Taytor—On the Photographing of the Solar Corona. 277 
in the present state of photography. That such lenses can be 
easily and efficiently mounted and used was clearly shown by 
Professor Schceberle ; and it seems to me, after the results of 1893, 
that it is only by such object-glasses as I have indicated that such 
an important occurrence as a total solar eclipse should be attacked 
in future, if we wish to add to our knowledge of the corona. 
There does not seem to be any great difficulty in mounting 
these long-focus instruments. Professors Todd and Bigelow at 
Cape Ledo, 8. W. Africa (1889, Dec. 21-22), adopted a tripod 
stand, with a sand piston for one leg, which was so arranged that 
the shortening of the leg as the sand ran from under this piston 
caused the telescope to be moved to follow the sun during the 
eclipse. As this necessitated the use of a long tube, supported 
on what looked to me an unstable mounting which would certainly 
shake in a wind, I do not think any great success can be anticipated 
from this form; certainly the risks of failure would be greater 
than if some form of fixed tube were adopted. 
We can use a fixed tube in any of three ways :— 
(a) With a heliostat, in which a mirror is carried on a polar 
and declination axis, and is driven by clock-work, so as to throw 
the sunlight always in the same direction, ¢.e., into the horizontal 
telescope tube. 
(6) With a mirror mounted on a polar axis driven by clock- 
work, the tube of the telescope being placed in the meridian, and 
inclined at an angle equal to the latitude of the place of obser- 
vation. 
(c) On the plan adopted by Professor Schoeberle, with the 
tube adjusted in altitude and azimuth so as to point to the sun at 
mid-eclipse, the photographic plate being moved so as to follow 
the sun during totality. 
The first two of these plans are open to the objection that the 
sun’s image, as reflected from a mirror, rotates as the polar axis 
carrying the mirror is moved in right ascension. This rotation 
would amount to over 20 minutes of arc in 100 seconds’ exposure, 
and would probably be sufficient to blur the finest details of the 
inner corona. The first is open to the additional objection that 
with a heliostat-mounting the motion of the polar axis is commu- 
nicated to the mirror by means of a jointed rod, and the motion 
through this is a series of slips, and not the perfectly even motion 
