JANUARY 31, 1907] 
NATURE 
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Perception of Relief by Monocular Vision. 
A STRIKING example showing how any large lens can 
see’’ in relief (see NaTuRE, January 3, p- 224) may be 
demonstrated to an audience. 
An electric glow-lamp is lit in an optical lantern, and 
the image of the filament projected on to a screen. This 
image is only sharp in parts. 
A card with a small hole in it (§ inch) is now placed 
close in front of the lens; this sharpens the image on the 
screen. 
The card should now be moved backwards and forwards ; 
the image changes in a remarkable way with every move- 
ment, showing that the lens sees the filament from a 
different point of view from each point of its surface. 
Photographs taken with the ‘“‘stop”’’ at either side of 
the lens make a good stereoscopic pair. 
“cc 
A. E. SmitTH. 
8 Farringdon Avenue, E.C., January 109. 
THE RUWENZORI BOUNDARY DISPUTE. 
at HE dispute which has arisen as to the ownership 
of the Ruwenzori Mountains between the British 
and the Congo State Governments is the latest ex- 
ample of the danger of a fixed and definite boundary 
agreement based on unfixed and most indefinite geo- 
graphical data. So long as an elementary knowledge 
of geography—especially of the conditions and 
methods which govern geographical map making— 
forms no part of the educational equipment of our 
political staff we shall have these unscientific and 
clumsy disputes which may easily cost the country 
as much as a small war. 
In this instance the agreement indicated, as the 
boundary between the Congo State and Uganda, ‘a 
frontier following the 30th meridian east of Green- 
wich up to its intersection by the watershed between 
the Nile and the Congo, &c.’’ Presumably some sort 
of a map was consulted, but in 1894, when that agree- 
ment was drawn up no map could have existed 
which could claim to be more than approximately 
accurate in respect to any position fixed relatively to 
the meridian of Greenwich, and in the absence of 
detailed topography it must have been impossible to 
foretell whether the demarcation of such a line was 
even practicable. For a political boundary to be of 
any value it must either be carried by some well- 
marked natural feature or pass through country 
where artificial demarcation is possible. Conse- 
quently, of all dangerous boundary definitions that 
which involves a straight line through unmapped 
regions is perhaps the most unsafe. It might be 
urged that in the absence of all topography it was 
necessary faute de mieux to make use of an hypo- 
thetical line. In that case, treating all existing maps 
as a blank (which would have been the safest course), 
it was only necessary to express a doubt as to the 
finality of the arrangement whilst drafting the agree- 
ment. 
In the present instance probably no one will be 
much the worse for an unscientific boundary muddle. 
A certain nervous anxiety to avoid international com- 
plications has led to our hasty abandonment of a strip 
of country which lies between what is now determined 
as the thirtieth meridian east of Greenwich and that 
line which was supposed to represent it when the 
agreement was made. The strip is about twenty-five 
miles in width, and the Congo officials have already 
taken possession and forbidden the entry of any white 
men unless engaged on scientific investigation. It 
may be a valuable strip for rubber production or it 
may not. Whatever it is, it has practically been 
given away (like many another more important field 
of international frontier dispute) for the want of a 
little scientific knowledge of the limitations of geo- 
graphical definition. 5 
NO. 1944, VOL. 75 | 
He AR On Elbe APT AR Y 
Boo dealing with precious stones which have 
made their appearance in the past may be 
divided into three classes. First, works of exact 
science written by competent mineralogists, like the 
well-known treatises of Church, Max Bauer, and 
Kunz; secondly, treatises of an antiquarian character, 
of which the well-known works of C. W. King are 
the most conspicuous examples; and thirdly, books 
written from the commercial standpoint, like those of 
Messrs. Streeter and Emmanuel. All these classes 
of books treat, it is true, more or less incidentally of 
the lapidary’s art, but the information on the subject 
is often second-hand and sometimes not very trust- 
worthy. 
It may be readily understood that a skilful lapidary, 
who is constantly handling different gems from the 
Fic. r.—D/amond Cleaving. From ‘‘ The Gem-Cutter’s Craft.” 
most varied localities, must acquire an eye- and hand- 
knowledge of the objects of his craft of a very special 
kind; and the opinions of such men, based as they 
are on the results of constant observation and experi- 
ence, may often be worthy of the attention of men 
of science. A book written by a practical lapidary 
may thus be expected to have a peculiar interest of its 
own—one of a totally different character from that 
which attaches to the kinds which we have enumer- 
ated above; and thus it comes about that the work 
before us constitutes almost a new departure in the 
literature of precious stones. 
It is only fair to the author to point out that 
—while justly insisting on the importance and 
value of the practical knowledge of gems gained by 
constantly -handling them and by noticing their 
1 “The Gem-Cutter's Craft.” By Leopold Claremont. Pp. xv+296. 
(London: George Bell and Sons, 1906.) Price r<s. net. 
