744 
DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
4 o? sin 6 sin 20), 
9 'q being the energy at tlie same ]:)lace when there was no drift. 
So the energy received per second by a given small area A at that place, facing the 
sonrce, i.e., normal to the rays, is 
r/V A cos e = P (cos e + a cos 6)^. 
q = 
P (cos e + a COS 6 )" 
TttY;- 
Pf) (I + 2a cos 0 a^ cos 20 — 
The radiation at distance r from the source is, in fact, the same as what the 
radiation would be at distance p in a stationary medium ; except for the small 
inclination c. 
So a pair of similar thermopiles, fore and aft, at equal distances from a source, 
will, or. this hypothesis, receive unequal radiation ; the difference being equal to 
4a (PA/47rr~), or proportional to 4a. 
Ftzeau suggested this method, Imt I am not aware of its having been tried yet.* 
Fig. 5. 
Tliermopile experiment suggested by Fizeau ; in two alternative forms. 
19. But it is a serious question whether the reasoning establishing the effect is 
quite sound. It is not unlikely that motion may affect the radiating power of a 
source. In fact, the theory of exchanges almost necessitates something of tlie kind, 
else the two faces of an enclosure woidd become unequal in temperature by reason of 
mere motion through the ethereal medium. 
Hence, if, as in fig. 5, we consider a pair of thermopiles with a hot body half-way 
* The suggestion is quoted in a comprehensive, but to me not very intelligible, treatise on the whole 
subject of aberration : ‘ Astronomische Undulationstheorie,’ by Professor Dr. Kettelek, of Bonn. 
t Balfour Stewart (‘ Brit. Assoc, Report,’ 1871, Sects, p. 45), argued that this inequality of tempera¬ 
ture actually occurred; and, since motion thus afforded an available heat engine, he deduced an 
ethereal friction, dissipating energy. But, as Lord Rayleigh points ont (in his Article on “Aberration,” 
‘ Nature,’ March 1892), it is far more likely that motion should alter radiating and absorbing powers 
than that it should disturb equality of temperature. 
