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128 
NATURE 
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* 
- [Dec. 19, 1872 j 
which are no longer parallel, you will see that there is a 
distinct alteration in the effect produced ; the beam is di- 
rected to another portion of the wall altogether. The ray 
strikes the first side of the prism, and is bent towards the 
thicker part, or towards a line perpendicular to this sur- 
face, and on reaching the second side of the wedge, the ray 
is again bent in the same direction towards the base of 
the prism, for in this case the ray is bent away from the 
perpendicular to the second surface, and the light emerges 
from the second surface in a totally new direction. Fig. 
7 shows the effect in three cases, the incident ray S I, 
the path in prism I E, and the refracted ray ER; NI 
and E N’ being the lines perpendicular to the surfaces. 
An experiment may easily be tried, which will confirm 
this. Let a triangular piece of glass be held, with one 
edge pointing upwards, between the eye and a lighted 
candle, as shown in Fig. 8; it will be found that the 
candle cannot be seen; but if the prism be gradually 
raised, the image of the candle will appear, the amount 
the prism will have to be raised depending on its angle. 
Now, we have here obtained a deviation or refraction of 
light—that is to say, it has been bent out of its course ; 
for we have to look upwards to see the candle. Another 
effect has also been produced : the light which was white 
on entering the prism is now made up of several colours, 
which are separated more or less from each other; the 
candle, as seen inthe last experiment, is not white, but 
is fringed round with colours. If we again take our beam 
of light in the dark room, as in Fig. 9, and allow it to 
strike on one of our prisms, so placed that its edges are 
horizontal, and also that the beam enters it obliquely by 
one of its surfaces, and then receive the image on a 
screen, we see a band of colours which reminds us 
strongly of the rainbow : the lowest colour, if the base 
of the prism be upwards, will be red, next above orange, 
passing by imperceptible gradationsto yellow, and after- 
wards green, which then passes through the shades of 
greenish blue till it becomes a pure blue, then indigo, 
and finally ends with a violet colour. The transition 
from one colour to another is not abrupt, but is made in 
an imperceptible manner, so that it can scarcely be said, 
for instance, where the yellow ends or the green begins. 
The cause of this band of colours, or spectrum as it is 
called, was first discovered by Sir Isaac Newton, who 
tortured this spectrum in several ways. He took one of 
the colours thus produced, say red, as is shown in the 
figure, and made it pass through a second prism, re- 
ceiving the image on a second screen; the image is 
found to be rather longer, but the colour remains un- 
altered. This experiment proves that this colour of the 
spectrum is simple, and the same has been found of all 
the others. As Newton in his experiment operated with 
sunlight, the band of colours was in this case called the 
solar spectrum. The rainbow itself is also in reality 
nothing more nor less than a solar spectrum, which is 
caused by refraction in the rain-drops. 
If, instead of getting one beam of white light, we take 
two of differently coloured lights, red and blue, and pass 
these two beams of different colour through the same prism, 
you will see that the action of the prism on these two diffe- 
rently coloured beams will be unequal ; in other words, you 
will get the red beam deflected to a certain distance from 
a straight line, and the blue deflected to a certain other 
distance. You see by this experiment that there is a 
distinct difference in the amount of refrangibility—that 
the red light is not diverted so far out of its original direc- 
tion by the prism as the blue. And this leads us to 
Newton’s first proposition, which is this :—“ Lights which 
differ in colour differ in refrangibility.” 1 think that re- 
quires no explanation. You will be able to translate it 
for yourselves thus: Lights which differ in colour are 
differently acted upon by a prism, which, as you have 
seen, gives us a considerable result of the action of refrac- 
tion, J. N, LOCKYER 
(Zo be continued.) 
THE GEOLOGICAL EXHIBITION IN a 
GLASGOW 
PERE is probably no town or city in the United 
Kingdom, out of London, in which the science of 
Geology has been studied more extensively and enthu- 
siastically, and to more purpose, than in Glasgow, during _ 
the last fifteen or twenty years. It is about fifteen years 
since the Geological Society of Glasgow was formed, and _ 
during the whole of that period the progress of the study 
and of the Society has never flagged, of which there was 
ample evidence afforded by a great exhibition of geolo- 
gical and mineralogical specimens which the Society held 
in the Corporation Galleries on the evening of Friday, 
December 6. 
The Geological Society of Glasgow is one of the very 
few provincial societies, the results of whose scientific 
labours are permanently placed on record, and consulted 
by geologists elsewhere. The “ Transactions” of the 
Society are now in the fourth volume, and in them 
there are embodied many valuable original memoirs 
bearing particularly on special departments of the geology 
of Lanarkshire and the West of Scotland. : 
The exhibition of the Society of which we are now — 
giving a brief account, was chiefly devoted to an — 
illustration of the fauna and flora of the Carboniferous 
system of the west of Scotland. Various members of — 
the Society have worked most successfully in other de- 
partments of geological inquiry, but the function of the — 
Society as a whole seems to have been especially the in- — 
vestigation of the Carboniferous system, and the elucida- 
tion of the many important physical problems connected 
therewith ; and when we consider the fact that the ex- 
hibition in question was only a representation of the 
geological collections from which the specimens were 
obtained, we cannot help concluding that the Society’s 
function has been performed with most surprising results 
to science. 
Mr. James Thomson, F.G.S., corresponding member of © 
the Royal Society of Liége, was certainly the chief ex- 
hibitor in the department of carboniferous fossils ; but he 
was well supported by Messrs. Young and Armstrong. — 
The first-named gentleman has done immense service 
during the last fifteen years, as a collector, particularly in 
connection with the fossil corals. His services in this 
respect have been extensively acknowledged at home— 
by the British Association and otherwise—and by Con- 
tinental and American geologists, museums, &c. It is — 
probable that, within the time named, Mr. Thomson has ~ 
made sections of not fewer than ten thousand specimens 
of his favourite fossil corals. Besides the corals, Mr. 
Thomson’s collection is peculiarly rich in reptilian re- 
mains, some of them quite unique and rare. Mr. Arm- 
strong’s specimens were generally representative of all — 
the groups of animals and plants contained in the coal, — 
ironstone, shale, and limestone series of the west of 
Scotland—Lanarkshire and the adjoining counties. Many _ 
of his cases excited great admiration. Besides being — 
generally representative of the carboniferous system, Mr. 
Young was very strong in the Entomostraca and Fora- — 
minifera of that system, the species of which he has — 
materially increased by his own discoveries. 
In the department of Post-Tertiary shells, Mr. David — 
Robertson, F.G.S., was without a competitor. Indeed, 
he has been such a devoted student of the Post-Tertiary — 
period, that his collection is probably unrivalled. For a 
number of years the Rev. H. W. Crossley, F.G.S., now — 
of Birmingham, was a zealous co-worker with Mr. Robert- 
son. The Ostracoda and Foraminifera of the Carbonife- 
rous system, and the recent Hydrozoa and Polyzoa, were 
also largely represented in Mr. Robertson’s cases. : 
Silurian fossils collected in the Girvan district, on the 
coast of Ayrshire, were shown by Mrs. Robert Gray, an 
enthusiastic naturalist ; and from the Silurian system 
