8 REPORT—1846. 
bs 
‘ 
colour at very great incidences. The compound pencil, thus reflected and coloured, 
consists of two oppositely polarized pencils,—one polarized in the plane of reflexion, 
and of a pale bluish white colour at all incidences; and the other polarized perpendi- 
cular to the plane of reflexion, and ofa golden yellow colour at small incidences, passing 
successively into a deeper yellow, greenish yellow, green, greenish blue, blue, and 
light pink, as the angle of incidence increases. This very remarkable property, which 
I have discovered also in some other crystals, is not caused by any film of oxide formed 
upon the natural surface of the crystal, nor is it the result of any change produced 
upon the surface by external causes. It is exhibited, under the usual modifications, 
if the surface of the chrysammate is in optical contact with fluids, and, by pressure, with 
’ glass :—and when the crystal is in the act of being dissolved, or when a fresh surface is 
exposed by mechanical means, the superficial action of the crystal upon light is in both 
cases the same. When the chrysammate is re-crystallized from an aqueous solution, it 
appears in tufts of prisms of a bright red colour, the golden reflexion being overpowered 
by the transmitted light ; but when these tufts are spread into a film by pressure, the 
golden yellow colour reappears. When the crystals of chrysammate are heated with 
a spirit lamp, or above a gas burner, they explode with a flame and smoke like gun- 
powder ; and, by continuing the heat, the residue melts and a crop of colourless amor- 
ee th is left. I have found the same explosive property in the Aloetinate of 
otash, 
Description of a Portable Equatorial Stand for Telescopes without Polar 
Axis. By Ricwarp Greene, M.D. 
All previous attempts to produce equatorial motion have (the author believes) been 
based upon the notion that the telescope should revolve upon a material axis, which 
of course must be adjusted parallel to the axis of the earth. The principle also is 
bad, inasmuch as the telescope is supported near the centre, and the moving power is 
applied to that point, instead of the extremity of the tube. 
In following any of the heavenly bodies either to the east or west of the meridian 
with the common stands mounted with altitude and azimuth movements, the observer 
is obliged to keep them both continually in action to prevent the object getting out of 
the field of the telescope. As the effect of these two powers acting at right angles to 
each other is to cause the tube to move in the diagonal between them, it occurred to 
the author that it would be more simple and equally efficacious to employ only one 
moving power in the direction of that diagonal, and thus obtain the same motion by 
one screw, which before was obtained by the two screws worked together. He also 
remarked that when the object is passing the meridian, for a certain time it will re- 
main in the field of view by moving the azimuth screw alone. 
The essential principle of the invention is simply to be able to place the horizental 
or azimuth screw in all situations of the heavenly bodies in a position similar to that 
in which it is placed when an object is passing the meridian, viz. parallel to a tangent 
of the circle the body is describing and touching the circle at the point where the body 
then is. The common azimuth screw fully answers the purpose when the body is on 
the meridian, as the tangent is then horizontal. When the body observed is to the 
east of the meridian, as it is then rising higher every instant, and at the same time 
moving westward, all that is required is to point the adjustible screw, which the author 
calls the equatorial screw, upwards from the observer to such an angle as appears to 
be parallel to the path the body is describing, which, from its altitude and distance 
from the meridian, can be pretty nearly guessed by an astronomer. If he has ele- 
vated the screw to the proper angle, the body will remain very nearly in the centre 
of the field of view during the time he is following it through the length of the screw. 
Tf, however, it appears to sink in field, it shows that the screw is not sufficiently ele- 
vated, and that it rises faster than the axis of the telescope, and he must raise the 
remote end of the screw a little more; if the star rises in the field, of course it shows 
that the screw is too much elevated. By two or three trials the angle may be found 
in less than a minute. If the body viewed be to the west of the meridian, the remote 
end of the equatorial screw is of course to be depressed, pointing downwards from the 
observer. 
The principle of this equatorial movement is easily applied to many of the stands 
now in general use, as well as to the Herschel stand, on which the author first tried it. 
