128 Prof. Knoblauch on the Interference of Heat. 
nesses of traversed glass is such that places of greater heat 
become colder, and vice versd. 
3. Difference of phase through unequal reflexion. 
If, according to the principle of the representation of New- 
ton’s rings, solar rays are reflected from a flint-glass convex at 
its lower surface, and from a plane glass under the former con- 
sisting half of flint- and half of crown-glass ; if, further, between 
the two a liquid be introduced which, like clove-oil, is inferior 
to flint-glass, but superior to crown-glass in refracting power, 
the rays will in the one case pass first from a greater to a less 
refracting substance, and then from a less to a greater; whilst 
in the other case these rays will pass twice, successively, from*a 
greater to a less refracting medium. The interference-phzeno- 
mena, which in the first case consist of a series of rings with a 
dark centre, and in the second of a series with a light centre, 
being thrown on a screen by means of a lens, and the screen 
being replaced by a thermo-electric pile, the temperature in the 
one centre is found to be so low that the needle of the multiplier 
is only deflected 0°°5, whilst in the other centre it is so high as 
to cause a deflection of 3°. Laurel, aniseed, calamus, and cassia 
oils deport themselves like clove-oil; whilst with lavender, ber- 
gamot, and citron oil, &c., as also with water and air, their in- 
dices of refraction being even less than that of crown-glass, both 
centres have a lower temperature. 
When the double plate of flint- and crown-glass is replaced by 
one of caleareous spar bounded by the ordinary surfaces of clea- 
vage, two groups of interference-phenomena are also obtained 
by employing the first-mentioned oils, since their indices of 
refraction lie between those of the ordinary and extraordinary 
rays in the calcareous spar; these phenomena can only be sepa- 
rated, however, by interposing a Nicol’s prism between the pile 
and the interference-apparatus. In the one case, corresponding 
to the dark centre, a deflection of 0°25 was obtained; in the 
other case, corresponding to the light centre, a deflection of 2°'5, 
and these according as the principal section of the Nicol’s prism 
and that of the rhomb of calcareous spar were inclined at 90° or 
were parallel to each other. By every position of the Nicol’s 
prism the centre of the rings had the same low temperature 
when, between the convex flint-glass and the calcareous spar, one 
of those substances were interposed whose index of refraction is 
smaller than that of the extraordinary ray in the calcareous spar. 
4, Difference of phase produced by unequal velocities of doubly- 
refracted rays. 
In order to obtain rectilinear bands by means of double refrac- 
tion in the polarizing apparatus, it is best to use two plates of 
