uring the distribution of energy in its spectrum. This is the dream of the experimenter, and it is the goal toward which he has turned his efforts. But, in order to accomplish useful results, the present investigation shows that the radiation sensitivity must be still further increased by a hundred-fold. In other words, the radiometric outfit (including the reflecting mirror) must be sufficiently sensitive to detect the radiation from a candle at a distance of more than 500 miles. The instrument must be 10,000 times more sensitive than the one used by Nichols fifteen years ago, and perhaps from 100,000 to 500,000 times more sensitive than those used in the earliest attempts by Huggins and others, almost half a century ago. This shows how insignificant has been the gain in sensitivity in comparison with what will be required in order to accomplish much on the radiation from stars. It will be a nerve-racking investigation; but it is not so appalling as the above figures may indicate. Large reflectors are becoming more common every year. However, a very large reflecting mirror may not be desirable. The gain in light-gathering power in a very large reflecting telescope is not at all proportionate to the cost of manufacture and convenience of operation. The writer used a three-foot reflector, which, although operated by hand, could be set very quickly. If a six-foot reflector had been at his disposal the sensitivity would have been increased by only four-fold. This shows how little is contributed by the mirror, and how much of the burden as regards gain in sensitivity falls upon the radiometer.
The aforementioned extra hundred-fold gain in sensitivity required may be attained by the use of a reflecting telescope having a mirror seven feet in diameter, and by increasing the radiometer sensitivity twenty times. If the sensitivity of the thermocouples used in the present work can be increased two-fold, this will leave the galvanometer sensitivity to be increased ten-fold. By using a special pier for the galvanometer, and by using a light galvanometer suspension in a vacuum, it will not be a very difficult task to increase the galvanometer sensitivity ten-to twenty-fold. It looks, then, as though everything were in our grasp—everything except a six-to seven-foot mirror, set apart primarily for astroradiometric work. When one thinks of all the money wasted in idle pleasure, and in the wars of nations, it is pathetic to realize that but for a few hundred thousand dollars the aforementioned "layman" of this generation might live to see something "practical" forthcoming from the investigation of the radiation from stars.
