Fune 9, 1870] 
that the paper is a valuable contribution to our knowledge 
of Eastern Neolithic implements, and that our present 
remarks are, like those of Mr. Theobald, “ merely tenta- 
tive, and designed to elicit additional information.” 
J. Evans 
M, FIZEADS EXPERIMENTS ON“NEWTON’S 
RINGS” 
COMPARISON of the values given by Professor 
‘Angstrém (in his magnificent Recherches sur le 
spectre solaire) for the wave-lengths corresponding to the 
two principal components of Fraunhofer’s line D, with 
some observations made eight or nine years ago by M. 
Fizeau, not only reveals a remarkable agreement between 
the results of these two distinguished investigators, but 
yields one of the most striking confirmations of the truth 
of the undulatory theory of light that recent optical re- 
search has afforded. 
The experiments of M. Fizeau to which we refer were, 
essentially, the following. He produced the phenomenon 
of “ Newton’s rings,” by laying a convex lens of very 
long focus upon a piece of glass with plane parallel sur- 
faces, and illuminating the combination by the yellow 
flame of spirit of wine containing a little common salt. 
The lens was so arranged that it could either be made to 
touch the glass plate or be separated a short distance 
from it, its position being regulated by a micrometer 
screw. On gradually separating the lens from the glass 
plate, the rings were seen to contract and move in to- 
wards the centre of the lens, where they successively dis- 
appeared, while their place was supplied by fresh rings 
which made their appearance at the circumference of 
the lens. So far, all was in accordance with what was 
well-known before. But M. Fizeau found that when the 
phenomenon was observed with sufficient care, nearly 500 
rings could be counted, flowing inwards one after another, 
but that after about this number the rings ceased to be 
visible, the surface of the glass showing a nearly uniform 
illumination all over instead of a sharply defined alterna- 
tion of light and dark bands. When, however, the dis- 
tance between the lens and the glass plate was further 
increased the rings re-appeared, getting gradually more 
and more distinct, until when nearly another 500 had 
passed they had become as sharp as at first; but a still 
further increase of distance caused them again to become 
confused, and they ceased a second time to be discernible 
at about the 1,500th. With a still greater separation of 
the glasses, however, they reappeared again, and became 
quite sharp at about the 2,oooth, after which they for a 
third time got gradually confused and became indistin- 
guishable at about the 2,50oth. 
So the phenomenon went on, the stream of rings in- 
wards towards the centre of the lens, followed by fresh 
ones from the circumference, continuing as the lens was 
moved further and further away from the glass plate; but 
the succession of rings was not uniform, but broken up 
into batches of about 1,000 each, separated by short in- 
tervals of confusion in the way that has been described. The 
rings did not finally cease to be distinguishable until 
Jjifty-two such batches had been counted, and the two 
glasses were at a distance of about fifteen millimetres 
(more than half an inch) from each other. 
This remarkable phenomenon of the alternate periods 
of distinctness and confusion of the rings is easily ex- 
plained, as M. Fizeau points out, when we remember that 
the light employed was not strictly homogeneous, but con- 
sisted of two portions of nearly, but not quite, equal 
degrees of refrangibility. If either of these two consti- 
tuent parts of the light had been used by itself, it would 
have produced a set of rings, but the rings of one set 
would have been a very little larger than the correspond- 
ing rings of the other. Hence if the two sets of rings are 
put together (as they were in Fizeau’s experiment), they 
NATURE 
105 
will nearly, but not quite, fit each other. If we examine 
a few rings at the centre, when the two glasses are in con- 
tact, they will appear to coincide precisely ; but if they 
are traced to a sufficient distance from the centre, the 
coincidence is seen not to be exact. For although the 
twentieth ring (say) of one set is not perceptibly bigger 
than the twentieth ring of the other set, the fve-hundredth 
of one set is perceptibly bigger than the five-hundredth of 
the other, and, when put upon it, falls almost exactly half 
way between the five-hundredth and five-hundred-and-first 
of this set. Consequently, at about this part of the phe- 
nomenon, the bright spaces of one set of rings will occupy 
the same position as the dark spaces of the other set, and 
they will mutually obliterate each other. But since the 
thousandth ring of one set is nearly the same size as the 
thousand-and-first of the other, the two sets of rings will 
appear to fit each other again about this point; the 
jifieen-hundredth of the first set, however, is larger than 
the fifteen-hundred-and-first of the second set, but not so 
large as the fifteen-hundred-and-second ; and hence, at 
about the position of this ring, the rings of the two sets 
will overlap each other, and mutually efface each other’s 
outlines. And, carrying such considerations further, it 
is evident that the apparent coincidence and overlapping 
of the two systems of rings would recur alternately at 
regular intervals. 
In order to simplify this explanation, we have tacitly 
assumed the lens to be so large that several thousand 
rings could be seen between its centre and its circum- 
ference. Practically, this would be impossible ; but, by 
gradually separating the lens from the plane glass, we 
can, as it were, draw in towards the middle the rings 
which, with a larger lens, would be formed at a great 
distance from the centre. 
Now, according to the explanation which the undulatory 
theory gives of the formation of “ Newton’s rings,” the 
distance by which the interval between the glasses must 
be increased, in order that a given ring may come into 
the position previously occupied by the next smaller ring, 
must be equal to half the wave-length of the kind of light 
used for the experiment ; and the distance of 0°28945 milli- 
metres, through which, as M. Fizeau found by actual 
measurement, it was necessary to vary the space between 
the glasses, in order to make the rings go through one of 
the recurrent periods above described, that is to say, pass 
from sharpness to confusion and become sharp again, 
must contain just one more half wave-length of one 
portion of the light by which the rings were formed than 
it does of the other. 
This brings us to the point of contact between M. 
Fizeau’s observations and those of Prof. Angstrém, to 
which we referred at the beginning. According to the latter, 
the wave-lengths of the two principal constituents of the 
light emitted by a flame containing the vapour of sodium 
(such as the flame employed by M,. Fizeau) are 
respectively — 
Millimetres 
070005 89513 
and 07000588912. 
Now, if we divide 028945 by half the former of these 
numbers, we get as the quotient 982 ; and if we divide it by 
half the second, we get as the quotient 983. That is to say, 
we find, precisely as the undulatory theory requires, that the 
distance measured by M. Fizeau contains exactly one more 
half wave-length of the more refrangible constituent of the 
light of a sodium-flame than it does of the less refrangible 
part. And, moreover, if we calculate, from Angstrém’s 
determination of the wave-lengths, the number of rings 
which must intervene between the positions of greatest 
confusion and greatest distinctness, we find 491 of the one 
set and 491} of the other, which agrees entirely with 
M. Fizeau’s estimated round number 500. 
G, C, FOSTER 
