THE SUN'S VISIBLE DISC.
The necessity of having pictures of the sun's disc, which are merely representa tions of the appearance of the surface, has now sunk into a position of secondary im portance. Monochromatic photographs of its surface reveal its chemical state, and the analysis of small areas by means of the spectrometer tells far more of the local characteristics or the nature of any disturbances in progress. Still, as the apparatus for the two latter methods of observation is very complicated, requiring great care and skill for its successful use, ordinary changes in the sun are traced by the usual photographic methods. In the solar surface there are centres of violent disturbance forming great chasms, which are always changing in shape. These parts are termed sun spots, but they are so large at times that they would easily hold many planets like the earth. The sun rotates about its own axis, thus caus ing every part of its surface to be visible in turn, and the spots are seen to be really great hollows, by the saucer-like appear- ! ance they possess when they get to the edge of the disc. The part of the sun appearing on a negative is not a picture of the extreme outside of the sun, but of that part called the photosphere, i.e. of
the part which is at such a high tempera ture and pressure that it forms the source of the light radiation that reaches the earth. The telescope in common use at Greenwich, up to 1897, for photographing the photosphere, is only 4 in. in aperture, and the focal length of the object glass is 5 ft. The telescope and camera together are termed a " photo-heliograph." The image of the sun at the principal focus is about 6"/10 in diameter. The slowest photographic plates are employed, and the exposure is made by means of a slit pulled by a powerful spring, which, of course, gives equal exposure to all parts of the image. To avoid missing the photo graphs on dull days, common enough in this climate, similar instruments in India and Mauritius are engaged in taking duplicates. The measurement of the area covered by the sun-spots is facilitated by using a piece of glass ruled over with • small squares, which can ,be placed over the photograph.