512 
by a micrometric screw with a graduated head. My ob- 
jection to the original construction was that the bright 
line was photographed on a small piece of glass, and the 
background was so far from being black as to much im- 
pair the spectra of substances that will not transmit a 
bright light. 1 suggested that in place of this glass plate 
asmall piece of tin-foil should be used, having a very 
minute hole in it. This shows far brighter than the line 
in the photograph, and the back-ground is quite black ; 
and thus the bright dot can easily be seen even when in 
the brightest part of the spectrum, and there is nO ex- 
traneous light to impair the faintest absorption-bands 
The only important objection to this method of measur- 
ing their position is, that a very slight movement in the 
apparatus, due to the loose fitting of moveable parts, 
alters the readings, and that the value of the measurements, 
as read off by the micrometer, depends on so many vari- 
able particulars, that nearly every instrument might have 
a different scale. The chief objection to my interference 
scales * is the difficulty of making all agree absolutely, 
but when accurately made they have not the above-named 
disadvantages. I therefore still adhere to that plan, but 
at the same time I have found the bright dot arrangement 
very usejul, not only as an indicator in showing spectra to 
others, but also asa fixed point in comparing, different 
spectra, or in counting the bands of the interference scale. 
Possibly without such help some observers might find this 
difficult, and would prefer in all cases to measure the posi- 
tion of bands by means of the graduations on the circular 
head of the micrometer, and therefore I was anxious to 
devise a prism that would have a dispersive power inter- 
mediate between the two extremes already mentioned, and 
at the same time have the upper face inclined at an 
angle of 45° to the axis, so that the bright dot micro- 
meter might be employed conveniently. To accomplish 
this, Mr. Browning made for me a prism composed of 
two rectangulars of crown glass, one rectangular of very 
dense flint, and one of less dense, cut at such an angle 
as to give direct vision. This combination gives what I 
consider to be as good a medium dispersion as could be 
wished, and at the same time enables us to measure the 
position of the bands with the bright-dot micrometer as 
accurately as is requisite in nearly all practical applica- 
tions. Subsequent trials have shown that the same advan- 
tages may be secured in a more satisfactory manner by 
replacing the less dense flint glass prism by two, one of 
flint and the other of crown, of such angles as give direct 
vision for the whole combination of five. The dispersion 
is very nearly the same as that of two prisms of ordinary 
flint glass of 60° angle. 
I have been thus careful in explaining the advantages 
and disadvantages of various arrangements, because the 
successful use of the spectrum-microscope depends so 
much on such particulars, and because so many who have 
not had experience in the practical working of the instru- 
ment seem anxious to see a wide spectrum, and overlook 
the practical importance of being able to recognise obscure 
absorption-bands. My own experience of this question 
agrees with that of most of my friends who have worked 
with the instrument, and yet I am quite prepared to 
believe that a different amount of dispersion might better 
suit some observers, and to admit the truth of the German 
saying, “ Eines schickt sich nicht fiir alle.” 
H. C. SORBY 


NOTES 
RirE in years and in honours, his work done and his fame 
world-wide, amid the regrets of all ranks of his countrymen, 
Sir Roderick Murchison has gone to his rest. It is nearly a 
year since he was seized with an illness which disabled him 
from further active work. Yet in the interval he has shown all 
his old interest in the affairs of which he has so long been the 
* Proc. Roy, Soc., vol. xv. p. 434. 
NATURE 
2 ee SS ee 

[ Oct. 26, 1871 

heart and soul, keeping up his intercourse with the world of 
science by reading, and with many of his associates by personal 
interviews at his own residence, and by correspondence. To the 
last his wonderful memory remained true, even to trifling details 
of place and date. Within the last few weeks, however, the 
disease made sad progress, and though he continued to enjoy 
frequent carriage exercise, his physical strength became less able 
to withstand any malign effects which the chills of autumn bring 
with them. On Thursday last he was seized with bronchitis, 
and gradually sank under the attack, till he died at half-past 
eight on Sunday evening, the 22nd inst. We shall offer next 
week a fuller reference to Sir Roderick’s Jife-work and scientific 
influence. For the present, and ere the earth closes over all of 
him that is mortal, let us only say that in him Science has lost 
a hard-working and distinguished cultivator, as well as an in- 
*fluential patron, and that to a narrower circle of mourners his 
loss is also one of a kindly large-hearted friendship. 
WE have to record the death, on Saturday last, at the age of 
seventy-nine, of Mr. Charles Babbage, the eminent mathema- 
tician and mechanician. The most important events of his life, 
as well as some of the eccentricities of his character, are familiar 
to the public through his autobiographical volume, ‘‘ Passages in 
the Life of a Philosopher.” Born in 1792, he entered Trinity 
College, Cambridge, in 1810, and was transferred to St. Peter's 
the following year. At his B.A. degree he did not take honours 
in mathematics, not having specially pursued that subject of study 
as a student, and was understood to have been disappointed at 
not being elected a fellow. In 1828 he was howeyer elected 
Lucasian Professor of Mathematics at Cambridge, a position once 
held by Sir Isaac Newton. He published no less than eighty 
volumes, but his claim to public notice rested chiefly on his 
invention of the Difference Engine, on which he spent immense 
labour and a large sum of money. Notwithstanding his eccen- 
tricities and his failings, Mr. Babbage was a mathematician and 
an inventor of whom England may be justly proud. 
Tue English Government Eclipse Expedition sailed this 
morning for Ceylon in the A/i7zepore from Southampton, Mr, 
Lockyer in charge, expecting to reach Point de Galle on Noy. 
27. They hope to confer with the Indian observers as soon as 
possible, and plan a concerted campaign. The experience of 
the last Expedition necessitated that the whole of the instruc- 
ions should be rewritten; and the Eclipse Committee of the 
British Association, consisting of the following gentlemen :—Sir 
William Thomson, L.L.D., F.R.S., President, Prof. J. C. 
Adams, D.C.L., F.R.S., GB. Airy, F.R.S., Astronomer 
Royal, Prof. Clifton, F.R.S., Warren de la Rue, D.C.L., F.R.S., 
Dr. Frankland, F.R.S., Captain Douglas Galton, C.B., F.R.S., 
George Griffith, M.A., J. R. Hind, F.R.S., W. Lassell, F.R.S., 
President R.A.S., Lord Lindsay, J. Norman Lockyer, F.R.S., 
General Sir Edward Sabine, K.C.B., President R.S., General 
Strachey, F.R.S., W. Spottiswoode, LL.D., F.R.S., Colonel 
Strange, F.R.S., Prof. Stokes, D.C.L., F.R.S., and Dr, Thomas 
Thomson, F.R.S., have had very hard work to get the arrange- 
ments completed, in which they have been most zealously assisted 
by the Government, and by the Peninsular and Oriental Steam 
Boat Company. Lord Lindsay placed at the disposal of the 
Expedition the whole of his valuable instruments, and has sent 
a photographic observer at his own expense. Several members 
of the Expedition have voluntarily given up a month of their 
time before starting to perfect themselves in spectroscopic and 
other observations at the Royal College of Chemistry, a most 
commendable example to others in similar situations. We have 
now only to wish the Expedition a prosperous voyage, and better 
fortune with regard to weather than was experienced in Sicily 
last year. 
WE have to announce the return of Mr, Gwyn Jeffreys from 
