630 FRAGMENTS OF SCIENCE. 
transverse to the direction of propagation. The example 
of sound was at hand, which was a case of longitudinal 
vibration. Now the substitution of transverse for longi- 
tudinal vibrations in the case of light involved a radical 
change of conception as to the mechanical properties of the 
luminiferous medium. But though this change went so 
far as to fill space with a substance possessing the proper- 
ties of a solid, rather than those of a gas, the change was 
accepted, because the newly discovered facts imperatively 
demanded it. Following Mr. Marti neau's example, the 
opponent of the uudulatory theory might effectually twit 
the holder of it on his change of front. " This ether of 
yours," he might say, " alters its style with every change 
of service. Starting as a beggar, with scarce a rag of 
' property' to cover its bones, it turns up as a prince 
when large undertakings are wanted. You had some show 
of reason when, with the case of sound before you, you 
assumed your ether to be a gas in the last extremity of 
attenuation. But now that new service is rendered 
necessary by new facts, you drop the beggar's rags, and 
accomplish an undertaking, great and princely enough in 
all conscience; for it implies that not only planets of 
enormous weight, but comets with hardly any weight at all, 
fly through your hypothetical solid without perceptible 
loss of motion." This would sound very cogent, but it 
would be very vain. Equally vain, in my opinion, is Mr. 
Martineau's contention that we are not justified in modify- 
ing, in accordance with advancing knowledge, our notions 
of matter. 
Before parting from Professor Knight, let me commend 
his courage as well as his insight. We have heard much 
of late of "the peril to morality involved in the decay of 
religious belief. What Mr. Knight says under this head is 
worthy of all respect and attention. ' I admit," he-writes, 
"that were it proved that the moral faculty was derived- as 
well as developed, its present decisions would not be 
invalidated. The child of experience has a father whose 
teachings are grave, peremptory, and august; and an 
earthborn rule may be as stringent as any derived from a 
celestial source. It does not even follow that a belief in 
the material origin of spiritual existence, accompanied by 
a corresponding decay of belief in immortality, must 
necessarily lead to a relaxation of the moral fiber of the 
