ACCOaDING TO THE UNDULATORY THEORY. 499 



A; minimum = a (1 — r) 



and 



1 + (1 -r)'- 





If we now compare the intensities in the maximum and minimum 

 with one another, we have 



A ; max. _ 2 r + m r- 1 + (1 — r)^ 



A; min. ~ 1 + (1 — r)^ — m r- 2r 



Bearing now in mind that r must be an extremely small magnitude 

 the above formula is reduced to 



A ; max. m r 



IT ~ = 1 + -ET- 



A; mm. 2 



From this then we see that the proportion between the intensities in 

 the maximum and in the minimum must become greater the greater 

 m is, i. e. the denser the absorbing medium is. Hence we obtain a 

 reason for the considerable difference between the maxima and minima 

 of the solar spectrum, when the atmospheres of the sun and 

 earth are the absorbing media. There are, however, many circum- 

 stances by which our estimate of the relative intensities in the maxima 

 and minima are rendered very uncertain. The flames of light, which 

 suffice to light our chamber in the evening, become by day, when 

 placed in the light of the sun, almost imperceptible. In the same 

 manner, when we step out of a lighted chamber on a dark evening, 

 the darkness seems to us so deep that we can scarcely recognise 

 any one of the surrounding objects ; after a few minutes we per-; 

 ceive them quite well. The heavenly bodies, which by night shine 

 8o brightly but by day are quite imperceptible, present a second 

 example of the same kind. Moreover we know that the eye itself 

 changes according to the greater or lesser intensity of the light : the 

 contraction and expansion of the pupils are very likely not the only 

 change which the eye undergoes in this respect ; for, indeed, I consider 

 it also probable that even the pellicle of the eye possesses the property 

 of greater or less irritability. For this reason it is easy to conceive that 

 the difference between the intensities in the maximum and minimum 

 may possibly appear to us very great without being so in reality, and 

 that the intensities in the black stripes may be very considerable, al- 

 though the proximity of the brigliter stripe makes it imperceptible to 

 our eye. 



Vol. I — Part IV. 2 m 



