106 E. C. Pickering—Measurements of the 
is perceived. This very remarkable result may be expressed 
y saying that ten plates of glass transmit more light obliquely 
than normally. The appearance to the eye confirms this result, 
but it deserves a careful photometric proof. At57° the reflected 
ray is, of course, in all cases, totally polarized; but at other 
angles the amount of polarization is greater the less the num- 
ber of mre itiene of the contrary, as might have been 
anticipate 
With the refracted ray quite a different law holds. For one 
surface the polarization increases from 0° to 5 fl ; with two sur- 
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ont the polarization of the refracted beam, we find it equal to 
To show how far these effects are due to internal reflection, 
another table was computed by the formula ae a)™, or suppos- 
ing that no internal reflection took pla A son 
aoe that while the reflected beam is aifected but little a great 
change takes place in the transmitted light. The results are 
eb by dotted lines in figs. 7, 8, and 10, and will be discussed 
To test the above conclusions, two experimental methods 
may be employed. First, by means of a photometer, to deter- 
mine the amount of light in any given case; and, secondly, by 
means of a polarimeter, to determine the percentage of polari- 
zation of the reflected and refracted rays. The latter method 
has been employed in the following experiments. The instru- 
ment commonly used to measure the amount of polarization 
