TRANSACTIONS OF SECTION A. 413 
3. The Instantaneous Propagation of a Disturbance in a Dispersive 
Medium, By Dr. T. H. Havenock. 
4. The Relative Motion of the Earth and Aither and the FitzGerald- 
Lorentz Effect. By C. W. CHaMBrruain. 
To explain the negative results of the Michelson-Morley experiment to 
test the relative motion of the Earth and ether FitzGerald and Lorentz 
suggested that the motion of translation of a solid through ether produces 
a contraction in the direction of the drift, with extension transversely, the 
amount of which is proportional to the square of the ratio of the velocities 
of translation and of light. 
Analysis shows that the total effect of the relative motion of Earth and 
ather is a displacement of the interfering rays in the line of sight, and 
at right angles to the line of sight. The displacement in the line of sight 
should have been detected if it was not counteracted by the FitzGerald- 
Lorentz effect. The displacement at right angles to the line of sight does 
not alter the distance of the interfering rays from the focal plane of the 
telescope, and therefore does not shift the interference system seen in the 
interferometer. 
The displacement at right angles to the line of sight, which is not 
counteracted by the FitzGerald-Lorentz effect, should be detected by means 
of the Diffractometer—a combination of an interference system and a diffrac- 
tion grating. It is proposed to produce interference between two rays of 
light which have travelled paths at right angles to each other. The inter- 
fering rays will be received by a diffraction grating. If one of the paths 
of the interference system is made longer than the other, interference fringes 
may be made to appear either in the spectra to the right or to the left, 
when the lines of the gratings are parallel to the interference fringes. If 
the length of path of the interference system is 55 x10’, as in the 
Morley-Miller experiment, and a grating having 30,000 lines to the inch 
is employed, a shift of one interference band may be expected when the 
apparatus is rotated through ninety degrees, 
5. On some New Methods under Trial for Tables of the Moon’s 
Motion. By Professor E. W. Brown, F.R.S. 
6. A Cemented Triple for Spectroscopic Use. 
By Lieut.-Colonel J. W. Girrorp. 
It was my privilege to exhibit at the Dublin Meeting an apochromatic 
triple for astronomical purposes. This objective had an aperture of three 
inches, a focus of 375 inches, and the focal lengths equalised were for 
wave-lengths 7066 (He), 5067 (Pb), and 4678 (Cd). The method of 
obtaining the refractive indices of the glasses used and the formule used 
for calculations have already been described.! | With this objective it was 
difficult to trace any residual colour, the secondary spectrum having been 
very perfectly eliminated, but the ratio of aperture to focus was as great as 
12°5. For spectroscopic purposes a greater light grip is desirable. This was 
obtained by a different combination of glasses, without, however, seriously 
sacrificing the practical coincidence of focus for all wave-lengths. 
The glasses employed were Mantois’s Borosilicate Crown, Schott’s 031393 
Borosilicate Flint, and Schott’s 03187 Baryta Light Flint. The wave- 
lengths equalised for focus were 7682 (Kd), 5607 (Pb), and 4341 (H). 
1 Proc. R.A,.S., vol. lxix. No. 2. 
