Now, allow me to go one step further: suppose that instead of 
a line, whether straight or crooked, we have a slit in the shape 
of a ring, shall we see a ring through the prism? You will see 
that we shall. And then comes this question: If, when we 
work in the laboratory we examine these various slits, illumi- 
nated by these various vapours, why should it not happen that if 
we observe the corona in the same way, we shall also get a ring 
built up by each ray of light which the corona gives to us; 
since we know, from the American observations, that there were 
bright lines in the spectrum of the corona, as observed by a line 
slit? In other words, the corona, examined by means of a long 
train of prisms, should give us an image of itself painted by each 
ray which the corona is competent to radiate towards us. — 
Now let us pass to the screen, the screen merely replacing the 
retina. We will first begin with the straight slit with which 
you are familiar—we now have our slit fairly focussed on the 
screen—we then in the path of the beam interpose one of these 
prisms, and there we get on the screen a bright line. ‘ 
Now, to continue the argument, we replace the straight slit 
by a crooked one, and you see we get a crooked image on the 
screen, We now replace this crooked slit by aring. We have 
now a ring-formed image on the screen. So that you see we 
can use any kind of narrow aperture we choose, and as long as 
we are dealing with light which is monochromatic, or nearly so, 
we get an image of the aperture on the screen, 
If we consider the matter further, it will be evident that we 
may employ a mixture of vapours, and extend this result. 
We will now, for instance, instead of employing sodium 
vapour, employ a mixture of various vapours. You see now 
that each ray given by these substances, instead of building up a 
line image, is building up for us a ring image—that we have 
now red, green, yellow, blue, and violet rings. 
Now that was the consideration which led to the adoption of 
one of the new attempts to investigate the nature of the corona 
used this time. It was, to use a train of prisms, pure and simple, 
using the corona as the slit, a large number of prisms being 
necessary to separate the various rings we hoped to see, by reason 
of their strong dispersion. On thescreen the rings to a certain 
extent intersect each other; and it would have been easier to 
show you the ring-form of the images if we could have used 
more prisms than one. 
If this is good for a train of prisms such as I have referred to, 
_ it is good for asingle prism in front of the object-glass of a tele- 
scope. Such was the method adopted by Prof. Respighi, the 
distinguished Director of the Observatory of the Capitol of Rome, 
who accompanied the expedition, 
Now you may ask how would this method, if it succeeded, be 
superior to the ordinary one? In this way. If we were dealing 
merely with reflection, then all the rings formed by vapours of 
equal brilliancy at the base of the chromosphere would be of the 
same height, while if reflection were not at work, the rings would 
vary according to the actual height of the vapours in the sun’s 
atmosphere, and the question would be still further advanced if 
the spectrum did not contain a ring representing the substance 
which underlies the hydrogen. 
Our ew spectroscopic equipment then was as follows :— 
1. A train of five prisms, 
2. A large prism of small angle placed before the object-glass 
of a telescope. 
3. Integrating spectroscopes driven by clockwork. 
4. A self-registering integrating spectroscope, furnished with 
telescopes and collimators of large aperture, and large prisms. 
(This instrument was lent by Lord Lindsay.) 
Now a word about the polariscopic instruments, referring you 
to my lecture given last year for a general notion of the basis of 
this class of observation. 
A new idea was that observations to determine the polarisation 
of the corona might be made with the same telescope and eye, 
both with the Biquartz and the Savart. 
By the kindness of Mr. Spottiswoode, who has placed his 
magnificent polarising apparatus at our service, I hope to be able 
to show you on the screen the mode of examining the corona by 
means of those two instruments, so as to enable you pretty well 
to follow what was actually done. 
Let me begin with the Biquartz polariscope. In the first 
instance I will throw on the screen a representation of the 
corona itself, and we will then insert a Biquartz, and see its 
effect when I flood the screen with polarised light. You now 
see an indication of what would be observed supposing the 
polarisation was due to polarised light diffused in the region be- 
tween us and the dark moon and eclipsed sun, in which case the 
NATURE 
polariscopic effect would be observed getterally over the dark 
moon, the corona and the region of the sky outside the corona, 
But this is not all ; not only does this arrangement enable us to 
determine the existence of such a general polarisation, but the — 
vertical line in the Biquartz called the line of junction indicates 
the plane of polarisation, when the colours on both sides of it 
are the same; so that we have two colours strongly contrasted 
in either half of the field when we are¥away from the plane of 
polarisation, and a uniform colouring of the whole field when in — 
or at right angles to that plane. By turning this prism through 
go degrees, you see I entirely change the colours. 
But we are not limited to the Biquartz in this inquiry. We 
can apply the Savart polariscope. Having still our image ofthe 
corona on the screen, I now replace the Biquartz by a Savart. 
We now no longer see a line of junction with the similar or 
different colours on either side of it, but lines of colour running 
across the image. Iturn the prism. We first see the lines with 
a white centre, then with a dark one; while at times they are 
altogether absent. And as a departure from the plane, when we 
use the Biquartz, gives us the strongest contrasts of colour, so 
you observe that with the Savart under these circumstances all 
indications of polarisation vanish. 
Now, if we assume polarisation to be general, and the plane of 
polarisation vertical, we should get those coloured bands, as you 
see them there, crossing the corona and dark moon, the lines 
being vertical and dark-centred. If the plane of polarisation 
were horizontal, we should find the lines horizontal and the 
central one white. 
But so far as we have gone, we have been dealing with polari- 
sation which is general, and we have not attempted to localise 
polarisation at the corona itself. But I have here an apparatus, 
by means of which, quietly, in this theatre one can see as 
admirable an example as we should desire of polarisation, 
assumed to be particular to the sun and not general—I mean 
radial polarisation. We have simply a circular piece of mahogany, 
or something else which polarises light equally well, with a hole 
in the middle with sloping sides, cut as you see this cut, and then 
we place behind it a candle, so that the light of this candle, after 
falling on oiled tissue paper stretched across the aperture, can be 
reflected to the eye by the sides, the direct light of the candle 
being stopped by a central metallic diaphragm. We have now 
a source of polarised light of a different kind from thé last. The 
next thing we have to do is to introduce into a small telescope 
exactly the same kind of apparatus we have there, though of 
course on a much smaller scale, and examine the ring of light 
seen when we put the candle behind the aperture. On examining 
the ring of light which is now visible by means of this telescope, 
which contains a Biquartz and analyser, I see the most exquisite 
gradations of colour on either side the line of junction which cuts 
the field of view and the bright ring in the centre into two. 
Now, instead of the candle, we will employ the electric lamp ; 
and instead of the eye, the screen; but I must inform you that 
the great heat of the electric lamp prevents the appearance being 
args successful on the screen, as the reflecting varnish is 
melted. 
In this experiment we cannot work with an image of the 
corona. We must make our corona out of the image of the ring 
we hope to get on the screen ; and then, by employing the 
Biquartz in the same way as before, instead of getting similar 
colours on either side of the line of junction, as we did when 
we were working in the plane of polarisation, and getting the 
greatest contrasts, as we did when we worked 45 degrees away, 
you observe we get different colours in each part of the ring. 
On the screen we now have a highly-magnified image of the 
hollow cone of iron which I am compelling to reflect the light 
from the lamp ; and by inserting this Biquartz I throw various 
colours over different portions of that ring, which I beg you to 
consider for one moment as the solar corona, and the colours 
change as I rotate this prism. You will at once be able to 
explain the different actions of this Biquartz in this instance. 
The reflection, and therefore the plane, of polarisation is no longer 
general, but varies from point to point of the reflecting surface. 
It Ds in fact radial, and hence the delicate radiate arrangement of 
colour. 
Such, then, were some of the new methods and new instru- 
ments we used for the first time in our researches. And I hope 
you will allow me to use this term, although our work was con- 
ducted a long way from the Royal Institution, the natural home 
of research in England. 
(Zo be continued.) 
[Wov. 21, 1842 
