ee —————- 
—— 
TRANSACTIONS OF THE SECTIONS. 25 
We may therefore still infer that when, as in the actual case, @ is small compared 
to o, the energy lost in the earth’s rotation is almost the exact equivalent of that 
consumed in tidal friction, or in any work done by tidal action. It would also 
appear that as tan (0+) is less than 3, and the mean value of A must be 
small, the coefficient of @ 18 positive ; and that consequently, as in the case where 
the equator and the plane of the moon’s orbit coincided, so in the actual case 
there is a small accompanying transfer of energy from the earth’s rotation to the 
orbital motion. 
All these conclusions apply nwitatis mutandis to the solar tides, if we regard as 
our binary system the earth and sun. 
In the case of nature, when we have to consider the three bodies acting together, 
the main conclusion that nearly all the energy is drawn from the earth’s rotation 
will not be invalidated. 
It would also appear that if, as is usually done, we assume the friction to vary as 
the velocity, the smaller effects (which consist in the transference of energy from 
the earth’s rotation to the energy of the orbit of the moon about the earth and 
that of the earth about the sun) will correspond to the values separately calculated 
for the binary systems, 
Liaut. 
Further Experiments on Light with Circularly Ruled Plates of Glass. 
By P. Brawam. 
On Extraordinary Reflection. By Professor Curtis. 
The author drew attention to the fact that in elementary treatises on Experimental 
Physics no mention is made of. extraordinary reflection, whereby students are 
frequently, although illogically, led to the conclusion that in the case of media 
which violate the ordinary law of refraction, the ordinary law of reflection is 
fulfilled ; while mathematical considerations show that Huyghens’s construction is as 
applicable to reflection as to refraction, and that a ray of light proceeding through 
a crystal, and impinging on the surface of contact of the crystal with a surrounding 
medium, will give rise to two reflected rays, accompanied by one or two refracted 
rays, according as the surrounding medium is ordinary or extraordinary. When 
the erystal is uniaxial, one of these reflected rays conforms to the ordinary law of 
reflection, while ix general the other does not; if the crystal is biaxial, neither of 
the reflected rays in general conforms to that law. If, then, a ray of light falls 
upon a crystal surrounded by air, part of this light is reflected and part is refracted, 
the latter being in general split into two rays: each of these rays will suffer double 
reflection at the point where it again meets the bounding surface of the crystal ; 
and in the case where the two portions of the bounding surface are parallel, it is an 
immediate consequence of theory that the planes of polarization of one pair are 
parallel to those of the other, while the intensities of the light in one pair is not in 
general the same as in the other, and in fact one or more may vanish without the 
others vanishing. 
These facts were illustrated by an apparatus consisting of a horizontal stage free 
to move round and along a vertical axis ; on this stage a uniaxial or biaxial crystal 
is te and a ray of light is allowed to fall on it through a tube properly adjusted 
and fitted by a cap, in which is a small orifice: at the opposite side of the stage is 
placed another tube properly adjusted; on looking through it, jive images of the 
small orifice in the cap of the first-mentioned tube are seen—one formed by re- 
flection at the upper surface of the crystal, and the other four by the double reflection 
of the two rays refracted at the upper surface. As the stage is rotated the images 
may be four, three, or two; if the cap in the first tube be replaced by a Nicol’s 
prism, the images as the stage is rotated may be four, three, two, or one, The 
Bi ie) e 
