n 
Proceedings of the Royal Society 
A 
is a straight stick passing horizontally immediately under the 
thread, and graduated in centimetres on its upper edge. A 
_ grating constructed by drawing trans- 
parent lines at the rate of 2000 to the 
inch upon glass covered with a dark 
ground, is held by the hand against the 
end E of the stick, cut square with its 
edges. The stick is then pushed in or 
out in the direction of its length till the 
red colour of the first spectrum to the 
right of A is seen to be directly above 
the same colour of the first spectrum to 
the left of B. A pencil mark is then 
made upon the stick directly below the 
thread. The stick is then drawn further 
out until the yellow colours of the two 
spectra are seen in the same vertical 
line, and another mark is made ; and so 
with the remaining colours. The dis- 
tance from centre to centre of the two 
slits, in a horizontal line, being 10 centi- 
metres, the distances marked off between 
E and the thread were read 99-5 centi- 
metres for the red, 107 for the yellow, 
116 for the green, and 135 for the blue. 
These numbers were taken for the dis- 
tance to the light in each case, being 
only about ^ per cent, less by calcula- 
E 
tion. The corresponding wave length 
by the formula X = (a + e) sin I, are 
Bed 
5 
i 
^ of an 
inch = -000638 millimetre. 
99-5 
* 2000 
39800 
Y ellow 
5 
107 
1 
* 2000 
1 
42800 
- -000593 
Green 
5 
1 
X 2000 
1 
= -000547 „ 
116 
46400 
Blue . 
5 
1 
1 
= -000470 „ 
135 
X 2000 ' 
54000 
