300 CAPTAIN' L. DARWIN, DR. A. SCHUSTER, AND MR. E. W. MAUKDER 
Hitherto we have put K = ; but supposing the rate of the clock is such that 
during the exposure K = 20, which was very nearly the case during the late eclipse, 
the above equation shows that the effective aperture was reduced to half, or that the 
photographs could not show anything which an eye-observer with a telescope of 
2-inch aperture might not have seen. 
Let us now turn our attention to the resolving powers which have hitherto been 
actually obtained in photographs taken of the corona. 
During the Eclipse of 1871 two prominences were separated in a photograph, 
which were at a distance of 15 seconds, and the corona itself gives no evidence of 
a finer structure. The aperture used was 4 inches.'" 
In the corona photograph of 1882 the diameter of one prominence subtends an 
apparent angle of about 15 seconds. As far as the photograph is concerned, we have 
no reason to suppose that an aperture resolving two stars at a distance of 15 seconds 
could not have shown everything that is seen on the photograph, for no detail of the 
corona gives evidence of smaller structure. This gives an aperture of 2 inches as 
sufficient to resolve all the detail shown in this photograph. 
An examination of the drawing made by Mr. Wesley of the present eclipse gives 
substantially the same result. 
The 4-inch glasses used in the eclipses which I have mentioned all give, therefore, 
a resolution equal to that obtainable with a 2-inch aperture on a stationary 
object. 
During the last eclipse the regularity of the clock motion was, as we have seen, not 
sufficient to give more perfect images ; the same cause has very likely stood in the 
way of more perfect images during previous echpses. We have at present no reason 
to suppose that the corona possesses any structural detail that could not be seen with 
an aperture of two inches, and it wmuld be very desirable if eye observations could be 
taken at some future eclipse which would give us some idea of the detail of the 
structure visible with larger instruments. 
But it is very remarkable that the definition obtained is just what we should have 
to expect, if we take the error of clock motion into account. If the motion of the 
clock is regular, we should by shorter exposures get in great part over the difficulty of 
adjustment, but in the instruments which have been at our disposal the mechanism of 
transmission from the clock to the telescope is not as perfect as it might be made, 
and the irregular vibrating motion produced by this cause 'would be sufficient to 
damage the sharpness of the image. 
There does not seem to be any advantage at present in using larger apertures in 
future eclipses, at least if these larger apertures are accompanied by increased focal 
length. Most of the adjustments have to be more accurate for them, in order to get the 
advantage of the increased aperture, and the difficulties of mounting and adjusting are 
* Owing to an oversight, the aperture was in the Preliminary Account stated to be 2 inches. The 
focal length of the lens was only half that used during the later eclipses, but the aperture was the same. 
