170 
NATURE 
[Fune 24, 1886 
— 
the study of ornithology presents many phenomena of far 
deeper interest than the mere search after the oldest 
name, resulting, as it does too often, in the unearthing of 
some utterly unknown title, to the confusion of the 
student. The same principles of nomenclature which we 
tried to follow in earlier days are those of the A.O.U. 
now, which result in Uszzator /iwmme as the name of the 
Red-throated Diver. And it is not as if there will be any 
finality about this nomenclature, for we have seen the 
treatment of too many monographs to make us believe 
this. When an ornithologist takes up a group of birds 
and monographs it, he spends months or even years of 
study on this particular group, obtains a grasp of his sub- 
ject, and does his level best to give finality to his work. 
Does he succeed? Seldom, if ever. We hold it as an 
absolute canon that the nomenclature of monographs 
should be followed, unless a definite reason is given why 
a name should be altered. But, instead of this being 
done, we find, over and over again, that the author 
of a small paper or of a faunal list will, by altering 
generic names and so re-shuffling the specific names, give 
a totally different aspect to birds which have only just 
before been carefully monographed with a hope of finality 
in their nomenclature. So will it probably be with the 
A.O.U. “ List,” when some ornithologist in America will 
rise up and (as we expect to see before long) declare 
the trinomial system unworkable or the nomenclature of 
the “ List ? too complicated, and will re-shuffle the names, 
and attain temporary renown. 
We think, however, that, now that the two leading 
Ornithological Societies of England and America have 
spoken with authority on the subject of the nomencla- 
ture of the birds of their respective countries, the 
British Ornithologists’ Union should endeavour, if 
possible, to confer with the sister Society in America, 
and see if a common ground of agreement cannot be 
arrived at. If these two bodies came to a settlement, 
the whole matter could be laid before an Ornithological 
Congress, and there would be some hope of unanimity 
for the future. The points of divergence in practice 
between English and American ornithologists are less 
than might be supposed. The two principal ones are the 
adoption by the A.O.U. of the 1oth edition of Linneus’s 
“ Systema Natura” instead of the 12th edition, ard the 
employment of trinomial nomenclature. So many English 
ornithologists are now using the latter mode that there 
ought to be no difficulty in conceding the latter point if 
any ornithologist like the method. Formulated as it is 
in the A.O.U. “Code,” there is no difficulty in under- 
standing what is meant by the trinomial titles, and the 
American Committee have given a clear definition of their 
object in Canon XI., though the difficulties which have 
been pointed out on this side of the water are still not 
disposed of, “ In a word, intergradation is the touchstone 
of trinomialism. It is also obvious that, the larger the 
series of specimens handled, the more likely is inter- 
gradation between forms supposed to be distinct to be 
established, if it exists.” So says the canon above quoted ; 
but, we would ask, if two forms absolutely intergrade, 
why are they not of the same species? and why will 
not a binomial title be sufficient ? and again, what name 
is to be given to the specimens collected at the point 
of contact? Or again, if a larger series of specimens 
proves that two species do not intergrade, as they were 
at first supposed to do, then they will each once more 
bear a separate specific name. Further, are trinomials 
to be used for insular forms, as is done by Mr. Allen 
for Loaigilla noctis sclateri from Santa Lucia, as there is 
no chance of intergradation between it and Z. soct7s from 
the neighbouring islands? Trinomial nomenclature has, 
however, taken such a place in American ornithology, and 
is adopted by so many naturalists in the Old World, that 
the principle must be conceded to all who like to avail 
themselves of it. The question with regard to the tenth 
edition of Linnzeus’s “ Systema” might also be got over, 
but the A.O.U. will have greater difficulty in convincing 
European naturalists that it is advantageous to the pro- 
gress of ornithological science to alter established nomen- 
clature by introducing Che/édon as the generic name for 
the Chimney-swallow instead of the feather-legged Martins, - 
which are to be henceforth Azrundo. This radical change 
is to be adopted in homage to Forster’s “ List of British 
Birds,” a mere list of names without a character for a 
single genus. Although similar lists have sometimes been 
accepted for specific names, their recognition in the case 
of genera is rare, although in many instances long-esta- 
blished usage has rendered some of them familiar. 
The few objections which we have made above must 
not be supposed to lessen our respect for the general 
tenour of the work now issued by our American con/réres, 
whose labours deserve our most careful consideration, 
while it cannot be doubted that the publication of this 
“ Code and Check-List” will have great influence on the 
future of zoological nomenclature. 
R. BOWDLER SHARPE 
PROFESSOR NEWCOMB’S DETERMINATION 
OF THE VELOCITY OF LIGHT} 
HE method selected for the important experiments 
described in the present memoir,” is that known as 
Foucault’s. The idea fundamental to it is that of the de- 
termination of the interval occupied by light in flashing 
from a revolving to a fixed mirror and back, by the 
| amount of deviation produced in its return path through 
the change meantime effected in the position of the re- 
volving mirror. The angle of deviation of the ray is 
double the angle of displacement of the reflector ; to this 
angle corresponds (since the mirror rotates at a known 
rate) a definite fraction of a second, which is the time of 
luminous transmission across twice the measured distance 
between the mirrors. 
But this theoretically simple means of ascertaining the 
velocity of light is complicated, in practice, with in- 
numerable difficulties. A choice demanding the utmost 
nicety of judgment must be made between various con- 
flicting conditions ; sacrifice in one direction is the price 
of advantage in another; a balance has to be struck,, 
giving the largest sum-total of facilities, with the fewest 
and least intractable drawbacks. The plan finally decided 
upon by Prof. Newcomb was the result of much anxious 
deliberation; we hope to render it, in its main outlines,. 
intelligible to our readers. 
A fundamental condition of the problem is to get an 
image of the light-source absolutely coincident with the 
light-source itself, so dong as the movable mirror zs at 
rest. And this, whatever be the position the mirror is at 
rest in, provided only that it be such as to permit the rays 
sent out by it to return, after due triple reflection, to the 
eye. This requisite is secured by locating the centre of, 
curvature of the distant concave mirror in the axis of the 
revolving plane one. All rays emitted from this point 
towards the former will return along the same paths ; dif- 
ferences of direction due to differing positions of the 
movable mirror will be eliminated by the return reflec- 
tion; and there ensues a “stationary image” of the light- 
source, occupying, when visible at all, an invariable 
situation. 
So far, all the operators by Foucault’s method have ; 
been unanimous ; but in the placing of the lens indispens- 
able for the management and concentration of the light 
employed, a material distinction obtained between the 
1 “ Measures of the Velocity of Light made under direction of the Secretary 
of the Navy during the years 1880-32,” by Simon Newcomb, Professor, U.S. 
Nayy. Astronomical Papers prepared for the use of the American Ephemeris 
and Nautical Almanac, vol. ii. parts iii. and iv. (Washington: Bureau of 
Navigation, 1885.) 
* For the historical notice serving 
as an introduction to it, see NATURE, 
May 13, p- 29. 
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