NATURE 
[JuLty 9, 1914 
SS 
Radio-activity and Atomic Numbers. 
LET Th,;, Ra;, Acr be the periods of half-change of 
corresponding members of the thorium, radium, and 
actinium family respectively, M the atomic number, 
M(Pb) that of the lead-group, and c a constant (+4°5); 
then for all substances emitting a rays— 
Th,= Ray. Ac;/cM-M(®b) 
For analogous #-radiators Ra;.Ac;/Th*,;, though not 
unity for group BIV. is >1 for B III., and <1 for 
BV. (the only three groups in which comparable 
values are known). 
The only exception here, as in all similar relations, 
is thorium-X (or actinium-X). For radiothorium, 
where a few months as well as two years are given 
for the period of half-change, the formula gives the 
first value. Of course, very accurate results cannot 
be expected from values like 2 min., 3 min., 0-002 sec., 
etc., but the differences are nowhere greater ‘than 
what from this lack of precision must be expected. 
Periods ot half-change: Calculated Experimental 
Radiothorium ... .., V 365 X10"19's/4°58 ad =6s days A few months 
Thorium emanation... V 3°86 x 85400 x 3°9/4°54 s =56'4 sec. 53 sec. 
Thorium A... : V 180 X0'002/4'52 § =0'134SeC. o'r4 sec 
Thorium Cf V 45 X24 X0'0333/4°5 =2°83 hours 2°87 hours 
Tonium... crf ars) 84e550Cb52/50 76 d =10° years Io years 
Radium emanation ... 4°54 532/3°9 5 =342days 3°86 days 
Radium A ... 475% X o'r 42/0002 S =3°31 min 3 min. 
Radium Cf... 4°5 X2°872/0'0333 hk =46°4 days 45 days 
Radio-actinium ... 4°58 X 652/365 X 109 @ =19'5 days 19's days 
Actiniumemanation... 4°54 532/3"84 86400 s =3°5 sec. 39 Sec. 
Actinium A... 4°52 X0'1 42/180 S =0°0022 Sec. 0°002 Sec. 
Actinium Cy 4°52X 1022/10 6 s =2"10-15 sec, ? 
Actinium C¢ 4°5X2°872/45 X 24 mt =2°03 min. 2 min. 
A. VAN DEN BROEK. 
Gorsel, Holland, June 26. 
Seeing and Photographing Very Faintly Illuminated 
Objects. 
THe question frequently arises, particularly among 
astronomers, whether it is possible to photograph 
Height Height Velocity 
apparatus), at the other end of which a 6 inch F/573 
telescope objective formed an image on the plate 
tested, or, with an ocular, on the retina. 
The results obtained are tabulated below :— 
Int. atsource On plate On retina Min. exp. Vision 
(1) 86 m.c. 0°24 m.c. 0'69m.c. 16sec. Comportable 
(2) 0°33 0°0092 0°026 7 min. Distinct,  un- 
adapt. 
(3) 0°0127. =—-0'00035 =: O GOIO2 2) late Distinct, after 
3 min. 
(4) 0700049 +0'000014 0°000039 over 50 hr. Invisible, adapt. 
In experiment (3) the plate illumination was just 
sufficient to produce a distinct image on a Seed 
30 plate after an exposure of three hours, while the 
illumination on the retina as viewed was three times 
as great, the source being just easily visible after 
resting the eye about three minutes in total darkness. 
In other words, an image on the retina just visible 
after partial adaptation to darkness would just produce 
an image on a photographic plate after an exposure 
of one hour. The retina fully adapted to darkness is 
still a thousand times more sensitive than this. 
P. G, NUTTING. 
Rochester, N.Y., June. 
June Meteors. 
A PARAGRAPH referring to some brilliant meteors 
observed at Bristol on June 25 appeared in Nature of 
July 2 (p. 464), and I am induced to send a few details 
of our June results, for they appear to me to exceed in 
importance and interest any obtained in any other 
month for a long period. There are a large number 
of double observations of the same objects, and I have 
been enabled to compute the real paths of fourteen, 
particulars of which are given in the subjoined table. 
They were all observed by Mr. S. A. Wilson and Mrs. 
Fiammetta Wilson (marked ‘‘W.’’), and some were 
recorded by Miss A. Grace Cook and some by myself. 
The very persevering and accurate observations by 
Mrs. Wilson and Miss Cook have been very successful 
in this branch of astronomy in the last few years. 
Radian 
Dae cor ean T. Mee. ae os 2 pee a : Observers Meteor appeared over 
{h. m miles miles miles miles 3 \ 
June 3 10 30 g 51 48 160 25 281-25 W. and others The Wash to Durham 
15 II 4% 4-1 60 52 26 19 260-22 W.and W.F. D. Alton to W. of Reading 
” IT) os 2 87 62 68 35 279-13 W.and W. F. D. Wilts to Ross 
” II 32 6-4 69 53 27 41 315+21 W, and W. F. D. Tunbridge Wells to Dorking 
16 II oO 2-1 69 43 29 29 270+50 W.andF. Denning Selsey Bill (nearly vertical) 
21 II 22 3-4 72 48 37 37 293+10 W.and A. G. C Sea 34m. E. of Broadstairs 
25 10 513 >I AS: Pan 14 20, 260-24 W. and W. F. D 12m. W. of Bristol to Usk [Harwich 
” Il 273 9 Bie, 25 45 30 342+39 W. and W. F. D. 12m. N.W. Chelmsford to 6m. N.W. 
> II 46 5-4 68 48 39 25 258+2 W. and W. F. D. 15m. W. Aldershot to Henley 
» DeS2y I= a 59 23 46 18 354+77 W. and W. F. D. Iom. S. W. Luton to $m. S.E. Reading 
» Il 573 rl Gye) 167 52° §2 9) 350-8 SeWe and W: F. D 4m. N.W. Salisbury to Axbridge 
26 II 114 4-2 78 67 II 25 260+70 W.and A. G. C S. of Bedford (nearly vertical) 
’ Il 173 4-2 WS 56 22 44 320+61 W. and A. G. C Halstead to Bishops Stortford [Hants 
29 II, 25 I 64 53 19 26 320+19  W. and W. F. D Eng. Chan. 32m. S. of Christchurch, 
objects too faintly illuminated to be seen. At the Now that the most attractive and prolific season for 
suggestion of Dr. Mees, the writer, assisted by Mr. 
Huse, has made some observations with measured 
illuminations giving comparative sensibilities of the 
human retina and an extra rapid photographic plate. 
The source used was a sort of artificial moon con- 
sisting of a 1o-candle Tungsten lamp in a metal box 
over the front of which were placed several layers 
of dense opal glass. The normal light flux from this 
surface measured equivalent to 86 metre candles. 
This intensity was further reduced by neutral filters 
transmitting 1/26 of the light. This source was placed 
at one end of a 20 ft. tube (our plate resolving power 
NOW 332.) VOL. .O34 
meteoric work is at hand I trust that some readers 
of NaturRE may be inclined to watch the sky and 
record the apparent paths of such meteors as may 
appear. They are usually unduly plentiful between 
the middle of July and middle of August, and the 
great Perseid shower can be favourably traced during 
nearly the whole of the period named. 
Any observations may be forwarded to the Rev. M. 
Davidson, director of the Meteoric Section of the 
British Astronomical Association, or to myself. 
: W. F. DENNING. 
44 Egerton Road, Bristol, July 6 . 
