340 The Astronomer Royal on Bands in the Spectrum. 



find the following numbers representing the intensity oflight 

 which comes to the eye : — 



7659, 9164, 10512, 11617, 12399, 12801, 12801, 12399, 

 11616, 10512, 9164, 7659, 6093, 4586, 3238, 2135, 1353, 

 949, 949, 1353, 2135, 3238, 4586, 6094, 7659, 9164, 10512, 

 &c. 



The bands here are extremely well marked, but not quite 

 so vividly bright and black as in the first experiment. 



The bands recur at an interval of 24 ordinates ; and as 4 

 ordinates are now included in the space C, the bands recur at 

 an interval of 6 C. 



Special condition for the third experiment. 



The aperture of the eye is less than that in the first experi- 

 ment, in the ratio of 3 : 2. 



Since each curve is to be drawn back by the constant space 

 C from that above it, and since C (as is above explained) is 

 now equal to two intervals of ordinates, it follows that we 

 must now combine the 1st number of the first column on page 

 4 with the 3rd number of the second column ; the 5th number 

 of the third column, and so on. Effecting this, and dividing 

 the sums by 18, we obtain the following numbers for the inten- 

 sity of the light which reaches the eye : — 



7871, 5880, 4157, 3161, 3161, 4157, 5880, 7871, 9594, 

 10256, 10256, 9594, 7871, 5880, &c. 



The bands here are well marked, but are not so vividly 

 bright and black as in either of the former experiments. 



The bands recur at an interval of 12 ordinates; and as 2 

 ordinates are now included in the space C, the bands recur at 

 an interval of 6 C. 



Thus it appears that, as result of theory, the interval of the 

 bands is precisely the same whether the aperture of the eye 

 be represented by 2, by 3, or by 4. I have gone through 

 these proportions in detail ; but any one who has watched the 

 operation sufficiently to see upon what the succession of values 

 in the numbers depends, will perceive that the recurrence of 

 bands depends only on the recurrence in the values of R, 

 and therefore that, whether a telescope or the naked eye be 

 used, whether the spectrum be produced by diffraction or by 

 refraction, and whatever be the refractive angle or dispersive 

 power which produces it, provided the same retarding plate 

 be used, the same bands will always be formed in the same 

 part of the spectrum. The theoretical effect of altering the 

 aperture of the eye (besides of course affecting the general 



