Chase.) 590 {April 18, 
Fra. 6. Palatopterygoid and mandibular arches of a fourth individual 
from right side, with Am, hyomandibular. 
Fie. 7. Superior tooth of external row, without apices of two of the 
cusps ; from the palatine bone of the specimen represented in fig. 5 ; one- 
half larger than nature, anterior view. * 
Fria. 8. Tooth of Didymodus platypternus Cope, nat. size, from above 
posteriorly. 
Fra. 9. Tooth of a second specimen of Didymodus platypternus from 
below. 
Photodynamic Notes, IX. By Pliny Harle Chase, LL.D. 
(Read before the American Philosophical Society, April 18, 1884.) 
411. _dithereal Oscillation. 
Some readers of the Photodynamic Notes have found a difficulty in ap- 
plying the laws of pendulum oscillation to the undulations of the luminifer- 
ous wether. It is well to guard against the conception of material pendu- 
lums, hung in or across the solar system, but it is also well to remember 
that the modern theories of molecular motion explain the rigidity of steel, 
and of all other solids, by the rapidity of motion, in ultimate discrete par- 
ticles. If this view is correct, all changes in molecular movement are 
probably transmitted in and through the same elastic medium as the un- 
dulations of light, and all oscillations are in some way dependent on ethe- 
real oscillations. 
412. Illustrations of Nodal Tendency. 
The well-known experiments of placing bits of paper on vibrating 
strings, sprinkling sand on Chladni plates, depositing fine powders in 
transparent musical tubes, and eliciting musical notes from glass vessels 
which are partly filled with water, illustrate the tendency of all vibrations 
to drive material particles towards musical nodes. These nodes are sub- 
ject to the same laws of inertia which determine centres of oscillation in 
ordinary pendulums. The nodal tendency is greatest where the relative 
elasticity and the consequent undulatory velocity are greatest. As we 
know of no other medium in which the ratio of elasticity to density is so 
great as in the luminiferous «ther, we can reasonably look in no other 
direction for such striking evidences of rhythmic influence as are to be 
found in cosmical and molecular arrangements. 
4138, Atthereal Rotation. 
The supposed properties of the luminiferous ether are so similar, in 
many respects, to those of ordinary gases, that we may suppose it to act 
and react on all grosser forms and aggregations of matter. The rotations 
and revolutions of suns, planets and satellites are not only in harmony 
