v.v. 



KNOWLEDGE. 



[June 1, 1896. 



Professor Turner will use well-tried instrumentB, which, 



although yiclcliiiL,' pictures on a small scale, will K've 

 results strictly comparable in every particular with those 

 of former eclipses. The images will be small and bright, 

 and a special endeavour will be made to register the very 

 delicate regions of the outer corona. One set of pictures 

 will be on a scale of a half-inch to the sun's diameter, and 

 another one and a half inches, the latter being enlarged 

 by a " telophoto " lens. 



At the Norwegian station the arrangements for photo- 

 graphing the corona will be in the capable hands of Dr. 

 Common. Part of his outfit will probably consist of a 

 modified Casscgrain reflector, giving large images of the 

 corona ; and other instruments which he will employ will 

 give relatively small but bright images similar to those 

 which Prof. Turner hopes to secure in .Japan. 



During the coming eclipse there will be a new departure 

 in the instrumental arrangements for photographing the 

 corona. In place of the equatorials which have almost 

 invariably been employed up to the present time, the 

 photographic telescopes will be fixed, and will receive the 

 light of the corona after reflection from the mirror of a 



the mouth of the Obi, and at Enontekis clockdriven 



cameras will be employed. 



If the photographs of the corona are as successful as 

 everyone desires, they should throw light on various points 

 of solar physics. The larger pictures should especially 

 enable us to learn something of the solar currents, "lines of 

 force," or whatever it may be that determines the peculiar 

 structure of the solar appendages. Tiie distribution of 

 coronal matter in different solar latitudes should also be 

 revealed, and an investigation of its connection with the 

 sunspot zones will become possible. Another point on 

 which the photographs permit inquiry is the low of decrease 

 in the intensity of the corona in passing outwards. Finally, 

 if success is met with all along the line, it will be possible 

 to determine if the corona changes its form in the interval 

 of about one and a half hours intervening between the 

 observations in Norway and Japan. It may be remarked, 

 however, that previous experience does not favour the idea 

 of such rapid changes. 



In addition to the photographic records of the appearance 

 of the corona, there will doubtless be many attempts at 

 sketching. Chief among the workers in this field will be 



Fig. 3. — Photogi-aiili of Eclipsed Sun, April 16tli, 1893. Taken with Six-inoli Prismatic Camera in West Africa by Mr. Fowler, 



" coelostat." The new instrument is an exceedingly 

 portable form of heliostat which has recently been brought 

 to the front by M. Lippmann. It consists simply of a 

 mirror which is made to revolve on a polar axis in its own 

 plane at the rate of one revolution in forty-eight hours, in 

 the same direction as the apparent diurnal motion of the 

 heavens. In a mirror so mounted the image of any 

 celestial object whatever appears stationary, and a tele- 

 scope pointed at the mirror in any direction will have a 

 constant field of view. 



The ofhcial observers, however, will not have the photo- 

 graphic field to themselves, although they will naturally 

 have a monopoly of work which can only be done by large 

 instruments. Many members of the British Astronomical 

 party will make use of their cameras and telescopes, and 

 Mr. Lunt will employ the three-inch photographic telescope 

 with which he has been so conspicuously successful in 

 stellar photography. The American observers in Japan 

 will be chiefly occupied in photographing the corona on a 

 large scale with objectives of long focus, and the same 

 method will be adopted by the Russian observers at 

 Olel.m'nsk. Several ordinary cameras will be at work at 



Mr. N. E. Green, a gentleman of high artistic ability, who 

 is well known to the readers of Knowledge. 



We now come to the spectroscopic part of the attack. 

 The spectrum of every prominence, every part of the visible 

 chromosphere, and of as many regions of the corona as 

 possible is to be observed or photographed, in order that 

 the duty of fully recording the phenomena shall be faith- 

 fully accomplished. It is true that the chromosphere and 

 prominences can be investigated without an eclipse, but it 

 would be unwise to completely neglect them in the bslief 

 that we perceive everything connected with them in our 

 daily observation?. At the same time, as the corona can- 

 not yet certainly be observed at all except during an 

 eclipse, and the lower reaches of the chromosphere only 

 very imperfectly, it is to these features that attention will be 

 chiefly directed. 



For recording the spectroscopic appearances, instruments 

 of two forms are employed — those which are provided with 

 slits, and those which utilize the eclipsed sun itself as a 

 virtual slit in the form of a ring. 



The ordinary slit spectroscope may be used in either of 

 two ways. It may be employed as an integrating spectro- 



