198 Growth of Trees, ivith a Note on Interference Bands. 



shortly as follows : As has been stated, the relative retardation of the 

 interfering rays is 2^/7. Here 7 (and in a less degree ?/) varies with jx, and 

 with the wave-length. If two rays, say, yellow and blue, are to reach the 

 eye in the same direction and with the same relative retardation, then must 

 2/y7y/^t = 3/b7b/'^b. The difference between ijy and ijs is small compared 



8oorO-8 



Total reflexion Total reflexion 

 Blue Yellow 

 >i=4,300 5,900 



Fig. 6. 



to the difference between 7y and 7b , and therefore there is approximate 

 achromatism when 7y / 7b = / ''^b • 



The diagram (fig. 6) will make this clearer. The two curves Y and B 

 (which relate to a sample of crown-glass) give the value of 7 in terms of r, 

 in the neighbourhood of the angle of total internal reflection, Y referring to 

 the yellow of the D line, and B to the blue near G, viz., at \x = 5900^and 

 Xb = 4300. 



It will be seen that there is one value for r (marked by * on the figure) 



