324 Mr. T. J. Bowlker on the Factors serving 
With Note of Wave-length 40 inches. 
Zero at 2° left. 
"Two-image point" at 40° left and 38° right. 
" Cross-over angle " from 90° right (wide image) to 90° 
left (wide image). 
Taking 39° for the mean position of the two-image point, 
we get 2 (31 sin 39°) = 39 inches as the wave-length. 
Note of Wave-length 27*6 inches. 
Zero at 4° left. 
"Two-image point"... 22° r. and 30° 1. (mean 26°). 
New images 51° r. and 55° 1. (mean 53°). 
Cross-over angle from 50° right to 40° left and from 40° 
left to 40° right. 
Wave-length from " two-image point " 
= 2 x 31 sin 26° = 27-3 inches. 
Wave-length from new images 
= 31 sin 53° = 24-8 inches. 
Note of Wave-length 19*4 inches. 
Zero at 0°. 
" Two-image point... 19° 1. and 16° r. (mean 17J°). 
New images 42° 1. and 46° r. (mean 44°). 
Cross-over angle ... 30° r. to 36° 1. 
and 36° 1. to 36° r. 
Wave-length from "two-image point" 
= 2x31sinl7i° = 18-6 inches. 
Wave-length from new images 
= 31 sin 44° = 21'4 inches. 
Note of Wave-length 13*8 inches. 
Zero at 0°. 
"Two-image point"... 17° 1., 17° r. 
New images 29° 1., 62\L, 29° r., QS° r. 
Cross-over angle 18° to 18°. 
Wave-length from " two-image point " 
= 2x31sinl7°=18 inches. 
Wave-length from new images 
= 31 sin 29° = 14-9 inches. 
i 31 sin 6d° , . - . , 
and =14*1 inches. 
