196 Sir David Brewster on the Anatomical and Optical 
distance as the angular distance of the first image from the cen- 
tral image. If we call this distance A, and put B for the 
breadth of each fibre, or the breadth of the transparent part, 
together with the breadth of the opake interval or line which 
separates the fibres, we shail then have B = C being a 
C 
7, 
constant quantity for the red ray to be determined by experi- 
ment. According to Fraunhofer’s experiments on Interfe- 
rence, C is equal to 0°0000256 of an English inch for the 
0:0000256 
5 ; 
In order, however, to save the trouble of calculation, and 
in cases where a general estimate only is required, I use Mr. 
Barton’s system of grooves already mentioned ; and upon look- 
ing through the lamina at the white image, or rather the cen- 
tral image of the iris surface, I can at once compare the di- 
stance of the first prismatic images of the one with the same 
distance in the other. If I am using, for example, the divi- 
sions of 5000 in an inch, and if I find the distances of the co- 
loured images the same with the laminze and with the steel, I 
infer that the breadth B is the 5000dth of an inch. This con- 
clusion, however, is not quite correct, for we may have been 
comparing the effect of a perpendicular incidence on the la- 
mina with the effect of an oblique incidence onthe steel. Now, 
Fraunhofer has shown that the coloured images separate as 
the incidence in a plane intersecting the grooves at right an- 
gles is increased, and that the value of B, to which this in- 
creased distance corresponds, is smaller in the proportion of 
radius to the cosine of the angle of incidence, I. Hence the 
. C 
formula becomes B= essen This property gives us the 
middle red ray; hence the formula becomes B= 
advantage of a variable scale; and in making the experiment, 
I found that at an incidence of 25°, when the divisions of 5000 
in an inch were employed, the coloured images of the laminze 
corresponded with those of the grooved steel. Hence the 
value of the quantity B, or the breadth of the fibres, is about 
the 5500dth part of an inch. 
I next detached laminz from parts nearer the pole, and I 
found that the coloured images gradually separated as the 
fibres approached to the poles; thus proving, what might have 
been inferred from other facts, that the fibres gradually taper 
in breadth from the equator to the poles of the lens. In order 
* Fraunhofer found that in a system of lines where B was 0:0001223 of 
a Paris inch, A was 11° 25! 20": and in another system, where B was 
00005919, A was 2° 20! 57’. 
