10 REPORT—1845. 
On the Rings which surround the Image of a Star formed by the Object-glass 
of a Telescope. By the Rev. S. Earnsuaw. 
The object is to draw the attention of persons, who take an interest in optical 
phenomena and their theoretical explication, to the fact, that the rings which are 
ordinarily seen round the image of a’star are not the whole of the phenomenon opti- 
cally due to the experiment. Ifa point of sunlight be substituted for the star, ad- 
ditional parts become visible, which before were not appreciable by the eye on account 
of the feebleness of the light emitted by the star. If the experiment-be varied (as was 
done by Fraunhofer and Sir John Herschel) by placing an annular aperture before 
the object-glass, two remarkable changes take place; viz. a great increase in the 
number of rings seen round the central spot, and the spreading of a diffused light 
over the field of view. ‘The diffusion is not uniform, but the brightness decreases 
from the centre to a minimum, then increases to a maximum; then it decreases to 
a minimum, and so on. The black rings occur at tolerably equal intervals, within 
the part comprehended by the first ring of minimum brightness just spoken of. The 
two circumstances which attend the substitution of an annular aperture for a com- 
plete circular aperture were pointed out as phenomena for which the undulatory 
theory has hitherto been unable to account, though the attempt has been made by 
one who has been perhaps more successful than any other person in applying the 
methods and principles of that theory to observed phenomena. 
A reference was also made to Fraunhofer’s experiments with annularvapertures, 
for the purpose of remarking that, on account of the narrowness of his annuli and 
the low power of his telescope, it was not possible for him to observe some part of 
the phenomenon seen by Sir John Herschel ; while, on account of the feebleness of 
starlight, the latter experimenter was not able to see the whole as a single pheno- 
menon, but was induced, by the imperfection of the image presented by his telescope, 
to refer the two portions which he did see to distinct causes. There can be no doubt 
however upon the mind of an observer, who employs sunlight, that the rings and 
halos are parts of a certain phenomenon, the whole of which is due to one cause; 
and as such must be both included in the explanation of theory, before the theory 
can be said to have succeeded. ‘The author considers that its not having been able 
to include these two parts in one investigation, is a sufficient proof that either the 
investigation is defective, or the theory in some of its details erroneous. 
An Improvement in the Method of taking Positive T. albotypes (Calotypes). 
By Sir Davin Brewster, F.R.S. L. §& E., Hon. M.R.LA. 
In the method now in use the face of the negative Talbotype is placed directly 
upon the side of the paper which has been brushed over with a solution of nitrate, 
or ammonia-nitrate, of silver, and which is to receive the positive picture. Instrong 
sunlight the picture is thus taken very quickly; but there is a roughness in the 
shades, owing to the formation of black specks, which destroys the softness of the 
picture, and in portraits gives a disagreeable harshness to the human face. In 
order to remove this defect, the author first interposed thin plates of glass, with their 
surfaces sometimes ground and sometimes polished; but, though the divergency or 
diffusion of the light, passing through the negative picture, produced great softness 
in the positive, yet the outlines were too indistinct, though the Talbotypes looked very 
well, when placed at adistance. He then tried the effect of interposing a sheet of 
writing paper, without the water-mark and of uniform texture. The result of this 
experiment fully answered his expectations. The diffusion of’ the light thus occa- 
sioned shaded off, as it were, all the sharp lines and points, and gave a high degree 
of softness to the picture. The effect was even improved by interposing two sheets 
of clean paper; and, with a very bright meridian sun, he found that three sheets 
may be used with advantage. A similar effect may be obtained, ina smaller degree, 
by placing the back of the negative upon the positive paper, so as to cause the light 
to traverse the thickness of the negative, and this may be combined with one or more 
sheets of clean paper. This, of course, will be appropriate only with portraits; and 
it has the advantage (sometimes required) of making the figure look another way. 
To those who see the experiments above described for the first time, the effect is 
