ON THE TOTAL SOLAR ECLIPSE OE AUGUST 29, 1886. 
321 
during expo-sure. The result of the shake is that there are two distinct sets of images, 
corresponding to two positions of the instrument. The second set of images is far 
more dense than the other, and was alone used in tlie measurements. The images in 
this case are lengthened out in one direction, either by vibration or through imperfect 
movement of the clock, and prominence No. I. presents a curious appearance in conse¬ 
quence. The image is wide, and is also separated into two parts by a band of vdrite, 
giving the appearance of another shake, but a comparison with other photographs shows 
that it is in reality due to the peculiar formation of this prominence. In this instance 
this white band was used in measuring the distances of the images. The first set of 
images is much more faint than the other, and, as far as tire prominences are con¬ 
cerned, is only visible as the H and K lines of No. I. prominence. The existence of 
two segments of circles has been mentioned; tliese were extremely puzzling at first, 
until it was noticed that they coincided in position with this at first unobserved faint 
set of images; that is to say, that if the circles had been completed they would have 
run through the bases of these prominences. No other rings corresponding to other 
lines can be seen. The H and K lines are in all cases by far the strongest, and it is, 
therefore, not surprising that these should be visible when the exposure is not sufficient 
to produce any others. But it is remarkable that, in the second or stronger set of 
images, the H and K lines of the same prominence should not have rings corresponding 
to them, for they are far more dense; that is to say, it would appear at first sight 
that the same object produced an image during a short exposure, and yet failed to 
produce one during a considerably longer exposure. It is probable from its position 
that this bright inner circle was covered by the Moon during the period when the one 
set of images was being formed, but was visible during the other exposure, wliich is 
thus presumed to be the first. It is also to be observed that the part of the limb 
where these rings occur was that which was least obliterated by clouds at the 
commencement of totality. 
VIII. The Spectroscopic Cameras. By Arthur Schuster. 
1. Resolving Power of the Instruments. 
In order ro form a clear idea of the amount of detail we may expect in a photograph 
of a spectrum, it is necessary to enter further than is generally done into tlie details of 
the construction and adjustment of the instruments used. 
Lord Rayleigh has given the theory of the spectroscope for the case of a sufficiently 
narrow slit. It is easy to extend his results so as to apply also to the case of slits of 
which the width cannot be neglected. If R is the horizontal width of the beam 
entering the prisms (the refracting edge being supposed vertical), r that of the beam 
leaving the prisms, the equation Sf = R8iyr defines the angle through which the 
emergent beam is turned, owing to a small rotation hi of the incidental beam. If a is 
MDCCCLXXXIX.—A. 2 T 
