SUPPLEMENTARY REPORT ON METEOROLOGY. 125 
former*. The distance of the neutral point from the sun varies 
with atmospheric contingencies. The position of this neutral 
point seems to have been at first inaccurately reported, for we 
find English observers searching for it in the sun’s neighbour- 
hood, and not in the opposite quarter of the skyf. The 
polariscopes of Arago and Savart, used for detecting the 
minutest quantities of polarized light, are little known in this 
country ; it is by their aid that the light of the moon is ascer- 
tained to be slightly polarized. 
261. On the phenomena of Mirage we have nothing new to 
state. The phenomena of Twilight and of Atmospherical Re- 
fraction, although connected with Optical Meteorology, we 
shall also omit. 
B. The Rainbow. 
262. There is no step in the progress of science more inter- 
esting than that which calls in the aid of comparatively abs- 
truse principles to explain the slighter outstanding variations 
between theory and observation, which were overlooked in the 
first unqualified satisfaction with which the announcement of a 
simple general principle, harmonizing with every-day experi- 
ence, is invariably received. Amongst such cases may be 
reckoned the law of double refraction in crystals with two axes, 
Laplace’s correction for the velocity of sound,—and we may 
now add, the phenomena of the rainbow, so far as these were 
not included in Newton’s general explanation. 
263. The diameter of the primary rainbow (caused by two 
refractions with one intermediate reflexion), and of the secondary 
(caused by two refractions and two reflexions), may be most 
easily found by the formule which M. Babinet has lately 
given { for expressing the radii 5, directly in terms of the re- 
fractive index of water m, viz. 
_ (4—m’)? 
27 m* 
. & m*+18 m*—2 
For the secondary, sin a eee 
* See Peclet, Zraité de Physique, 4me edit. art. 1448. I am unable to 
state where M. Arago’s original account of these experiments is to be found. 
Compare Quetelet’s Notes to Herschel on Light, French Translation, ii. 554. 
+ Airy and Chevallier, Philosophical Magazine, N.S., iv. 312, 313. I must 
add, however, that a very recent communication by M. Babinet to the French 
Academy of Sciences (Comptes Rendus, 19th October, 1840) states the exist- 
ence of a second neutral point, 20° or 30° distant from the sun. 
$ Comptes Rendus, iv. 646. The demonstration is given in Peclet, Zraiié 
de Physique, 4me édit. art. 1489. 
: yao 
For the primary, sin? > 
