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address of Prof. B. Sahni delivered at the Indian 
Science Congress in 1921 (Journal and Proceedings 
of the Asiatic Society of Bengal, vol. 17, No. 4, 
Pp. 152-75), that Indian botanists are taking an 
interest in the Indian fossil flora, as yet but little 
explored since the earlier work of Feistmantel, 
embodied in the ‘ Fossil Flora of the Gondwana 
System.”’ Prof. Sahni points out that so far Indian 
strata have given little but plant impressions, but 
with the recent microchemical methods for the 
microscopic study of such impressions, developed 
in the Cambridge laboratories and demonstrated by 
Mr. J. Walton at the British Association Meetings 
at Hull, fossil impressions may become as valuable 
and as definite in the results they yield as the plant 
petrifactions permitting anatomical study. } 
“it 
MEASUREMENT OF VERTICAL DIMENSIONS WITH 
Microscope.—In the Journal of the Quekett Micro- 
scopical Club (Ser. 2, vol. 14, No. 88, November 1922) 
Mr. F. Addey gives a note on the measurement of 
the vertical dimensions of objects by the use of the 
graduated fine adjustment, in which he shows from 
mathematical considerations that the true thickness 
of the object is its apparent thickness multiplied by 
the refractive index of the mounting medium, the 
cover glass making no difference. This result has 
been confirmed by actual measurements. 
Focus APERTURE RATIOS OF Microscope OBJEC- 
TIvES.—In the Journal of the Quekett Microscopical 
Club (Ser. 2, vol. 14, No. 88, November 1922) 
Mr. E. M. Nelson discusses the focus aperture ratios 
of microscope objectives. If the values of the numer- 
ical apertures of objectives now available be plotted 
against the magnifying powers the resulting graph 
reveals several inconsistencies. In the present paper 
a new set of power aperture curves drawn up on a 
definite plan are given for the construction of objec- 
tives. The ideal value for the power aperture ratio, 
obtained from a consideration of the resolving power 
of the eye, cannot always be realised in practice. 
This ideal ratio expressed as an “ optical index” 
(that is tooo times the N.A. divided by the initial 
magnifying power) is shown to be 25, and in the 
proposed curve for achromats the low powers up to 
ro in. have an optical index of 20, after which the 
optical index is reduced and the curve becomes steeper, 
rising to a ¢ in. with N.A. 0-9. In the apochromats 
the optical index in the curve is maintained at 20 
up to a N.A. of o-8. For oil immersion lenses the 
optical indexes have to be reduced, and the proposed 
curve begins with a } in. of N.A. 1-0 (optical index 
14°3) and ends with a yy in. of N.A. 1-4 (optical index 
11-7). If such schemes of ratios of aperture to power 
were adopted the initial magnifying power and the 
numerical aperture would become practically syn- 
onymous terms and a lens could then be accurately 
designated by its numerical aperture instead of by 
the focus, thus avoiding ambiguity where different 
tube lengths are used. _ 
Contact Catatysis.—No. 30 of the Reprint and 
Circular Series of the National Research Council 
contains the first report of the committee on Contact 
Catalysis. The report, which has been drawn up by 
Prof. Bancroft, gives a summary of recent work and 
suggests that the two fundamental things to be done 
in the study of contact catalysis are: (1) To determine 
in what cases definite intermediate compounds are 
formed and what they are; (2) To determine what 
bonds and contravalences are opened when adsorp- 
tion takes place, and to show that the opening of these 
WO./278r° VOL: 121] 
NA TURE 

[FEBRUARY 17, 1923 
bonds and contravalences accounts for the formation 
of the reaction products. 
BACTERIA AND CONDENSER CORROSION.,—An investi- 
gation on the influence of the fermentation products 
of bacteria on corrosion in engine condensers, con- 
ducted by Messrs R. Grant, E. Bate, and W. H. Myers, 
originated during the systematic examination of 
possible factors in the causation of corrosion, par- 
ticularly pit-hole corrosion, ih condensers of two 
power-houses of the Government Railways and 
Tramways, Sydney, N.S.W. (Rep. of the Director- 
General of Public Health, N.S.W., for the year 1920, 
Sydney, 1922, p. 171). It had been noticed that 
tube failures often occur after a long period of shut- 
down, even when a condenser has previously been 
immune from trouble. The authors point out that 
condensers generally retain a considerable quantity 
of water, complete drainage never being obtained 
with the usual horizontal setting. This stagnant 
water always contains a very high proportion of 
free and albumenoid ammonia and nitrates: Various 
micro-organisms of ammonia-producing types were 
isolated from the circulating and stagnant waters 
of condensers. Plates of copper, brass, and zine 
introduced into cultivations of these organisms 
underwent corrosion and pitting, photographs of 
which are reproduced. It is concluded, therefore, 
that the activity of micro-organisms as a factor in 
starting or causing corrosion must be seriously 
considered. If corrosion were a purely thermal or 
chemical effect, the pitting might be expected to 
increase steadily with temperature; actually, it is 
found to be more in evidence in low temperature con- 
densers, which supports the micro-organismal theory. 
FRENCH STREAM GAUGING APPARATUS.—In a 
notice recently issued from the gauging station of the 
University of Toulouse at Ponts-Jumeaux, a descrip- 
tion is given of the log used by the French Service 
des Forces hydrauliques and the method of calibra- 
tion adopted. The log is essentially a screw of a 
special form, attached to a revolving axis mounted 
on ball-bearings in the body of the log. The appar- 
atus is designed in such a way that the axis lies in the 
direction of the current, and: the screw encounters 
the liquid filaments in front. The relationship 
between the rotations ” of the screw and the velocity 
v of the water is in the following form: v=a+bn. 
The determination of the speed of the screw in 
revolutions per second is carried out as follows. The 
axis of the screw engages by a worm in a cogged 
wheel, designed so that the screw makes N revolu- 
tions for a single revolution of the cogged wheel. 
This number, N, is fixed for any particular log and is 
generally equal to 25 or 50. A cam carried by the 
cogged wheel comes in contact at a fixed point of 
each turn with a spring plate connected with an 
insulated electric terminal on the body of the appar- 
atus and thus closes an electric circuit actuating a 
dell. By measuring the time T, which passes between 
two consecutive signals, there is deduced therefrom 
the number of revolutions of the screw per second 
(x=N/T) and the movement of the water can be 
calculated. The coefficients a and b are determined 
in the process of calibration. For the purpose of 
calibrating the apparatus, a carriage with a platform 
is propelled at a certain speed while the instrument 
it supports is drawn through still water. During a 
sufficiently long series of runs, the speed of the 
carriage is related to the number of revolutions of 
the screw, and a curve, which is generally a straight 
line, can be drawn. 
metres long, 2 metres wide, and 1 metre deep is used 
for calibration. 
A cement-lined channel 75— 
———— 
