16 REPORT—1850. 
ness of the field are preserved, and the achromatism is not disturbed. It is however 
desirable to make the inner surface of the field lens a little convex, as the ray now 
passes out of glass into water, and not into air. The Astronomer Royal of Scotland, 
after trying the eye-piece upon Saturn, double stars and clusters, expressed a very 
decided opinion as to its admirable performance generally, as well as the increased 
blackness of the field, owing to the absence of all false light. ‘To avoid some little 
trouble arising from the use of water, the author proposes to substitute glass or 
rock-crystal for the water, and to cement the surfaces together with Canada balsam ; 
but in this case the inner surfaces of the eye and field lens must have a diminished 
radius of curvature. It was added, that the use of an eye-hole, exactly as in the 
eye-piece of a Gregorian telescope, is not only desirable, but for large object-glasses 
indispensable. Without it, the aperture of an object-glass must be reduced to three 
or four inches when turned upon the sun, or the dark glasses will infallibly be cracked ; 
but with it, all injurious heat is stopped out, and the full aperture can be used as in 
the case of a Gregorian of seven or eight inches in diameter. This arises from the 
different refrangibility of the rays of light and heat. In the ordinary use of a prism, 
it is well known that the rays of heat are less refrangible than the rays of light, and 
are in fact at a maximum beyond the red rays of the spectrum; but when the sun’s 
rays are brought to a focus by means of an achromatic object-glass, the author finds 
that the point of most intense heat is within the focus of the compound lens. Ina 
direct experiment with a six-inch object-glass of Tuliey’s, he found that black glazed 
paper was not burnt, but only smoked, when held two inches beyond the focus ; at 
one inch it took fire in thirty-nine seconds, at half an inch in twenty-seven seconds, 
at the focus in twenty-four seconds, at a quarter of an inch within the focus in eleven 
seconds, at half an inch within in fourteen seconds, and at one inch within in nine- 
teen seconds. Hence it follows, from the different position of the principal foci of 
light and heat, that the eye-piece which makes the image rays parallel, leaves the 
hot rays divergent and passing to some extent on the outside of the illuminating 
rays, and the eye-hole becomes essentially important, not only for the general pur- 
pose of stopping out false light, but particularly for stopping out all injurious heat 
during the examination of the sun with large telescopes. 
On the Expansion of Solids by Heat. By Ricuarp Roserts. 
The author stated, that having some years since added the manufacture of clocks 
to his business of machinist, he, in order to be able to construct a good and cheap 
compensation pendulum, consulted tables of the expansion of solids published by 
many eminent men of science, but found them to differ very considerably ; he there- 
fore determined to make a series of experiments upon the various metals specified in 
the annexed tables, in which the rods were all thirteen feet long; the metal and 
glass rods were three-quarters of an inch in diameter, whilst those of wood had a 
cross section of an inch by one and five-eighths. The metal rods were wrapped in 
listing to prevent, as far as possible, any change of temperature during their removal 
from the stove to the measuring apparatus. ‘Lhe first experiment was made about 
6 a.M., summer and winter, the rod being at the temperature of the atmosphere, 
through having lain all night in a shed open at one side. The second experiment 
was made about noon, the rods and thermometer having previously been three or 
four hours ina stove over asteam-engine boiler. To ensure uniformity of expansion 
throughout their length, the rods, whilst in the stove, were placed in a box, which 
was suspended at different heights from the boiler, according to the degree of heat 
required. The rods were taken out of the stove separately and measured, which 
operation, as the apparatus was only a few yards from the stove, occupied only 
forty seconds. ‘The third experiment was made three or four hours later in the day, 
the rods having again been stoved. The apparatus used for measuring was as 
follows: on the outer side of a fire-proof brick building is a flight of steps, the hand- 
rail of which is fastened to the wall (27 inches thick) at an angle of 40° to the 
horizon, a little beyond the lower end of the hand-rail a piece of planed cast-iron is 
wedged securely into the walls, and to it is fastened an angular piece of iron, likewise 
planed, whose higher edge coincides with the middle of the first-mentioned piece, 
and is at right angles to the wall and hand-rail. A little beyond the upper end of 
the hand-rail another piece of cast-iron is wedged into the wall, upon which piece 
eS ll lee 
