11. 



in one focus , and even the secondary spectrum is practically eli- 

 minated. 



We have now to pass on to another colour phenomenon, which 

 is independaut of the achromatic or apochromatic correction of the 

 lens. Let us suppose we are using a perfectly corrected combination. 

 Referring to figures 3, 4, 5, we have noticed how the differences 

 between P' and (), i?, 6', T, depends upon the number of half 

 wave length, ^ a wave length being represented by the distance 

 YY'. Suppose we had been dealing with light whose whole wave 

 length had been only equal to YY\ then denoting the minima and 

 maxima of light i?, /S, T, as before, it is clear that the point B 

 would have fallen where at present the point Q is situated ; in fact 

 the whole diso and ring would have been coniined withiu just half 

 the previous limits. The ditference in wave length between the 

 mean red and violet light being roughly as 3 to 2, if we take our 

 original disc to have been due to red light , then the violet disc 

 would be f as wide, and all the other colours somewhere between. 



