ACCORDING TO THE UNDULATORY THEORY. 499. 
m f m r> 
=a (1—r) ( uma caaoy 
and 
A; minimum = a (1 — r)" G dae + Oo r):) 
1+(1—r) 
a m r> 
=a(1—7r) 1— pasa): 
If we now compare the intensities in the maximum and minimum 
with one another, we have 
A; max. _ Qr+ mr? 1+(1 =a 
A; min. 1+ (1—7r)?— mr? Qr 
Bearing now in mind that r must be an extremely small magnitude 
the above formula is reduced to 
A;max. _ mr 
A; min. 12h 
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, beeome 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 searcely recognise 
any one of the surrounding objects; after a few minutes we pers 
ceive them quite well. The heavenly bodies, which by night shine 
so 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 brighter stripe makes it imperceptible to 
our eye. 
Vor. I.—Panrrt IV. 2M 
