254 
THE SOLAR ECLIPSE OF 1898 . 
Another instrument that has been successfully employed is a parti¬ 
cular form of spectroscope known as the slitless spectroscope or pris¬ 
matic camera. This instrument depends on the principle that, instead 
of using a slit to get a narrow beam of light, we can make use directly 
of the fine sickle of light surrounding the eclipsed sun, and thus get a 
spectrum composed, not of lines, but of rings, each ring corresponding to 
a bright line in the spectrum of the chromosphere. The great advantage 
of this form of instrument lies in the fact that we can utilise the whole 
amount of light falling on the lens, and we are thus enabled to reduce 
the exposure and take a correspondingly greater number of photographs 
during a given time. 
With an instrument of this class Mr. Shackleton succeeded last year 
in photographing the spectrum of the "reversing layer,” and thus 
proving that it really existed, which was contrary to the opinion of 
some astronomers. 
Several other instruments are also used, but time does not allow us 
to describe them. We must, however, say a few words with regard to 
the mounting of eclipse instruments, obviously a very important 
point. 
In all cases where the exposure is to be something more than an 
instantaneous one some means must be adopted to keep the image in 
a fixed position on the plate. This can be done either by mounting 
the camera equatorially and driving it by a clock to follow the sun’s 
motion, or we may place the telescope in a fixed position and reflect 
the light of the sun into it by a plane mirror. In the latter case the 
mirror only is moved, which, inasmuch as it is much lighter than the 
telescope, is the preferable method. We must, however, have a truly 
plane mirror, which up till lately was not an easy thing to procure. 
How, however, thanks to the great skill of Dr. Common, late President 
of the Royal Astronomical Society, we are in possession of a num¬ 
ber of plane mirrors, whose perfection leaves nothing to be desired. 
Having thus briefly passed in review the various observations of eclipses 
that are made by the use of good instruments, and whose object is to 
elucidate the physical constitution of the solar surroundings, parti¬ 
cularly the corona, we may now say a few words on the secondary 
phenomena. By the term secondary we understand a phenomenon, 
the observation of which has no direct bearing upon solar physics, but 
which mayjbe interesting from other points of view. 
Among these we may mention that appearance known as "Baily’s 
beads.” These are seen just at the moment when totality is all but 
complete. The rapidly narrowing ring of light is seen to run into a 
number of luminous points or drops. The explanation of this lies in 
the fact that the limb of moon is not quite smooth, but presents a 
slightly serrated appearance, which serrations cut into the narrow 
crescent of light. The phenomenon is of no importance, but, as it 
will be found alluded to in many accounts of eclipses, it is as well to 
mention it. 
A more interesting phenomenon, and one of which it would be very 
desirable to secure a permanent record, is that of the so-called shadow 
