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NATURE 
81 

about N. and S. (magnetic). The evidence suggests 
movement of the ice from south to north, but we have 
no knowledge as to whether there did not exist, in 
Palzozoic times, an ex mass of ancient rocks to 
the northward of the present Islands. 
HERBERT A. BAKER 
(late Government Geologist for 
the Falkland Islands). 
Wood View, Grosmont Road, 
Plumstead Common, London, S.E.18, 
January 3. 

The Determination of pH of Microscopic Bodies. 
NeEvutrav red has the almost unique property of 
being both an intra-vitam stain and a fairly good 
indicator. It has also low salt and protein errors, as 
Homer has shown (1917, Biochem. gee II, p. 283). 
If therefore cells are stained with neutral red, the 
colour of the stain as observed with the microscope 
enables one to judge roughly the pH within the cell. 
Working on certain marine protozoa, I have found 
a method of greater accuracy than that of merely 
judging the colour as seen down the microscope. 
method is simple and, so far as I am aware, has 
not been recorded before. 
A series of tubes containing solutions of increasing 
PH is made up in the ordinary way, and a few drops 
of neutral red are added as indicator. A stout card- 
board strip is taken and holes are cut in it at intervals 
so that the strip will carry the series of tubes (each 
tube fitting tightly). 
The strip with the tubes hanging freely below it is 
now suspended in the window. ith the aid of the 
microscope condenser the series is focussed sharply 
in the plane of the object which is being examined. 
The appearance down the microscope now consists 
of the stained object and by its side the image of the 
series of tubes: both are seen against the same 
background of sky. By simply tilting the mirror 
the images of successive tubes of different fH can be 
brought into juxtaposition with the object examined. 
In this way the fH of the stained body can be deter- 
mined by direct comparison. 
I have found that the pH determined by the above 
method can be checked roughly as jelous, The 
mirror is tilted so that the image of one of the tubes 
forms a background against which the object is seen. 
In these circumstances the object is illuminated by 
ct of the particular quality corresponding to the 
colour of the tube. A succession of tubes is used in 
this manner as a background for the object. 
When the background transmits the same quality 
of light as the stained object, the latter appears 
relatively light and transparent: this occurs when 
the colour of the ie and the background cor- 
respond to the same pH. _ If the object is illuminated 
light from a tube of higher or lower pH, the 
object appears darker owing to the fact that the light 
transmitted by the background is not exactly of the 
Same quality as the light transmitted by the object. 
The pH of the background tube against which the 
oo appears lightest corresponds to the pH of the 
ec 
t must be admitted that the colour change with 
the pH in the case of neutral red does not render the 
latter an ideal indicator for the second method, but 
_ the effect is quite good enough to be used as a check. 
- 
Perhaps a better intra-vitam indicator will be dis- 
covered in the future, and in that case the method 
might be developed to a fair degree of accuracy. 
n all this work a good achromatic condenser is 
essential, for the diaphragm must be widely open in 
NO. 2777, VOL. 111] 

order that the colours of both the object and the 
image of the tubes may be well defined. 
When the light is bad or when artificial light is 
used, the definition of the colours is greatly increased 
if the light is first filtered through a dilute solution 
of copper sulphate. Using this filter, the red tint 
due to the presence of acid appears darker and is 
more easily seen in lightly stained bodies. 
C. F. A. Pantin. 
The Marine Biological Laboratory, 
Citadel Hill, Plymouth, December 14. 

Divided Composite Eyes. 
Ir would appear from Mr. Mallock’s letter (NATURE, 
December 9) that our knowledge of the Aleyrodide 
or ‘‘ White Flies "’ is not so exact as it might be. 
This, however, takes too pessimistic a view of the 
situation. Whilst, undoubtedly, much remains to 
be done, even with some of the British species, the 
specific limits of those to which he refers are quite 
well known to students of the group. Indeed, as 
a result of my own researches I have been able, in 
recent communications to the Entomologist and the 
Vasculum, to assign our British species to no fewer 
than four distinct genera, Aleyrodes, Tetralicia, 
Aleurochiton, and Asterochiton, and the forms 
mentioned by Mr. Mallock reveal themselves as 
comprising two genera and three species, namely, 
Aleyrodes proletella L., A. brassice Walk., and Astero- 
chiton vaporariorum West. 
Clearly, as no hint is given that any of his insects 
were bred from Chelidonium majus, the figures given 
cannot represent A. proletella as indicated by the 
legend; they must be referred either to Aleyrodes 
brassice or to Asterochiton vaporariorum. If A. 
brassice is the insect intended, then as a larva it 
feeds on cabbage, as a pupa it lacks well-developed 
dorsal papilla, and in the perfect condition has spotted 
wings with the median nervure appearing as a short 
spur. On the contrary, the larve of Asterochiton 
vaporariorum can be collected from any of the plants 
enumerated, its pupe have dorsal papilla, and its 
imago possesses immaculate wings displaying no 
trace of the media. 
To the latter insect belongs the notoriety gained 
by the so-called ‘‘ White’ or ‘‘ Tomato”’ fly during 
the past twenty years. Unfortunately, this Aley- 
rodid, although a native of neotropical regions, is 
so adaptable in its food habits as to be nearly poly- 
phagous and, furthermore, has acquired the habit 
of wintering at ordinary air temperatures even in 
this rather bleak locality on the north-east coast. A 
colony with which I was experimenting in 1921 
successfully withstood all the frosts of the winter 
of 1921-22, the first brood of the present season 
emerging in May. 
A further point I cannot understand is Mr. 
Mallock’s comparison of the life-cycle of the Aley- 
rodide with that of the Aphidide. So different are 
the two cycles that I feel sure that some mistake 
has arisen here. In every detail of their structure 
and life history their affinities lie rather with the 
Psyllide (particularly with some of the Trioze 
possessing scale-like larva) or toward the Coccide. 
Finally, I should like to point out that I am 
preparing a monograph of the British Aleyrodide 
and should therefore be extremely glad to receive 
species of the group, more especially if they are 
accompanied by their respective larvee and pupe. 
J. W. Hestop Harrison. 
Armstrong College, Newcastle-upon-Tyne, 
December 12. 
C2 
