1919.]} The Sixth Indian Science Congress. cxv 
If, for instance, as was suggested by Fitzgerald, either of the 
tubes contracts in the process of the experiment by a suitable 
amount, a contraction which will again require explanation in 
its turn, the null-effect is cage but if this is the case it 
follows that it will not be ible, by means of any such 
experiment, to find this slative velocity, if it exists at all. 
Moreover, experiments show that no optical, electrical or 
mechanical effects are observable that can be ascribed to the 
contraction, on Fitzgerald’s hypothesis, so that there must be 
causes nullifying the effect of contraction, as deep-seated as 
e mechanism of these phenomena themselves. We have 
seen that neither Maxwell’s simple theory nor Hertz’s modifi- 
cation of it for a moving media is competent to account for 
aberration, even up to first order effects, as observed in 
s experiments. With the help of the electron theory, 
however, the Fresnel formula can be obtained on the simple 
postulate that the total differential with regard to time, 
involves the velocity of the moving medium, as in hydro 
dynamics. ut in order to explain the null- effect of Misheleon 
and Morley’s experiments, we have to adopt the procedure of 
Lorentz and Larmor. This is to transform the equations of 
the electro-magnetic field, in terms of new co-ordinates referred 
to moving origin; the new X, Y, Z, T being related to the 
the equations remains unchanged. This requires not only that 
the new co-ordinates should involve To, but that the new T 
should involve Xo, Yo, Zo. This amounts to the assumption 
that the charge of an electron and the associated magnetic 
field are invariants for all such transformations. It is then 
found that the Fitzgerald contraction can be explained as 
well as the various null effects, already referred to. 
An altogether different and novel pee here of these has 
this form, the principle does, by no means, appear to be new. 
But the special implication contained in the principle is that 
time in the moving system depends on the time as well as the 
corresponding scan ah emmy of the first cog as in the 
