848 
Separation of Isotopic Ions. 
In the issue of NaTuRE for June 2, p. 763, there is 
a reference to a paper by Kendall and Crittenden 
(Washington : National Academy of Sciences, vol. 9, 
No. 3) which describes a method for separating isotopic 
ions. This method was first described by Prof. F. A. 
Lindemann at the Royal Society conference on iso- 
topes (March 1921). A considerable number ot 
experiments on this subject have been carried out 
here during the past year, but it seemed desirable’ 
to postpone publication until a definite result had 
been achieved. Joun G. PILLEY. 
Clarendon Laboratory, 
University Museum, Oxford, 
June 5. 


Haze on Derby Day—June 6. 
THERE was a dense haze overlying Southern England 
on June 6, becoming worse towards evening and 
greatly interfering with visibility. It was very 
marked in Surrey in the neighbourhood of Epsom, 
where the race-goers found it difficult to see clearly. 
Records of this haze taken with my dust-counter at 
Cheam, between 7 and 7.30 P.M., gave a greyish 
deposit of dust particles upon the cover-glass. These 
particles varied in diameter from 1} microns down 
to ultra-microscopic size; the average diameter was 
about $ micron. Most of them were irregular in 
shape and insoluble in water, but scattered among 
the irregular dust particles were a number of small 
spheres. The proportion of these spheres present 
was about 3 per cent. of the total number of dust 
particles. They were transparent and usually colour- 
less, but some were distinctly brown or reddish. The 
maximum diameter of the spheres found was I} 
microns, but most of them were less than this. They 
were insoluble in water. The haze was unusually 
dense for a country district, and the number of dust 
particles per cubic centimetre was between go0o0 and 
10,000. 
It will be remembered that on Derby Day there 
was very little wind, but what there was was from 
the north. It is difficult to avoid the conclusion that 
a large proportion of this dust travelled south from 
the manufacturing districts of the Midlands. The 
presence of coloured and colourless transparent 
spheres points towards ash particles ejected from 
chimneys, while the grey colour is not what one 
would expect if domestic smoke were the origin. 
The records obtained during London fogs are black, 
and a dense fog gives 40,000 to 50,000 particles per 
cubic centimetre. 
For comparison with June 6, a dust record taken 
at 7 P.M. on the evening of June ro at Cheam gave 
less than 100 dust particles per c.c. The wind was 
strong and blowing from the west, and visibility 
very good. J. S. OWENS. 

Perseid Meteors in July 1592. 
Wiru reference to Dr. Fotheringham’s interesting 
comments in NATURE, June 9, p. 774, on the probable 
shower of Perseids in 1592, I thought it best to 
accept the date kindly sent to me by Mr. Beveridge, 
as it fell near the time when a shower might be 
expected to occur. However, the shower of 1592 
appears to have been 19 days earlier than the correct 
time, and this (with another reason stated later) at 
once throws doubts on the identity of the display | 
with the true Perseids. 
NO. 2799, VOL. IIT] 
NATURE 


[JUNE 23, 1923 
The near correspondence in epoch may still, how- 
ever, occasion some suspicion that the Perseid 
shower formed the incident recorded in history, 
though the exact date and direction of the meteor 
flights are incorrectly given. This idea is encouraged 
by the fact that in two other cases (A.D. 784 and 865) 
the shower dates differed 10 dayS from the normal. 
It is perhaps important to remark in this connexion 
that several rich showers of non-Perseids have been 
frequently observed in modern times which do not 
differ materially from the date of the Perseids. Three 
of these may be mentioned as possibly the same as the 
ancient showers recorded which failed to conform 
with the exact Perseid dates. 
(1) There is a strong shower at 303°-9° near a 
Capricorni on July 25-August 6. 
(2) A rich display from 339° -11° in Aquarius on 
July 26-August 2. 
(3) A fine shower seen in 1879 from Draco 291° +60° 
on August 21-25. 
If the meteors of 1592, to which Mr. Beveridge has 
directed attention, “ traversed the heavens from west 
to east,” as stated in the ancient chronicle, they could 
scarcely have been Perseids, for the latter are moving 
ne from east to west, and this seems an important 
etail. 
The direction of the meteors of 1592 from west to 
east means that their apparent motions must have 
been slow and that they were overtaking the earth 
in its orbit. The Perseids belong to another class ; 
they are swift objects meeting the earth at a velocity 
of 38 miles per second. I was not aware until I saw 
Dr. Fotheringham’s letter that the direction of the 
meteors had been described as from west to east. 
I adopted a period of 11°75 years (Observatory, 
May 1923) as agreeing with a large number of 
abundant returns of the Perseids and as it seemed the 
best to be derived. I directed attention to it in the 
hope that future observers would bear it in mind and 
test it in the light of additional observations. 
W. F. DENNING. 

44 Egerton Road, Bishopston, 
Bristol, May 30. 

Tactile Vision of Insects and Arachnida. 
Wiru regard to Father O’Hea’s letter in NATURE 
of May 26, p. 705, I wish to point out— 
(1) That I originally questioned the statement that 
the house-fly and certain spiders avoided the approach 
of one’s hand by detecting ‘‘ convection currents.” 
(2) That experiments in this direction can only be 
made with fofally blind insects. 
(3) That I have not stated that vision is universal 
or even general among insects and arachnida possessed 
of eyes, and I offer no explanation (at present) of the 
use or purpose of “‘ sightless eyes.” Neither can I 
enter a discussion on “‘ vision and light-sensitiveness.” 
I do maintain, however, that many species form 
comparatively clear images and can judge distances. 
The fact that a male Attid (and some Lycosids) will 
perform for the benefit of a female in an adjacent 
glass tube is at present only explicable on the assump- 
tion of vision. Father O’Hea has not, he says, 
worked on this point, and I persist in offering it as 
a preliminary objection to his hypothesis. This dis- 
cussion cannot, however, proceed to any satisfactory 
conclusion until we have his further evidence for a 
large number of species; and until this is forth- 
coming I should suggest that a generalisation on the 
question of vision among arthropods cannot be made. 
G. H. Locxer. 
Salmon’s Cross, Reigate, Surrey. 
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