Physics. 223 
this portion of the Blue Ridge has been formerly spanned by 
a grand arch, or series of arches, of Cambrian age, upturned per- 
haps at the time of their upheaval—the broken fragments o 
which have been carried away, and only the abutments left to 
tell the story of a great catastrophe. 
Washington and Lee University, August, 1884. 
SCIENTIFIC INTELLIGENCE. 
I. PuHysics. 
1. Photographing Colored Objects in their Natural Shades.— 
Professor H. W. Voce. has laid before the Physical Society of 
erlin the results of his long continued researches on this subject. 
It is well known that unnatural pictures of colored objects are 
generally obtained by photography, the darkest shades of blue 
appearing white, the brightest tints of yellow and red as black, 
and so on. Professor Vogel has endeavored to render his plates 
less sensitive to less refrangible rays by alloying the silver coat- 
ing with some substance capable of absorbing these rays. He 
has obtained in eosine, and more especially in its various deriva- 
tives, coloring substances which possess merely a broad absorp- 
ro 
tion band in the yellow, and which give the desired results. By 
mixing these bodies in the right proportion on the dry gelatine 
plates, the yellow of the colored objects appeared quite clear on 
the photograph; but the blue was brighter. Thereupon Professor 
Vogel inserted between the object and the camera a yellow glass, 
which partially absorbed the blue rays, leaving the yellow unim- 
paired, In this way photographs can be obtained, in which the 
green and yellow and to some extent the red portions of colored 
objects present the same vivid effects as the originals.—ature, 
June 19, 1884, p. 188. oY, 
2. Measurement of Magnetic Forces by means of Hydrostatic 
Pressure.—G. QuinckE has shown that electric force produces a 
pressure upon insulating liquids placed in an electric field. e 
effect is the production of a tension parallel to the lines of electric 
force, and of a nearly equal pressure at right angles to the lines 
of electric force, which are proportional to the square of the elec- 
tric force at the point of the electric field where the experiment is 
made, and also proportional to the dielectric constant of the fluid 
under consideration. He has now extended this investigation to 
magnetic and diamagnetic fluids placed in magnetic fields. From 
analogy we should have in this case p= a H,’ (Maxwell’s Elec- 
tricity and Magnetism, 2d ed., ii, p. 257, § 642), in which H, is the 
magnetic force at the point in question in the magnetic field, and 
