Samuel Pierpont Langley . 1834 - 1906

or merely theoretical interest, but that it is justified by the fact that the whole subject of terrestrial radiation, the temperature of the surface of our planet, and the conditions of organic life upon it are intimately related to that of our present research. The entire radiation of the soil of our earth towards space goes on in a spectral region of which we have hitherto known nothing. These observations, in connection with those recently published on invisible spectra and NO. 8 SAMUEL PIERPONT LANGLEY—ABBOT 21 the wave-lengths of extreme infra-red rays " give us our first knowledge of this terra incognita. I say ' knowledge,' with the admission that this knowledge is as yet alloyed with those imperfections which are inherent in the most painstaking work in an utterly new field. All here is so new that the difficulties themselves are of a quite unfamiliar kind ; for it is well to bear in mind that though all our observations, from first to last, are made on an amount of heat which may be well called infinitesimal, it is still the kind of radiations which produce this heat rather than the amount which forms the greatest difficulty. This, as we shall see, is because this heat seems to be largely that absorbed and reradiated from the substance of the lunar soil, and whose temperature is consequently so low as to be in constant danger of being confused with the heat from the terrestrial media it has passed and from the different parts of the apparatus itself—a difficulty which, when the thing in question is to ordinary sense both invisible and inappreciable, constitutes an obstacle almost insurmountable, when we design to go beyond those features which Lord Rosse succeeded in noting. We notice, in particular, that however successfully we may protect our apparatus from the radiations of surrounding objects, we must always, in the nature of the case, either actually or virtually, interpose a screen at intervals to interrupt the heat we are measuring. In ordinary spectrothermal work, as in that on the sun, the radiations of this screen are perfectly negligible, and would be so if the sun's heat, while the same in kind as now, were no greater in amount than the moon's. Here, on the contrary, because they are of the same kind as those radiated from the moon's cold surface, they become of the first importance, so that a special study of the radiation of the screen becomes a necessity. " There are three princi})al methods of investigation : First, the measurement of the total heat of the moon with a concave mirror of short focus, concentrating it as much as possible and admitting the interposition of a sheet of glass to rudely indicate the quality of lunar rays as compared with those of the sun. This method, which was that employed by Lord Rosse, has been very thoroughly practiced here with results which have been partly given in the previous memoir. The second method has been to form, usually with this same mirror, an image of the moon, but this now falls upon the slit of a special spectroscope "See Am. Journ. of Sci., XXXII, August, 1886, 'On Hitherto Unrecognized Wave-Lengths ' ; also an article in Annales de Qiim. et de Phys., 6 ser. T. IX December, 1886, ' Sur les spectres invisibles.' 22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92 provided with a train of rock-salt lenses and a salt prism of exceptional size and purity ; and after expanding this excessively minute heat in this way it has been found possible, with late improvements in the apparatus, to measure by the bolometer the different degrees of heat in the different parts of this lunar spectrum ; and the doing of this, with its results, forms the principal subject of the present memoir. .... Third. Since such a mirror as that just mentioned, owing to its short focus, forms an extremely small lunar image, in certain observations, carried on, however, only during a limited time, we have taken advantage of the sensitiveness of our apparatus to explore a large lunar image with the bolometer in spite of the diminished heat in such a one. For this purpose a special mirror 303 mm in diameter and 3,137 mm focus, giving a lunar image of about 30 mm diameter, has been employed.' On the special occasion of a lunar eclipse the last-named apparatus has also been used. " Let it be remembered that every observation on radiant heat, however conducted, whether by the thermometer, the bolometer or thermopile, on the sun or moon, or on a neighboring candle—every observation in radiant heat, we repeat, involves the use of a screen at some stage in the process ; since its use is inherent from the very nature of the observation. Again, let it be remembered that, in this peculiar case, the screen itself not only intercepts other rays, but contributes radiations of its own of like quality and amount to those which we would study, and the importance of the investigation to be shortly given on its theory becomes manifest. It will be seen later that the screen is used as little as possible, and that to this end every observation on the moon is preceded by one on the adjacent sky to the east and followed by one on the adjacent sky to the west ; and that the lunar radiation is compared in every case immediately with the mean of the last two and only mediately with that of the screen, whose use we might here appear to be able to dispense with, but which is in fact imposed upon us, we repeat, at some time in the course of the observations by conditions inherent in the nature of the observations