In stellar photography we must go back to about 185o, when pictures of , Castor and Vega were obtained by Bond and Whipple. Since this time the uses of photography in celestial investigations becomes every year more manifold and apparent. A star may be so weak that the beam of light it emits is too feeble to render it visible even with the most powerful telescope. But this faint beam .---,ii.„ of light, acting upon the more sensitive dry plate, will in time (it may be hours) succeed in making an impression made visible by b development. So it will at once be seen how useful it must be to r astronomers to have the means of recording faithfully the position of stars which he will never see. Some idea of the increase of astronomical knowledge may be gained from the fact that in Cyg nus 17o stars had been carefully mapped out by hand by a lengthy and laborious process. An exposed dry plate, however, revealed 5,000 on a single plate taken in a few hours, and if we were to con struct a telescope that would make these 5,000 stars directly visible to the eye there is no doubt that another dry plate exposed at the \ end of this telescope would reveal many thousands more worlds, rolling and shining beyond, veiled in the obscurity of immeasur able distance.
Not only have many thousands of invisible stars been 0 revealed by photography, but many strange novelties. The great nebula in Andromeda, for instance, was for ages a mystery to astronomers until the photographs obtained by Mr. Isaac Roberts revealed the fact that it was composed of a number of rings of gaseous material apparently in a highly heated ' state, and undergoing some process of transformation Pho tography has, too, been the means of informing astronomers of the existence of a mighty nebula, hitherto unknown, sur rounding the well-known group of stars, the Pleiades.
Not only in recording the position and appearances of heavenly bodies has photography been useful, but also as a co-worker with the spectrum in ascertaining the physical condition of these bodies. A photographic investigation on a novel system of the spectra of all the higher northern stars is now being prosecuted at Harvard College Observatory. The spectra are taken, as it were, wholesale ; a large prism placed in front of the object glass analyses at once the rays of all the stars in the field. Captain Abney's remarkable extension of the powers of the camera
in photographing the red and intra-fed rays has made it possible to explore a vast unknown and forever invisible region of the solar spectrum. And it will soon be possible to ascertain correctly the condition of the various kinds of matter ignited in the solar atmosphere.
The value of photography to astronomy is admirably described in the following words :* " The observer must stand aside while photography takes his place, and works with a power of which he is not capable, and I feel sure that in a very few years the observer will be displaced altogether, while his duty will be done by a new sensitive being—a being not subject to east winds, to temper, and to bias, but one above all these weaknesses, calm and unruffled, with all the world shut out, and living only to catch the fleeting rays of light and tell their story." Among the most recent astronomical discoveries by photography may be mentioned those of Dr. Max Wolf, who, with an ordinary photographic objective, discovered five new planetoids in a .few months, besides a great many nebula. The planetoids appear as streaks on the photo graphic plate, and the length of the streak determines the course during the exposure. By this means photography has been instrumental in discovering and proving the motion of heavenly bodies.
The largest telescope now in existence 08g6) is the Yerkes, with a diameter of 4o inches, thus eclipsing the great Lick telescope by 4 inches.
ASTRO-PHOTOMETRICAL.—Pertaining to the measurement of the light from the stars. See Photometric.
ATMOMETER.—An instrument to measure the quantity of water evaporated in a certain time under ordinary atmospheric condition.
ATMOSPHERE.—The gaseous vapor that surrounds the globe. See Air.
ATOM.—An atom is the smallest particle into which an element can be divided. Theo retically an atom cannot exist in a separate state, but unites with one or more atoms to form a molecule. The atoms of different elements have definite relative weights, fixed and invariable for each. The weight of an atom of hydrogen is regarded as unity. See Chemistry.