50 
exclude the low promontories and ranges of hills 
around the great mass, since the vegetation of these, 
both geologically and botanically, agrees with those 
of the terraces of the Elbe and the Weser. The uni- 
formity of structure of the great central mass of the 
Hartz, the rarity of lime, &c., causes a comparative 
poverty of peculiar forms of flowering plants, but, on 
the other hand, a subalpine character may be detected, 
and this at a height above the sea which would not 
lead one to expect it. Another feature, long ago 
pointed out, is the depression of the climatic tree-limit. 
Dr. Drude gives the following list of the twenty-four 
rarer or characteristic species of the Brocken flora :— 
Listera cordata, Epipogon aphyllus, Trichophorum 
caespitosum, T. alpinum, Carex pauciflora, ? C. 
Heleonastes, C. rigida, C. limosa, C. sparsiflora, 
Geum montanum, Linnaea_ borealis, Hieracium 
alpinum, H. nigrescens bructerum, Andromeda poli- 
flora, ? Pinguicula alpina, Pulsatilla alpina, impe- 
trum nigrum, Rumex arifolius, Thesium alpinum, 
Salix bicolor, Betula nana, Lycopodium alpinum, Sela- 
ginella spinulosa, and Athyrium alpestre. 
Saxifraga Hirculus was found by Kohl in the neigh- 
bourhood of Zorge in 1809, but does not seem to have 
been gathered by anyone since that date, and many 
other interesting notes on individual species are given. 
Hampe, in his ‘‘ Flora Hercynica,’’ gives 1343 
vascular plants as occurring within his area, and there 
is a good deal of additional information on the dis- 
tribution of plants in this region in A. Andree’s little 
pamphlet ‘‘ Die Flora des Hartzes und des Ostlichen 
Vorlandes bis zur Saale.”’ 
The Erzgebirge, a chain of mountains mostly of 
primary formation, are on the south-east of the area 
taken by Dr. Drude. A long list of the characteristic 
species is given, from which we select the following 
as particularly worthy of note:—Orchis_ globosa, 
Herminium Monorchis, Coeloglossum viride, Gymna- 
denia albida, Listera cordata, Corallorhiza innata, 
Lilium bulbiferum, Streptopus amplexifolius, Poly- 
gonatum verticillatum, Luzula silvatica, L. sudetica, 
Trichophorum caespitosum, Carex pauciflora, C. 
rigida, C. supina, C. limosa, Calamagrostis montana, 
Poa sudetica, Scheuchzeria palustris, &c. 
Meum athamanticum, Orchis globosa, Gentiana 
spathulata, and Phyteuma orbiculare, which are present 
in the Erzgebirge, are wanting in the Upper Bohmer 
Walde, and only the Meum and the Phyteuma reach 
the Hartz. Senecio crispatus, in the Thuringen Wald, 
reaches its northern limit in Hercynia, but it is im- 
pessible in a brief notice to give any idea of the mass 
of detailed information which Dr. Drude has here 
collected together. 
Much good work has recently been published on the 
distribution of plants in various portions of the globe. 
In Central Europe there have been the preceding 
volumes in the present series, Prof. Moritz Will- 
komm’s ‘‘ Iberian Peninsula,’’ Dr. Pax’s admirable 
work on the Carpathians, Dr. Gustav Radde on the 
Caucasus, Dr. Beck on the various countries included 
under Illyria, and Dr. P. Graebner on north Germany. 
In the United States we have had Prof. MacMillan’s 
‘““Minnesota Plant Life,’’ *‘ The Plant Life of Ala- 
bama’”’ by Dr. Mohr, and a report on the Dismal 
NO. 1777, VOL. 69] 
NATURE 
[ NOVEMBER 19, 1903 
Swamp region by Mr. Thomas H. Kearney. In this 
country the late Robert Smith mapped out three dis- 
tricts in Scotland, and his brother, Dr. W. G. Smith, 
of Leeds, and his colleagues have already mapped out 
two districts in Yorkshire, and given lists, illustrated 
by photographs, of the characteristic plants of the 
different stations. A first instalment of a botanical 
survey of the basins of the rivers Eden, Tees, Tyne 
and Wear, by Mr. F. J.. Lewis, was lately read at the 
British Association, and we hope the contemplated 
survey of the Pennines, from Derbyshire to the 
Cheviots, will be successfully carried out. 
Dr. Drude in Hercynia has done his work most 
fully and conscientiously. Every possible plant- 
association connected with every varying physical con- 
dition of the country has been carefully noted, and both 
its phenogamous and cryptogamous constituents have 
been determined. But we feel that the work may 
somewhat bewilder the ordinary reader by reason of 
its excessive elaboration.” E. GB: 
MEASUREMENT BY LIGHT WAVES. 
Light Waves and their Uses. By A. A. Michelson. 
The Decennial Publications of the University of 
Chicago. Pp. 166. (Chicago: University Press, 
1903.) Price 2 dollars net. 
HE University of Chicago, in commemoration of 
the completion of the first ten years of its exist- 
ence, is publishing a series of volumes dedicated ‘‘ to 
the men and women of our time and country who by 
wise and generous giving have encouraged the search 
after truth in all departments of knowledge.’’ 
The publication committee is to be congratulated 
in that it has persuaded Prof. Michelson to contribute 
a volume to this series. Anything that he writes is 
sure to be worth careful and attentive study, and while 
the actual scientific results recorded do not, as a rule, 
in any way claim to be new, Prof. Michelson has 
succeeded in putting the important consequences of 
his own inimitable work in a manner which will render 
them known to many who could hardly be expected to 
follow the original papers. 
The volume contains eight lectures delivered at the 
Lowell Institute in 1899. It starts with an elementary 
account of light waves and their properties, and in the 
first lecture some of the consequences of the principle 
of interference are skilfully developed. 
But the distinctive tone of the work is not noticeable 
until we come to Lecture ii., which deals with a com- 
parison of the microscope and telescope with the 
interferometer. 
An account is given of the action of a lens, and the 
theory of the diffraction fringes formed by a micro- 
scope or telescope objective is outlined. This leads to 
the theory of the resolving power of a microscope, 
and to the conclusion that, while 1/250 of an inch is the 
limit of resolution for the human eye, that of the 
microscope is one four-hundredth of this, or about one 
hundred-thousandth of an inch. It is then shown that 
by limiting the aperture of a telescope to two parallel 
slits near the opposite ends of a diameter, the fringes 
formed become more distinct, though with a consider- 
able loss of light, and from this the action of various 
