Mr J. Janssen, a member of the Institute and Director of the Observatory of Physical Astronomy of Meudon, made use of the headlamps of the Tower of Experiments that the Proceedings of the Academy of Sciences of May 20, 1899 recount. The text is repeated below, it again evokes the interest of scientists for the Eiffel Tower.
M. Eiffel, having very obligingly placed the Tower of the Champ de Mars at my disposal for the experiments and observations that I would like to institute there, I thought to take advantage of the powerful source of light that has just been installed there for some studies of the telluric spectrum and, in particular, that which relates to the origin of the spectral lines of oxygen in the solar spectrum.
We know today that there are several groups of rays in the solar spectrum that are due to the oxygen in our atmosphere; but we may ask ourselves whether these groups are due exclusively to the action of our atmosphere, and whether the solar atmosphere enters into it for nothing, or whether their origin is double; in a word, if they are purely telluric or telluro-solar.
To solve this question, a number of methods can be used.
One of the most reliable is that of vibration, whose origin goes back to the fine design of Mr Fizeau and which was applied by Mr Thollon and perfected by Mr Cornu. It seems a rather difficult application in this case.
We can also observe the decrease in intensity that groups experience as we rise in the atmosphere and, by comparisons as careful as possible, and especially by the great practice of observations, judge whether the The decrease in intensity of the lines makes it possible to conclude that they disappear completely at the limits of the atmosphere. This is the method used in the last expedition to the Mont Blanc Massif (Grands-Mulets).
We can still proceed by a comparison of equality by installing a powerful continuous spectrum light at a distance from the analyzer that is such that the atmospheric thickness traversed represents the action of the Earth's atmosphere on the solar rays around the zenith.
But this last circumstance was very happily found realized by the respective situations of the Eiffel Tower and the Observatory of Meudon.
The Tower is at a distance of about 7,700 m from the Observatory, which is about the thickness of an atmosphere having the same weight as the earth's atmosphere and a uniform density equal to that of the surrounding atmospheric layer. soil.
In addition, the considerable power of the luminaire currently installed at the top of the Tower allowed the use of the instrument that had served me in Meudon and Grands-Mulets for the sun. However, I used a collector lens in front of the slot, to bring the spectrum to have an intensity quite comparable to that of the solar spectrum in the same instrument.
Under these conditions, the spectrum was extremely lively. The spectral field extended beyond A (Groups A and B are due to oxygen uptake in the air, Group B seemed to me as intense as the summer meridian sun. Group A was also highly charged, and there were still other groups, particularly those of water vapor, whose intensity seemed to me to correspond to the hygrometric state of the atmospheric column traversed.
No oxygen band showed up in the visible spectrum. However, the thickness of the oxygen layer crossed was equivalent to a column of more than 260 m of oxygen at 6 atmospheres of pressure, that is to say at the pressure for which the tube of our laboratory shows them. with a length of only 60 m, or four times smaller. This shows that for oxygen the lines obey a different law than the bands.
Indeed, while for the rays the experience of last Sunday shows us that it seems indifferent to use a column of gas with constant density or acolumn equivalent in weight but variable density; for the bands, on the contrary, the absorption taking place according to the square of the density, the computation shows that iton the surface of the ground, an atmospheric thickness of more than 50 km is needed to produce them.
I only consider the experience of last Sunday as bringing one more fact to a set of studies, a fact that needs to be clarified and developed. But it is certain, for me, that the height at which the tower of the Champ de Mars makes it possible to place the luminous focus and the power of this hearth promise us experiments of the order of those which have just been made and of the most high interest. "
Explanatory note on the presence of oxygen in the Sun
Since the admirable application of spectral analysis to astronomy, we know that the sun contains most of our terrestrial metals, and everything indicates that it is the great reservoir where all the bodies that make up our system planetary are together. However, we have not found the presence of a body of immense importance for the production and maintenance of life on the surface of our earth, namely: oxygen.
Mr. Draper had thought he could announce the presence of oxygen in the sun, according to certain experiments; but this conclusion was found to be inaccurate. However, oxygen lines appear in the solar spectrum and they form important groups named A, B, & alpha; (We know that these groups of lines belong to the oxygen gas because they are obtained by passing a light beam through long enough containing only pure oxygen).
Are these groups solely due to the action of the oxygen contained in our atmosphere that the solar rays must necessarily pass through, or are they already pre-existing in the solar spectrum that would be obtained before the entry of the solar light in the Earth's atmosphere, and does it only increase the intensity?
This is the question to be solved if we want to be able to affirm that oxygen, at least as we know it in our laboratories and in the Earth's atmosphere, exists or does not exist in the solar atmosphere. Since we can not carry our instruments to the limits of the atmosphere, we are obliged to use the method which consists in showing that the diminution of the intensity of the oxygenated groups of the solar spectrum is related to the atmospheric thickness crossed. (as can be achieved by the use of a high station, Mont Blanc for example, or even by showing that if we pass through a beam of light, an atmospheric thickness equal to or equivalent to that which the rays At a certain time of the year, in June, for example, and at noon, the groups thus obtained are artificially equal in intensity to those of the solar spectrum under the above-mentioned conditions.
It is precisely this last condition that could be realized by analyzing at the Observatoire de Meudon a light beam produced at the top of the Eiffel Tower, because the distance between these two points is very close to that which represents a thickness atmospheric equivalent as quantity to that of the Earth's atmosphere, that is to say that a vertical radius crossing the Earth's atmosphere must experience an absorption equivalent to that of the same radius from the Tower to Meudon, admitting well It is understood that the absorption is proportional to the weight quantity of air through which, moreover, it has been ascertained with respect to the group of lines A. B, & alpha ;. This is what gives a particular interest to the experiment made in 1889 between the Tower and the observatory of Meudon, an experiment that would be very interesting to resume in more rigorous and conclusive conditions of accuracy.