Marcu 24, 1904] 
NATURE 
489 
In a letter in Nature of March 17 Prof. Herdman 
‘mentions that *‘ the plaice in the open-air ponds at the 
Port Erin Biological Station started spawning on March 3, 
-and those at the Piel (Lancashire) Sea Fish Hatchery (under 
cover) on March 1.’ He would like to know how this 
‘record compares with that of fish in the sea. As the pro- 
fessor suggests that ‘* probably’ the officials of the Inter- 
‘national Investigation will be able to speak as to the 
condition in the North Sea and... English Channel,”’ 
I herewith state my experience. 
About the middle of February I was informed by fisher- 
men that plaice in the North Sea (southern part) were for 
the most part spent. From personal observation I can state 
that the larger plaice trawled by the s.s. Huxley in the Great 
West Bay on February 26 were spent. Further, the eggs 
procured in the tow-nets at the various hydrographic 
stations in the English Channel in the latter part of 
February can hardly fail to prove, on examination, to be 
those of plaice. Prof. Herdman is probably aware that 
investigations conducted by the Marine Biological Associa- 
tion’ in the south Devon bays during 1901 and 1902 estab- 
‘lished the conclusion that ‘* the maximum spawning period 
lies between the third week of January and the second week 
of February.’’ 
In regard to the North Sea, I may mention that I ex- 
amined on board about 300 plaice trawled by the s.s. Huxley 
on March 11 and 12 on the south-west part of the Dogger 
Bank, and that of these fish the larger were spent, the 
smaller immature. Only one female had a considerable 
remnant of ripe eggs in its ovary, but none of the males 
were ‘‘running.’’ ‘Lhe eggs procured in the tow-nets in 
the southern part of the North Sea are apparently of several 
species, but plaice are almost certainly present, and prob- 
ably predominate. From present facts and previous records 
I conclude that, as compared with the North Sea and 
English Channel, Prof. Herdman’s plaice are late in 
starting to spawn. Wn. WALLACE. 
Marine Laboratory, Lowestoft, March 20. 
Euclid’s Definition of a Straight Line. 
Ow p. 409 your reviewer states that ‘* Euclid says nothing 
about the extreme points of the line’’ in his definition of 
a straight line, but regards “* all the points on it.” 
Will he kindly look at p. 410 of your vol. lvii., where I 
have given reason for considering the older translation of 
Euclid’s words to be correct. IRE 1S 1835 
I HAVE read the passage referred to, but for many reasons 
cannot admit that the argument is conclusive. In the first 
place, the quotation from Aristotle’s *‘ Ethics’’ has no 
authority whatever ; it has no grammatical connection with 
the previous context, and shows every mark of being a | 
marginal annotation which has been wrongly incorporated | 
with the text. Then the use of the same symbol for different 
points (AA, BB, 'f denoting segments) is very unusual, and 
is not what we should expect from a competent mathe- 
matician, so it is rash to infer that the use of éf’ ay TA in 
the sense ‘‘ of whichT, A are the extremities ’’ was a current 
technical practice at the time of the writer. But even if 
this be admitted, it does not follow that Euclid means the 
same thing in his definition of a straight line; all the 
evidence, it seems to me, points in another direction. 
Euclid has just defined points, and stated that the extremi- | 
ties of lines are points; if he had intended what the current 
English translation makes him say, would he not have 
written, “‘ 
with respect to its extremities’’? Again, in i. 9 he says, 
“‘on AB let any point D be taken”’ (eiAnpAw emi tHs AB TuXbY 
onuerov To A); now if D is taken ‘‘ on’? AB, surely it is 
included in ‘the points on AB” (7a ém tHs AB onpeia). 
Moreover, Euclid explicitly recognises infinite, or inde- 
finitely long, straight lines; the enunciation of i. 12 is 
"Em thy SoGeicay evOciar kmeipov, &c.; see also the scholia in 
Heiberg’s edition of the ‘* Elements,’ v. 78-83, 136-9. 
The real difficulty, I think, is in the interpretation of 
é& Yoov; what this is intended to mean can hardly be settled, 
unless new documents should be discovered. Personally, I 
Tbelieve that what Euclid had in his mind was something 
NO. 1795. VOL. 69] 
A straight line is a line which lies evenly (e& %oov) | 
of this sort :—if we stand at any point A-on a straight line 
and look towards any other point B on it, the appearance 
of AB is always the same, and the same as that which we 
get by going to B and looking towards A. But this is 
only a conjecture; my principal contention is that ra ém 
ypauuns Tivos onuera naturally means ** the points on a line,”’ 
namely, all the points on it, including its extremity or 
extremities if it does not extend indefinitely both ways, and 
that this is the sense which the phrase bears in Euclid’s 
definition of a straight line. Your REVIEWER. 
Respiration in Frogs. 
THE respiratory movements of frogs have been studied by 
several observers, especiaily Gaupp and Baglioni, of recent 
years. These zoologists have pointed out that the rapid 
up and down movement of the floor of the mouth, so typical 
of most batrachians, is a kind of aspiration, and does not 
force air in or out of the lungs to any great extent. 
In a frog recently under observation I noticed these move- 
ments taking place while the creature was entirely sub- 
merged, and not engaged in croaking. I should be glad 
to learn whether this is an occurrence well known to your 
readers. ‘To the best of my knowledge neither of the above- 
mentioned naturalists allude to it, but I have not here access 
to their original papers (Gaupp, Archiv. fiir Anat. und 
Physiol., Anat. Abtheil, 1896; Baglioni, ibid., Physiol. 
Abtheil, 1900). 
Is it not possible that in certain circumstances, supposing 
the nostrils to be open and the glottis closed, even the adult 
| frog may take in water into its mouth for the purpose of 
breathing? The remarkable condition of the blood 
capillaries in the mouth points to it as being normally a 
place for interchange of gases when air is admitted. If 
this suggestion be possible, the power possessed by frogs 
of undergoing prolonged immersion may be partially 
explained. M, De Hiner: 
Zoological Laboratory, Eton College, Windsor, March 20. 
Subjective Colours. 
Ix the discussion on this subject in Nature I have not 
seen any mention of a phenomenon which I have now and 
then noticed of late years, but never before, nor have I seen 
it described anywhere. When I have been reading and 
have become sleepy, just as I was about to fall asleep 
portions of the print in patches in different parts of the page 
turned a brilliant red. It is impossible to make any exact 
observations on the subject, because the moment one rouses 
oneself to do that the printing resumes its ordinary black. 
I do not find that this phenomenon is affected by the amount 
of light in any way. 
The phenomenon mentioned by Mr. E. Hubbard (p. 318) 
is true in my case, and I attribute it to the ‘act that the 
eye that is exposed to the greatest light is more or less 
dazzled, or else has light reflected into it from the eyelids, 
and so the field of view is suffused with red or orange light 
which combines with the tint seen with the shaded eye. 
T. W. Backuovuse. 
West Hendon House, Sunderland, March 18. 
Secondary Radiations of Radium. 
I ENCLOSE two prints I have taken with a radium 
screen. No. 1 shows the impression of a steel pen-nib, a 
steel screw, and an ordinary paper fastener. . These articles 
were laid on a photographic plate and exposed for 6 days. 
No. 2 shows the impression of two bronze coins (half- 
pennies) similarly placed, but exposed for 135 days. One 
coin rests partly on the other, and at this part the edge of 
the lower coin is very much blurred, pointing to rather 
great secondary radiation from the upper coin. 
Another peculiarity seems to be that while in one , 
photographs the articles are shown as shadows, in the wanys 
the coins show bright on a darker background. Does th\+ 
point to the possibility of bronze exposed to radium rays 
for such a period as 133 days becoming more radio-active 
than radium itself? Jy. S.* Davis. 
Culham College, Abingdon, Berks, March 19. 
