The slit of the spectroscope can be moved anywhere on the disk of the sun; so that if the observer sees a tornado begin, he moves the slit along with it, measures the length of its tract and velocity. With the telescope, micrometer, heliostat, and spectroscope came desire for more complex instruments, resulting in the invention of the photoheliograph, invoking the aid of photography to make permanent the results of these exciting researches. This mechanism consists of an excessively sensitive plate, adjusted in the solar focus of the telespectroscope. In front of the plate in the camera is a screen attached to a spring, and held closed by a cord. The eye is applied to the spectroscopic end of the complex arrangement to watch the development of solar hurricanes. Finally an appalling outburst occurs; the flames leap higher and higher, torn into a thousand shreds, presenting a scene that language is powerless to describe. When the display is at the height of its magnificence, the astronomer cuts the cord; the slide makes an exposure of one-three thousandth part of a second, and an accurate photograph is taken. The storm all in rapid motion is petrified on the plate; everything is distinct, all the surging billows of fire, boilings, and turbulence are rendered motionless with the velocity of lightning. At Meudon, in France, M. Janssen takes these instantaneous photographs of the sun, thirty inches in diameter, and afterward enlarges them to ten feet; showing scenes of fiery desolation that appalls the human imagination. (See address of Vice President Langley, A. A. A. S., Proceedings Saratoga Meeting, p. 56.) This huge photograph can be viewed in detail with a small telescope and micrometer, and the crests of solar waves measured. Many of these billows of fire are in dimensions every way equal in size to the State of Illinois. Binary stars are photographed so that in time to come they can be retaken, when if they have moved, the precise amount can be measured. Another instrument is the telepolariscope, to be attached to a telescope. It tells whether any luminous body sends us its own, or reflected light. Only one comet bright enough to be examined has appeared since its perfection. This was Coggia's, and was found to reflect solar from the tail, and to radiate its own light from the nucleus.