

352 
NATURE 

containing titanium was prepared. Titanic acid proved to be 
equal or superior to molybdic in its power of extending the blue 
end of the spectrum more than corresponds to the dispersive 
power of the glass ; while in every other respect—freedom from 
colour, permanence of the glass, greater abundance of the element 
—it had a decided advantage; and a great number of titanic 
glasses were prepared, cut into prisms, and measured. Some 
of these led to the suspicion that boracic acid had an opposite 
effect to titanic, to test which Mr. Harcourt formed some simple 
borates of lead, with very varying proportions of boracic acid. 
These fully bore out the expectation ; the terborate, for instance, 
which in dispersive power nearly agrees with flint glass, agrees 
on the other hand in the relative extension of the blue and red 
ends of the spectrum with a combination of about one part (by 
volume) of flint glass with two of crown. 
By combining a negative (or concave) lens of terborate of 
lead with positive lenses of crown and flint, or else a positive 
lens of titanic glass with negatives of crown and flint, or a posi- 
tive of crown and a negative of low flint, achromatic triple com- 
binations free from secondary dispersion might be formed, with- 
out encountering formidable curvatures, and by substituting at 
the same time a borate of lead for flint glass, and a titanic glass 
for crown, the curvatures might be a little further reduced. 
There is no advantage in using three different kinds of glass 
rather than two, to form a fully achromatic combination, except 
that the latter course might require the two kinds of glass to be 
made to order, whereas with three we may employ for two of 
them the crown and flint of commerce. It is probable that | 
enough titanium might be introduced into a glass to allow the 
glass to be properly achromatised by Chance’s “*light-flint.” 
In a triple combination of lenses the middle lens may be made 
to fit both the others, and be cemented. ‘Terborate of lead, 
which is somewhat liable to tarnish, might thus be protected by 
being placed in the middle. Even if two kinds only of glass 
be used it is desirable to divide the concave lens into two for 
the sake of diminishing the curvatures. On calculating the cur- 
vatures so as to destroy spherical as well as achromatic aberra- 
tion, and at the same time, to make the adjacent surfaces fit, | 
very suitable forms were obtained with the data furnished by Mr. 
Harcourt’s glasses. | 
After encountering great difficulties from strie, Mr. Harcourt 
at last succeeded in preparing discs of terborate of lead and of | 
a titanic glass, of about 3in. diameter, almost homogeneous, 
and with which it is intended to attempt the construction of an 
actual object glass, which shall give images free from secondary | 
colour. 
This notice extends to a greater length than I had intended, 
but still it gives only a meagre account of a research extending | 
over so many years. It is my intention to draw up a full 
account for presentation to the scientific world in another way. 
I need but say that the small grant made to Mr. Harcourt for 
these researches has been expended over and over again, but it 
was his wish, in recognition of that grant, that the first notice of 
the results he obtained should be made to the British Associa- | 
tion, 



THE BRITISH ASSOCIATION MEETING AT | 
EDINBURGH 
SECTION A. 
| the meridian. 
what Mr. Lockyer and M. Janssen had said. M. Janssen had 
asked that Britain should join France and Germany in their 
friendly struggle. There was a challenge from France and Ger- 
many, and it would be a disgrace to England if it did not accept 
that challenge, and do its best to beat its rivals in the struggle. 
Report of the Committee for discussing Observations of Lunar 
Objects suspected of Change. 
Mr. W. R. Birt, to whom the execution of the work was 
confided, read the report on behalf of the committee, consisting of 
Edward Crossley, Esq., and the Rey. T. W. Webb. The report 
stated that much attention having of late years been given to 
lunar objects, the purpose for which the committee had been 
appointed would be best carried out by confining the discussion 
to the observations of a small but well-known portion of the 
moon’s surface ; and as the spot plate had presented during the 
last two years a variety of interesting and important features 
which had been well observed, it had been chosen as the most 
likely to yield results contributing to the advancement of seleno- 
graphy. Time having permitted the discussion of the observa- 
tions of the bright spots only, it was requested that a further 
grant of 20/. should be placed at the disposal of the committee 
for the discussion of the observations of the streaks and markings 
on the floor which were intimately connected with the spots. 
Mr. Birt, in alluding to the work which he had executed on 
behalf of the committee, said that as his report was voluminous, 
he would content himself with a brief description of the results 
at which he had arrived. The area of Plato in which the spots 
exist measured about 2,700 square miles ; as many as thirty- 
seven spots had been observed, but he wished it to be particularly 
understood that the whole had never been seen together; the 
greatest number observed on any one occasion was twenty-seven, 
the mean or average number being not more than eight. With 
the aid of diagrams drawn on the black board, he showed that 
the mean number seen at intervals of twelve hours of the luni- 
solar day varied during the progress of the day, so much so as to 
indicate that the number of spots visible at any given interval 
| does not depend upon the angle at which the sun’s light falls 
upon the floor of Plato. Some spots, he said, had been seen 
more frequently at about sixty hours after sunrise upon the floor 
of Plato than at any other portion of the Iuni-solar day; the posi- 
tions of these spots on the floor were pointed out, and it was 
remarked that they were situated in the western part of the 
crater, and they agreed in having been more frequently observed 
in August 1869, than at any other period of the observations. 
Other spots were observed more frequently at a later period of the 
observations than in August 1869, and they had been seen more 
frequently at a later period of the day, or alter the sun had passed 
Daylight at the moon is equal to fourteen terrestrial 
days and nights. These facts Mr. Birt argued were incompatible 
with the assumption that variations of aspect were entirely de- 
| pendent upon variations of illumination, and rather pointed to 
the existence of activity on the moon’s surface, the exact nature 
of which required further observations to elucidate. 
Report on Thermal Conductivity of Metals, by Prof. Tait. 
Prof. Tait, on the part of the committee appointed to report 
on this subject, drew attention to the relation that exists between 
electric and thermal conductivity of metals, and the effect on 
conductivity of a very small amount of impurity. He also 
sketched the apparatus made use of in the determination, and 
said that, as a new gasometer had been introduced, he had re- 
| commenced the whole of his investigations under better auspices 
| and with hopes of very great accuracy. 
On the Recent and Coming Solar Eclipses, by J. N. Lockyer, | 
F.R.S. The substance of this has already appeared in these 
pages. 
Prof. Tait remarked, after the reading of Mr. Lockyer’s paper, 
that the photography which had been exhibited left no doubt in 
his mind that the greater part of the solar corona is produced in 
the earth’s atmosphere. The rays are pretty obviously to be 
attributed to ice-crystals, and the various irregular protuberances 
sometimes seen may be due to germs and light particles blown off | 
from meteorites before they become incandescent, which, accord- 
ing to a beautiful investigation of Stokes, descend with extreme 
slowness towards the earth. This simple consideration is suffi- 
cient to show the utter absurdity of the sneers with which Sir W. 
Thomson’s suggestion has been received, and to justify it asa 
scientific possibility—all it pretended to be. : 
On the Coming Solar Eclipse, by M. Janssen. In the discussion on | 
these communications, Sir W, Thomson said he joined warmly in | 
On a New Steam Gauge, by Prof. Zenger. 
Experiments on Vortex Rings, by H. Deacon. 
Observations of Parallax of a Planetary Nebula, by D. Gill. 
On a New Form of Constant Galvanic Battery, by Latimer 
Clark. 
On a Method of Testing Submerged Electric Cables, by C. F. 
Varley. 
Description of Experiments made in the Physical Laboratory of 
the University of Glasgow to determine the Surface Conductivity 
Jor Heat of a Copper Ball, by Donald M‘Farlane. The experi- 
ments described in this paper were made under the direction of 
Sir W. Thomson during the summers of 1865 and 1871. A hot 
| copper ball, having a thermo-electric junction at its centre, was 
suspended in the interior of a closed space kept at a constant 
temperature of about 16° C. The other junction was kept at the 
temperature of the envelope ; the circuit was completed through 

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[| Aug. 31,1871 — 
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