AN APPARENT NEW POLARITY IN LIGHT. 
23 / 
band strongly marked, with bright and dark bands on each side faintly marked. 
And the whole of these phenomena will be independent of the thickness of the mica 
(within wide limits). The reader may very easily verify these conclusions by experi- 
ment ; and the whole will be found strictly in agreement with observation. 
If the eye had been supposed too near to see the line of light distinctly, the investi- 
gation would have been precisely the same, but the place where the bands are 
c b 
sensible would have been determined by making -j — g small, or b nearly equal to 
o! cr % # 
-f- -y ; from which it will appear that, upon moving the plate of mica, the bands 
will appear to move in the opposite direction; which agrees with observation. 
2. Suppose that the linear origins of the various kinds of homogeneous light are 
separated, either by prism-refraction, or by the diffraction of a grating, or in any 
other way which arranges the colours in an order corresponding to the order of the 
values of X ; suppose that the eye is too distant to see the lines of colour distinctly ; 
a thin plate of mica, with its edge parallel to the lines, is gradually brought across 
the pupil of the eye: to describe the appearance of the spectrum. 
First, suppose the red end of the spectrum to be on the same side as the plate of 
mica, or on that side on which b and g are considered positive. 
Let k be the ordinate measured from a fixed point on the retina to the centre of the 
confused image of any one colour ( k therefore is a function of X) ; and let l be the 
ordinate, measured from the same fixed point, of any point of which the intensity of 
light is to be ascertained. Then k + b = /, or b = / — k. Substituting this in the 
general expression for intensity, it becomes 
^ ( / 2a cl ck\ 
■ c°s 9 G/ —. T + g - T ) 
+ G 
Cx/M 
- + 
a ' 
Now the second term of this expression may at once be neglected, without consider- 
ation of the position of the spectrum. For as k is a function of X, and as the spec- 
tral separation is considerable, • will vary rapidly with X, and therefore 
the angle <p (^/ 
2 a 
Kc e 
cl , c k\ 
H +£ “ n) 
may have all the values included in several cir- 
cumferences, for the variation of X included in those rays which fall on the same point / ; 
and the positive and negative values of the cosine will sensibly destroy each other. 
With regard to the third term, it must be remarked that, in the image formed 
upon the retina, the blue end of the spectrum is on the same side as the plate of 
mica, or that k is greatest for the most refrangible rays, and therefore 
f L/H.. c J +g - c -*) 
r V V \ce a 1 o a / 
