Beyond this distance the image would be less than the junction. The conclusions to be drawn from the foregoing examples are: (1) that the amount of energy received by the junction does not vary, provided the image overlaps it; and (2) that the limiting distance at which a correct reading can be secured is that at which the size of the image is equal to that of the junction. Thus, taking distances of 10 and 20 feet, as in Example II; at the former distance the energy striking the mirror is four times as great as with the latter; but, on the other hand, the area of the image at 10 feet distance is four times as great as that obtained at 20 feet. Hence, at the greater distance, the proportion of the image impinging on the junction is four times as great, and the fact that only ¼ the amount of energy strikes the mirror is thus counterbalanced. All the reflected rays which fail to strike the junction are ineffective, and pass out through the entrance of the tube. [Illustration: FIG. 46.—FÉRY’S SPIRAL.] The two-scale form of instrument described above is extremely useful for general purposes, but when all the temperatures to be controlled fall within the limit of one of the scales, it is simpler and cheaper to dispense with the diaphragm, and to use an indicator furnished with one scale only. The single-scale mirror pyrometer is for this reason more generally employed for industrial purposes; and the Cambridge and Paul Instrument Company now make a pivoted indicator for use with full aperture, which is less liable to damage than one which possesses a suspended coil. [Illustration: FIG. 47.—FÉRY’S SPIRAL PYROMETER. SECTION.] =Féry’s “Spiral” Radiation Pyrometer.=—This instrument differs from the preceding merely in the fact that the rays are focused on a small spiral, formed of a compound strip of two metals, fixed at one end and furnished with a pointer at the free-moving end (fig. 46). The effect of alterations of temperature on this spiral are to cause it to coil up or uncoil, according to whether the temperature rises or falls. This movement is magnified by the pointer, the end of which moves over a dial graduated to read temperatures directly. This arrangement is shown in section in fig. 47, where C is the mirror, E the eye-piece, S the spiral, P the pointer, and D the dial, viewed through the window W. The appearance of the apparatus when viewed from the front is shown in fig. 48. The advantage gained by the use of the spiral is that the instrument is self-contained, no galvanometer being necessary; but, on the other hand, the indications are not so exact, an error of 20° C. being probable at temperatures over 1000° C. In using this pyrometer, it is observed that after focusing the hot substance, the pointer moves rapidly for a time and then pauses, after which it again commences to creep along the scale. The temperature indicated at the moment the pause occurs is generally taken as the reading, but this is not always correct.