
Aug. 31,1871] 

density of the stream diminish as its length increases, its inter- 
Jacing wreaths will give rise to a group of meteor-showers, more 
and more difficult to distinguish from each other, as their 
number becomes greater, until at last the condition of a 
meteor-belt so formed becomes that of innumerable meteor- 
particles revolving in orbits apparently independent of each 
other, and intersecting each other in all possible directions 
within the general boundaries of the elliptic ring. The 
appearance presented by a meteor-group of this description, 
during its first encounters with the earth, will be a periodic star- 
shower (like that of the November meteors), diverging, whenever 
it is visible, from a nearly exact and single radiant point. At 
the end of a certain number of cyclical returns, the star shower 
will be annually visible on a particular date, diverging from the 
same, or nearly from the same, radiant-point, but much less 
abundantly than at first ; and a twin meteor-shower with a time 
of maximum, and a radiant point closely adjacent to the former 
ones will, at intervals, make its appearance with the original 
shower. This also, like the latter, after an equal lapse of time, 
will become annual ; and both diminishing together will present 
the appearance of a double meteor-shower, appearing simultane- 
ously, or very nearly together, with a double or twin radiant- 
point; while at intervals, a third meteor-shower, of the 
same general features as the previous two, will begin to be 
added to the group. Proceeding in this manner, as the 
antiquity of the meteor-ring increases, the star-shower will 
resolve itself intoa more or less well-defined group of slender 
streams, producing alternate short lulls, and flights of meteors 
from a great multiplicity of radiant points, contained within a 
limited region of diffuse or multiple radiation. The ordinary 
appearance of the star-shower on the nights of the 9th and 11th 
of August, answering very closely to the description of a meteor- 
stream in an already far-advanced stage of its development, the 
much higher antiquity of the August than that of the November 
star-shower, already shown by its regular annual return, and by 
the ancient times in which it appears to have been recorded, 
must now also be regarded as satisfactorily confirmed by the fre- 
quently-recorded multiple, and more commonly observed diffuse 
character of its radiation. Among the star-showers of less 
ancient date, of which the November meteors appear to present 
a conspicuous example, Prof. Schiaparelli includes a meteor- 
shower observed by Zezioli on October 12, and two others on 
November 10, 1868 ; one star-shower on each of these dates 
radiating very exactly from points in the neighbourhood of the 
constellation Taurus, as well as the star-shower of October 18, 
and 20, 1864 and 1865, the radiant point of which was very 
exactly marked in those years in Orion. 
Continued observations of the best-known star-showers being 
calculated to afford such important information on the present 
conditions, and on the probable antiquity of their connection 
with the solar system, the committee propose to re-examine the 
principal meteor-showers during the coming year, with suitable 
means for registering the meteors observed on each of the follow- 
ing dates, viz., August 9 to 11, October 18 to 21, November 13 
to 15 (A.M.), December 11 to 13, 1871, and January I to 3, and 
April 19 to 21, 1872, and to determine, as exactly as possible, 
the moments of maximum frequency, the rates of appearance, 
and the principal points of radiation of the meteors visible oa 
those days. 

THE LATE REV. W. V. HARCOURT’S 
RESEARCHES ON GLASS* 
HE subject of the preparation and optical properties of glasses 
of a great variety of chemical positions, formed, fornearly forty 
years, a favourite study with the late Mr. Harcourt. As stated 
in a report published in the British Association Reports for 1844, 
some experiments on the subject were commenced in 1834, which 
he was encouraged to pursue further by a request published in 
the fourth volume of the Transactions of the Association. A re- 
port on a gas furnace, the construction of which formed a pre- 
liminary inquiry, was published in the reports, but the results of 
the actual experiments on glass have never yet been published. 
My own connection with these experiments commenced at the 
meeting of the Association at Cambridge in 1862, when Mr, 
Harcourt placed in my hands some prisms formed of the glasses 
which he had prepared, to enable me to determine their charac- 
ter as to fluorescence. I was led incidentally to observe the fixed 
lines of the spectra formed by them; and as I used sunlight 
* Paper read by Prof, Stokes in Section A, British Association, 1871, 
NATURE 

351 
which he had not found it convenient to employ, I was enabled 
to see further into the red and violet than he had done, which 
was favourable to a more accurate determination of the dispersive 
powers. This inquiry being in furtherance of the original object 
of the experiments, seemed far more important than that as to 
fluorescence, and the increased definiteness caused Mr. Harcourt 
to resume his experiments with the liveliest interest, an interest 
which he kept up to the last. Indeed, it was only a few days 
before his death that his last experiment was made. To show 
the extent of the inquiry I may mention that at least 166 masses 
of glass were formed, and cut into prisms for measurement, each 
mass doubtless involving in many cases several preliminary ex- 
periments, besides discs and masses for other purposes. 
It is well known how difficult it is, in working on a small 
scale, to make glass which is free from strize and imperfections of 
the kind. Of the first group of prisms, 28 in number, 10 only 
showed a few of the principal dark lines of the solar spectrum ; 
the rest had to be examined by the bright lines in artificial 
sources of light. These prisms seemed to haye been cut at ran- 
dom by the optician from the mass of glass furnished to him. 
Theory and observation alike showed that striz interfere com- 
paratively little with an accurate determination of refractive in- 
dices when they lie in planes perpendicular to the edge of the 
prism. Accordingly, in the rest of the research the prisms were 
formed from the glass mass that came out of the crucible by cut- 
ting two planes passing through the same horizontal line a little 
behind the surface, and inclined 223° right and left of the vertical, 
and polishing the enclosed wedge of 45°. In the central portion 
of the mass the strize have a tendency to arrange themselves in 
nearly vertical lines by the operation of currents of convection, 
and by cutting in the manner described the most favourable 
direction of the strize is secured for a good part of the prism. 
This attention to the direction of cutting, combined no doubt 
with increased experience in the preparation of glass, was attended 
with such good results that now it was quite the exception for a 
prism not to show the principal dark lines. Some of the latest 
prisms were almost equal to prisms of good optical glass, 
On account of the difficulty of working with silicates, arising 
from difficult fusibility and the pasty character of the glasses, 
Mr. Harcourt’s experiments were carried on with phosphates, 
combined in many cases with fluorides and sometimes with 
borates, tungstates, molybdates, and titanates. The glasses 
formed involved the elements potassium, sodium, lithium, barium, 
strontium, calcium, glucinium, aluminium, magnesium, manga- 
nese, zinc, cadmium, tin, ‘lead, thallium, nickel, chromium, 
uranium, bismuth, antimony, tungsten, molybdenum, titanium, 
vanadium, phosphorus, fluorine, boron, and sulphur. A very 
interesting subject of inquiry presented itself collaterally with the 
original object, namely, to ascertain whether glasses could be 
formed which would achromatise each other so as to exhibit no 
secondary spectrum, or a single glass which would form with 
crown and flint a combination achromatic in that sense. This 
inquiry presented considerable difficulties. The dispersion of a 
medium is small compared with its refraction, and if the dis- 
persion be regarded as a small quantity of the first order, the 
irrationality between the two media may be regarded as depend- 
ing on small quantities of the second order. If striae and imper- 
fections of the kind present an obstacle to a very accurate deter- 
mination of dispersive power, it will readily be understood that 
the errors of observation thus occasioned go far to swallow up 
the small quantities, in the observation of which the determi- 
nation of irrationality depends. Accordingly little success 
attended the attempt to draw satisfactory conclusions as to irra- 
tionality from the direct observation of refractive indices ; but 
by a particular mode of compensation, in which the experimental 
prism was achromatised by a prism built up of a combination of 
slender prisms of crown and flint, I was enabled to draw trust- 
worthy conclusions as to the character, in this respect, of these 
prisms, which were good enough to show a few of the principal 
dark lines of the solar spectrum. 
Theoretically any three different kinds of glass may be made to 
form a combination which shall be achromatic as to-secondary as 
well as primary spectra ;_ but for a long time little hope of a prac- 
tical solution seemed to present itself. A prism containing molyb- 
dic acid was the first to give fair hopes of success. Mr. Harcourt 
warmly entered into the subject, which he prosecuted with un- 
wearied zeal. The earlier molybdic glasses prepared were many 
of them rather deeply coloured, and most of them of a perish- 
able nature. At last, after numerous experiments, molybdic 
glasses were obtained nearly free from colour, and permanent. 
Titanium had not yet been tried, and about this time a glass 

