Partial Interception of the Prismatic Spectrum. 339 



eve be enlarged in the proportion 3 : 4, the length of each 

 curve will be diminished in the ratio of 4 : 3, and there will 

 be four intervals of ordinates in the constant space C. If, in 

 another change, the aperture of the eye be diminished in the 

 proportion 3 : 2, the length of each curve will be increased in 

 the ratio of 2 : 3, and there will be only two intervals of ordi- 

 nates in the constant space C. 



1 shall now proceed to apply these considerations to the 

 circumstances of experiment. 



General Condition for the three following 

 Experiments. 



The constant C is supposed to be equal to three intervals 

 of ordinates, as at present marked in the diagram of page 10. 



Special condition for the first experiment. 



The aperture of the eye is such as to make the horizontal 

 extent of each curve exactly equal to that marked in the dia- 

 gram. 



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

 C from that above it, and since C is equal to three intervals of 

 ordinates, it follows that the aggregate of light will be obtained 

 by combining the 1st number of the first column on page 4 

 with the 4th number of the second column, the 7th number 

 of the third column, and so on; continuing the succession of 

 columns as long as numbers can be taken by this rule (the 

 seventh column being exactly similar to the first). Effecting 

 this, and dividing the sums by 12, we find the following num- 

 bers to represent die intensity of light which comes to the 

 eye: — 



13646, 12829, 11295, 9227, 6875, 4524, 2456, 921, 105, 

 105, 921, 2456, 4524, 6875, 9227, 11295, 12829, 13646, 

 13646, 12829, &c. 



The bands here will be most vividly bright and black. 



The bands recur at an interval of 18 ordinates or 6 C. 



Special condition for the second experiment. 



The aperture of the eye is greater than that in the first ex- 

 periment, in the ratio of 3 : 4. 



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

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

 equal to four intervals of coordinates, it follows that we must 

 combine the 1st number of the first column with the 5th num- 

 ber of the second column ; the 9th number of the third column, 

 and so on. Effecting this, and dividing the sums by 9, we 



2 A 2 



