Fuly 8, 1886 | 
NATURE 217 
Ampere’s Rule 
BEGINNERS are certainly, as Herr Daehne says (NATURE, 
June 24, p. 168), liable to get a little “‘ mixed” in reference to 
the above mem. technica ; chiefly, I think, for want of some idea 
sufficiently prominent to fix itself on the mind to the exclusion 
of others. 
T have found the following slight modification of the original 
rule pretty easily remembered and applied, 
It may be taken as agreed— 
(1) That the Aead is more important (‘‘ more worthy,” as the 
old grammars put it) than the cer. 
No one except an acephalous mollusk will deny this ; and 7 
is not a fair judge. 
(2) That the 77gA¢ hand is more important than the left hand. 
The left-handed people are a mere minority (and a nuisance 
at cricket) ; and minorities are, according to modern Radical 
ideas, ‘‘ une quantité négligeable.” 
3. That the N-seeking pole is that part of the compass-needle 
to which attention is mainly directed. 
Now,—If a person places himself so as to face the needle, and 
a current goes from HEAD to foot, the N-SEEKING pole moves to 
his RIGHT hand, 
This is practically the form in which the rule is given in 
Prof. Balfour Stewart’s ‘‘Lessons in Physics.” One of the 
small articulated wooden figures used as models in drawing is 
very useful for illustrating the above rule. Its right arm may be 
stretched out sideways at right angles to the body, and it may 
then be held close to the wire in various positions ; paper arrows 
being tied to the latter, to mark direction of current. 
Eton College H. G. MADAN 
Halos 
As the atmosphere appears recently to have reassumed in a 
marked degree some of the peculiar conditions which pertained 
to it during the time of the great sun-glows, I have thought it 
worth while to send you notes from my diary of some effects 
observed by me :— 
June 14.-—Between 10 and 11 a.m. Complete solar halo of a 
coppery colour, It lasted more or less distinctly for some time, 
and gradually faded. I saw no trace of mock suns. 
June 23.—Between 10 and 10.30 p.m. there was a curious 
pearly green light in the north-north-east, and some peculiar 
pearly green clouds (?) floated from north to west. At first I 
thought this was an auroral display, but probably it was due to 
the same cause as the ‘‘ glows.” 
June 15, 23, 30, July 1, 2, and 3.—After-glows of the usual 
pinkish hue. J. H. A. JENNER 
4, East Street, Lewes, July 3 
The Microscope as a Refractometer 
I HAD no idea that the short paper you did me the honour to 
print on this subject would have led any one to stippose that a 
claim was made for the discovery of a new principle in physics, 
or that the microscope was to be used for the first time in ques- 
tions on refraction, 
In so short a space it was impossible to tell over again the tale 
of progress in this branch of physical optics, and to signalise 
every worker in the field by name. So much has already been 
done in the perfecting of optical instruments, that the utmost 
one can now hope to do is, by a slight improvement here and 
there, to render them still more serviceable. 
All that was claimed as new in my paper of June 17 was— 
(a) The use of the marked slip, structure of cell, superposed 
cover-glass. 
(8) The measurement of the linear distances between the 
images by a finely graduated ‘‘ fine adjustment ” screw. 
(y) The use of an objective of high amplifying power (a 
1/20-inch homogeneous immersion may be used if the shoulder- 
Le of the cell are made with talc, and the cover-glass very 
thin). 
It is of course possible that one or all of these details is not 
new ; but, in spite of the authorities quoted by Dr. Gladstone 
to show the previous employment of the microscope in questions 
of refraction, I still maintain their claims to novelty to be valid ; 
and, even supposing they are not new (which has yet to be 
shown), my greatest offence is that of independently arriving at 
a previously known method. And, considering the attention 
that our most eminent physicists have bestowed upon the 
subject, the wonder is that this has not more frequently been the 
case. E 
As to the efficiency of the method, the only objections urged 
against it by Dr. Gladstone are: (1) its results cannot be relied 
upon beyond the third decimal figure ; (2) the temperature of 
-the drop of fluid under examination cannot be taken. 
As to the first objection, if we take 44, #. as the tabulated in- 
dexes of refraction of two known substances, 5 as representing 
the difference of distance between the images of the marks 
viewed through them, and measured by the fine adjustment, « 
and @ the corresponding symbols for the fluid under examina- 
tion, we have the following equation to determine u :— 
And Isee no more reason to limit the exactness of this to the third 
decimal figure than in the formula used with the hollow prism. 
Moreover, if a vernier is attached to the fine adjustment the 
result may be relied upon with still greater accuracy. 
(2) As to the temperature. In the case of most fluids this 
may be taken from the bottle containing the fluid; no grave 
-scientific error will arise from the difference in temperature of a 
drop of fluid in contact with glass on the stage of the micro- 
scope and the same fluid in a glass bottle by its side. In the 
case of ethers, &c., the cell may be temporarily sealed. 
As to the practical use of the method, the opinion of so known 
an expert as Dr. Gladstone is of the greatest weight, but as any 
recognition of the novelty of my method escaped acknowledg- 
ment in his notice, I may still hope that its practical use 
escaped observation also. So thin astratum of fluid is employed 
that the index of refraction of black ink may be obtained, a 
result which would puzzle any one to arrive at who restricted 
himself to the use of the hollow prism. 
That the microscope has been previously used for experiments 
in refraction no one ever doubted; if Dr. Gladstone, before 
writing, had had the time to go step by step through my 
method, he could scarcely have refrained from acknowledging 
that in its essentials it was hitherto unpublished. 
GorpDON THOMPSON 
St. Charles’s College, Notting Hill, July 3 
The Bagshot Beds 
As you have given publication (NATURE, July I, p. 210) 
to the abstract of the paper recently read by Messrs. Monckton 
and Herries before the Geological Society, in which they assert 
that their object was to ‘‘disprove”’ the view lately propounded 
by me, as to the relation of the Bagshot Beds of the London 
Basin to the London Clay, perhaps you will kindly afford me 
space to point out to the readers of NaTuRE (1) that these 
authors have ignored, in dealing with the question, whole 
chapters of the evidence upon which my view is based—evidence 
which is continually accumulating, as two forthcoming papers 
(one in the press for the Pvoc. Geol. Assoc., the other in the 
hands of the editor of the Geo/. AZzg.) will make mani- 
fest enough; (2) that in directing their attention merely to 
sections at the outcrop of the beds they have added Ittle, 
if anything, substantially, to that on which the old view was 
based, while the lithological distinctions of the Upper and 
Lower Bagshot Beds (where the latter have been for ages under- 
going oxidation) are not sufficiently marked to furnish, in discon- 
nected sections, evidence which can be anything more than, to 
say the least, equivocal. A. IRVING 
Wellington College, Berks, July 3 
The Enemies of the Frog 
IN connection with this subject the following incident may be 
of interest to some of your readers. One day, near the kitchen 
area, an unusual noise was heard: it seemed like the mewing 
of a cat combined with a well-sustained whistle. On going to 
the spot, it was found that the noise proceeded from a cat and 
a frog, but it was difficult to decide from zwwhich of the two. 
Every time the cat touched the frog the sound was produced 
and the frog hopped away. The cat exhibited in his attitudes 
and motions a sort of enjoyment mingled with awe. He would 
just touch the frog very gently with the tips of his paws, then 
watch it most attentively, and when the frog would emit its 
peculiar loud squeak—not the usual croak—he would give a 
sudden bound, as if both sirprised and amused; but he never 
