All the iron rings were forged. The cobalt ring was cast.

Four of the iron rings were made from the same bar of Norway iron, and were of nearly the same size. Nos. 1 and 2 are two of them. The third was cooled down to -25° C. or -40° C. by means of liquefied nitrous oxide, and a partial curve taken. The fourth was heated up to 280° C. in liquid paraffine, and a few measurements taken. Although complete curves were not obtained at either of these temperatures, the part obtained differed but slightly from the corresponding parts of the curves of Nos. 1 and 2.

In order to prevent the insulation from being destroyed at the high temperature, the ring was completely covered, except in three places,

with thin asbestos paper, and each turn of the coil was separated from the next by an asbestos string wound around the ring. Only one layer of wire was used. The small uncovered portions of the ring allowed it to take the temperature of the paraffine without materially affecting the uniformity of the winding.

From the curves of the other rings there does not seem to be any definite difference between the values of M due to the variation of the diameter or of the cross-section of the rings.

The temper of the metal seems to exercise a much greater influence upon the magnetic permeability than any other physical condition. This is shown by an inspection of Tables IV., V., and VI. for iron, and XI. and XII. for cobalt.

The iron ring 4 was heated to red heat and allowed to cool slowly, and it gave Table IV. It was then heated and plunged into cold water, and then gave Table V. We wished to see if it would regain its permeability on being softened again. It gave Table VI., in which the maximum value of M is greater than after the first annealing.

The maximum value of M for soft iron is about three times its maximum value for the same iron hardened.

The maximum value of M for soft cobalt is ten times as great as the maximum value for the same cobalt hardened.

SHIELDING OF MAGNETIC INFLUENCE.

We next wished to determine whether or not the outer layers of the ring shielded the inner layers from magnetic influence.

Professor Bosanquet* endeavored to decide this question by comparing the magnetic induction, for equal magnetizing forces, in rings of different cross-section. He said there was no shielding. It is evident from the preceding tables, that the quality of his metal would make such a large difference in the values of the induction as to completely mask the shielding effect if there were any.

We think the method tried by us more suitable to determine the question. A solid and a hollow ring were made from the same bar of iron, and of nearly the same diameter.

The hollow ring was made by cutting off a bar of the proper length, and drilling a hole along its axis. Then the tube thus formed was bent around and forged without closing the hollow. The radius cross-section was 0.698 cm., and the radius of the hollow, calculated from the equation weight divided by the density,

r2 π p2 :

=

*Phil. Mag., February, 1885.

was 0.360 cm. This is what was used in the tables. After completing the experiments upon the hollow ring, it was sawed open, in order to see that it was hollow, and to measure the thickness of the shell. The diameter of the hollow was almost, but not exactly uniform, and the average of the measured values of its radius was a little larger than 0.360 cm. It seemed that the hollow had been made a little larger than the drill at and near the place where the ring had been forged. This would make B, T, and M a trifle larger in Tables IX. and X., and would make the magnetic induction and the permeability more nearly equal to those of the solid ring in Tables VII. and VIII., while it would make the difference in the temporary magnetism of the two rings greater.

The mirror galvanometer used in Tables VII. and IX. was so sensitive that resistances had to be placed in the induced current circuit. This made the deflection of the earth inductor very small, and may probably have introduced an error. But as these tables were made especially to get the difference between the solid and the hollow ring, and as they were tried under exactly similar circumstances, the same errors would appear in both tables.

As far as this single experiment goes, it shows that the temporary magnetism is greater in the hollow than in the solid ring, consequently the permanent magnetism has a larger value in the solid ring. It also shows that the axis of the curve of the hollow ring is more inclined to the axis of M than that of the solid ring is.

JEFFERSON PHYSICAL LABORATORY.

INVESTIGATIONS ON LIGHT AND HEAT, MADE AND PUBLISHED WHOLLY OR IN PART WITH
APPROPRIATION FROM THE RUMFORD FUND.

XXIII.

CONTRIBUTIONS FROM THE PHYSICAL DEPARTMENT OF THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY.

XVII. - PHOTOGRAPHY OF THE INFRA-RED REGION OF THE SOLAR SPECTRUM.

By WILLIAM H. PICKERING.

Communicated May 14, 1884.

IT has been generally assumed, and indeed distinctly stated by Abney and some others, that the gelatine dry plate is insensitive to that region of the spectrum lying beyond A. On trial, however, this proved not to be the case, as the following results distinctly show. It was found that there was a great difference in the plates, those made by Allen and Rowell, and those by Walker, Reid, and Inglis, giving the best results, the latter being somewhat better than the former. This result was indicated by experiments on the sensitiveness of the plates to daylight and gas-light, the two abovementioned kinds being the most sensitive of all to the latter, while only moderately so to the former light. The object of the research was to determine to how great a wave-length the plates were sensitive, rather than to obtain a good representation of the lines. A very broad slit was therefore used, and a camera lens of large diameter and short focus. The condensing lens, collimator, and camera lens were each 10 cm. in diameter, and the last of 30 cm. focus. The first two lenses were each of about 90 cm. focal length. The prism measured 10 cm. on a side, and had a refracting angle of 30°. It was so placed that the rays struck the first surface at a slightly oblique angle, thereby obtaining a dispersion equal to that which would be had ordinarily with a 60° prism, and employing only half the thickness of glass. The camera lens had an angular aperture of 19°, and the slit, as usually used, of 1' 20". It should be stated here, however, that in the earlier experiments, made with a common spectroscope, using the object-glass of the