108 M. W. Wernicke on the Refractive Indices 



sin D acquires the values -f 1 and — 1 for the minima of intensity 



6+ -*-S )-=T- 



becomes equal to an indefinitely large multiple of ir; conse- 

 quently 



e+ V =m -r (6) 



in which m may be anj' positive whole number. The magnitudes 

 £j and £ 2 cannot be experimentally determined with sufficient 

 accuracy by means of the formula which the theory of elliptical 

 polarization gives ; but even if they could, in order to obtain the 

 most correct value of \ possible we should nevertheless have to 

 proceed according to the following reasoning. Let e, and e 2 be 

 two values of e, and m x and m 2 the values of m, which satisfy 

 equation (6); then by subtraction, since 6\ and S 2 in both cases 

 remain the same, 



X=2. 6q ~ ?1 (7) 



According to this formula, the differences of the thicknesses e 2 

 and e l of the layer for which the minima of the intensity of light 

 occur are proportional to the wave-length, which may be veri- 

 fied by observation. 



The observations are now conducted in the following manner. 

 If by any of the following methods a thin layer of the body to 

 be examined is prepared and then observed in reflected light 

 through the spectroscope, the solar spectrum is at first seen un- 

 changed in the field of view; when the layer has reached a cer- 

 tain thickness, a dark band appears at the more refrangible end, 

 which, with increasing thickness of the layer, moves through the 

 spectrum, and after a definite period again appears in the original 

 place. If the thickness of the layer is still further increased, 

 then two, three, and more dark bands gradually appear in differ- 

 ent places of the spectrum. The bands increase in sharpness, 

 and, when more than three are together at the same time, are 

 sometimes so dark that direct sunlight is needed in order to re- 

 cognize Fraunhofer's lines : generally cloud-light is sufficient to 

 fix their position in the spectrum. If we wish to determine the 

 wave-length of the light in the substance for any particular 

 Fraunhofer's line, then the layer must be made so thick that a 

 dark least band appears in this place; the layer with its support 

 is placed in a balance which is very delicate for small weights, 

 and is then strengthened until the bands appear for the second, 

 third, or fourth time in the same place of the spectrum. The 

 increases in weight give the magnitudes e 2 --e i , e 3 — e Xi e 4 — e x and 



