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Using a polarising photometer, he found that only 13 per cent. of the violet rays escape at the edge of the solar disc, 16 of the blue and green, 25 of the yellow, and 30 per cent. of the red.
Instruments are all about, on walls, tables, and shelves, the photometer is covered up; induction coils of various capacities, with other electrical paraphernalia, lie around, almost as if the experimenter were absent for a few days but would soon return and resume his work.
With a meridian photometer of augmented power, the surprising number of 473,216 settings were made during the years 1891-98, nearly all by the indefatigable director himself, and they afforded materials for a "Photometric Durchmusterung," published in 1901, including all stars to 7·5 magnitude north of declination -40°. A photometric zone, 20° wide, has for some time been in course of observation at Potsdam by MM. Müller and Kempf.
After all, however, as Professor Pritchard observed, "the eye is the real photometer," and its judgment can only be valid over a limited range. Absolute uniformity, then, in estimates made by various means, under varying conditions, and by different observers, is not to be looked for; and it is satisfactory to find substantial agreement attainable and attained.
His photometer was probably suggested by this appliance. It enables two lights to be compared by the relative brightness of their reflections in a silvered bead, which describes a narrow ellipse, so as to draw the spots into parallel lines.
It was succeeded in 1885 by Professor Pritchard's "Uranometria Nova Oxoniensis," including photometric determinations of the magnitude of all naked-eye stars, from the pole to ten degrees south of the equator to the number of 2,784. The instrument employed was the "wedge photometer," which measures brightness by resistance to extinction.
A wedge of neutral-tint glass, accurately divided to scale, is placed in the path of the stellar rays, when the thickness of it they have power to traverse furnishes a criterion of their intensity. Professor Pickering's "meridian photometer," on the other hand, is based upon Zöllner's principle of equalization effected by a polarising apparatus.
Every star in the manual was listed by light-frequency waves, to be checked against a photometer for a specific reading, and it almost drove Bart mad to go through the ritual when the Mentorians were off duty and could not call off the color and the equivalent frequency type for him.
In order, therefore, to obtain a correct idea of the extent to which this variation of the position of the wick might influence the readings of the photometer scale, I took a continuous number of photographs of the flame of a candle while it was burning in a room quite free from draught; no other person being in it during the experiment except a photographer, who placed sensitive dry plates in a firmly fixed camera, and changed them after an exposure of 30 seconds.
"I wish medic would find a way to keep them alive through warp," he said. "My Mentorian assistant could watch that frequency-shift as we got near the bottom of the arc, and I'll bet she could see it. They can see the changes in intensity faster than I can plot them on the photometer!" Bart felt goosebumps break out on his skin. Rugel spoke as if the certain death of humans, Mentorians, was a fact.
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