Heliometer

“ A heliometer is the most accurate astronomical instrument for relative measurements of position, as a transit circle is the most accurate for absolute determinations. It consists of an equatorial telescope with object-glass cut right across, and each half movable by a sliding movement one past the other, the amount by which the two halves are dislocated being read off by a refined method, and the whole instrument having a multitude of appendages conducive to convenience and accuracy. ”

“ The fact remains that before 1820 [2] Fraunhofer had completed one or more of the five heliometers (3 in. aperture and 39 in. focus) which have since become historical instruments. In 1824 the great Königsberg heliometer was commenced, and it was completed in 1829.
To sum up briefly the history of the development of the heliometer. The first application of the divided object-glass and the employment of double images in astronomical measures is due to Savary in 1743. ”

“ They are liable to the objection that their employment is limited to the measurement of very small angles, viz. 13′′ or 14′′ when the magnifying power is 100, and varying inversely as the power. Yet the beautiful images which these micrometers give permit the measurement of very difficult objects as a check on measures with the parallel-wire micrometer.
On the theory of the heliometer and its use consult Bessel, Astronomische Untersuchungen, vol. i.; Hansen, Ausführliche Methode mit dem Fraunhoferschen Heliometer anzustellen (Gotha, 1827) ”

“ Representing this motion on such a scale that the distance of our planet from the sun shall be one inch, we find that the nearest star, on the same scale, will be more than four miles away, and scarcely one out of a million will be at a less distance than ten miles. It is only by the most wonderful perfection both in the heliometer, the instrument principally used for these measures, and in methods of observation, that any displacement at all can be seen even among the nearest stars. ”

“ The measures on a single night with the heliometer should not have a mean error greater than one-fifth of a second, and we might reasonably expect that observations could be secured on about twenty-five nights during the opposition. ”

“ Greenwich Observatory has been here selected for tracing the progress of accurate measurement. But there is one instrument of great value, the heliometer, which is not used at Greenwich. This serves the purpose of a double image micrometer, and is made by dividing the object-glass of a telescope along a diameter. Each half is mounted so as to slide a distance of several inches each way on an arc whose centre is the focus. The amount of the movement can be accurately read. ”

“ As the planet pursued its course, fresh star-couples came into play, during the weeks that the favourable period lasted. In these determinations, only heliometers were employed. Dr. Elkin, of Yale college, co-operated throughout, and the heliometers of Dresden, Göttingen, Bamberg, and Leipzig, shared in the work, while Dr. Auwers of Berlin was Sir David Gill's personal coadjutor at the Cape. Voluminous data were collected; meridian observations of the stars of reference for Victoria occupied twenty-one establishments during four months ”

“ Evidently the farther asunder on the sky are the two stars under observation, the greater will be the separation of the semi-lenses necessary to make a single image of their light. Thus, measurement of the lenses' separation becomes a means of determin [158] ing the separation of the stars themselves upon the sky.
The two slides in the heliometer head are supplied with a pair of very delicate measures or "scales" usually made of silver. These can be examined from the eye-end of the instrument by looking through a long microscope provided for this special purpose. ”

“ The displacement is64 determinable by noticing Mars’s position with respect to stars which chance to be close to him. For this purpose the heliometer is specially suitable, because, having first a view of Mars and some companion stars as they actually are placed, the observer can, by suitably displacing the movable half-glass, bring the star into apparent contact with the planet, first on one side of its disc, and then on the other side—the mean of the two resulting measures giving, of course, the distance between the star and the centre of the disc. ”

“ An arrangement, afterwards described, has been fitted in modern heliometers for placing the screen in front of either segment by a handle at the eye-end. ↑ This heliometer resembles Bessel’s, except that its foot is a solid block of granite instead of the ill-conceived wooden structure that supported his instrument. The object-glass is of 7.4 in. aperture and 123 in. focus. ↑ Description de l’observatoire central de Pulkowa, p. 208. ↑ Steinheil applied such motion to a double-image micrometer made for Struve. ”

“ To Bouguer in 1748 is due the true conception of measurement by double image without the auxiliary aid of a filar micrometer, viz. by changing the distance between two object-glasses of equal focus. To Dollond in 1754 we owe the combination of Savary’s idea of the divided object-glass with Bouguer’s method of measurement, and the construction of the first really practical heliometers. To Fraunhofer, some time not long previous to 1820, is due, so far as we can ascertain, the construction of the first heliometer with an achromatic divided object-glass, i.e. ”

“ The helium atom (or, for that matter, the uranium atom) can on occasion masquerade in the scanty attire of the hydrogen atom. Normal helium has two planet electrons; but if one of these is lost, it becomes hydrogen-like and copies the simple hydrogen system on a different scale. It is significant that the Pickering Series appears only in the very hottest stars—in conditions likely to cause loss of an electron. The difference between hydrogen and hydrogen-like helium is firstly the difference of atomic weight; the helium nucleus is four times as massive. ”

“ The slit of the spectroscope can be moved anywhere on the disk of the sun; so that if the observer sees a tornado begin, he moves the slit along with it, measures the length of its tract and velocity. With the telescope, micrometer, heliostat, and spectroscope came desire for more complex instruments, resulting in the invention of the photoheliograph, invoking the aid of photography to make permanent the results of these exciting researches. This mechanism consists of an excessively sensitive plate, adjusted in the solar focus of the telespectroscope. ”

“ The helium formations do not reach the suns limb, and it is another puzzling detail that the spectrum of the disk shows no absorption line of anything like an intensity to correspond with the emission line of helium in the chromosphere. The prominences are of two kinds, quiescent and eruptive. Some of the former are to be seen at the limb on most occasions; they may hang for days about the same place; they reach altitudes of which the average is perhaps 20,000 m., and show the spectral lines of hydrogen and helium. ”

“ This conclusion could be indirectly tested by examining whether helium is produced in other substances besides radium, for example, in actinium and polonium.
The experimental determination of the origin of helium is beset with great difficulty on all sides. If the a particle is a helium atom, the total volume of helium produced in an emanation tube should be three times the initial volume of the emanation present, since the emanation in its rapid changes gives rise to three products each of which emits α particles. ”

“ But on applying the spectroscope to examine the character of the light which this gas emitted when submitted to the electric current, it yielded, to their amazement, the characteristic light of helium. Thus was the sun-element at last shown to be a terrestrial body, though no doubt a rare one. The circumstances that I have mentioned make helium for ever famous among the constituents of the universe. It will never be forgotten that though from henceforth it may be regarded as a terrestrial body, yet it was first discovered, not in the earth beneath our feet, but in the far-distant sun. ”

“ In a similar manner we find that the second lightest gas, helium, at a temperature of 150° C. (300° F.) , possesses a molecular velocity of 1.62 km. (1.1 miles) per second. This is less than the 2 km. (1.24 miles) per second necessary to leave the Moon’s sphere of attraction. But all helium molecules do not move at the same speed; some are faster127 and some slower than the average. Those moving at a higher rate than 2 km. (1.24 miles) per second constitute a considerable fraction of the total. ”

“ Therefore, we of Puffyloaf are taking today what may be the ultimate step toward purity: we are aerating our loaves with the noble gas helium, an element which remains virginal in the face of all chemical temptations and whose slim molecules are eleven times lighter than obese carbon dioxide—yes, noble uncontaminable helium, which, if it be a kind of ash, is yet the ash only of radioactive burning, accomplished or initiated entirely on the Sun, a safe 93 million miles from this planet. ”

“ If the helium were the final product, the amount present in the emanation tube after a few days or weeks would be insignificant, since the product radium D intervenes, which takes 40 years to be half transformed. Since the helium cannot be the final product of the series of changes, and since all the other products are radio-active, and almost certainly of high atomic weight, it is difficult to see what position the helium atom occupies in the scheme of transformation, unless it be the α particle expelled during the successive changes. ”

“ In a similar experiment, in which a very minute trace of helium had been added to the argon, there was no difficulty in detecting the helium on heating the tube. The conclusion therefore follows that no helium is present in our atmosphere.
It is, besides, exceedingly improbable that helium should be present. For, as shown by Dr. Johnstone Stoney, the rate of motion of a molecule of hydrogen is so rapid that, on finding its way to the confines of the atmosphere, it would escape, and travel through space until it found a [Pg 180] planet of sufficient attractive force to hold it. ”

“ The greatest dignity we may assign them is meta-element.
The formation of helium from the emanation was first shown by Ramsay and Soddy, and later confirmed by Indrikson, by Débierne, by Curie and Dewar, by Giesel and by Himstedt and Meyer. Helium, a conventional element which is devoid of any evidence of chemical affinity, is produced by or from radium, a conventional element, but the most active substance known. This occurs when the emanation is dry, and we have reason for assuming that the emanation may in reality be an active allotropic form of helium, as ozone is of oxygen. ”

“ The radioactive atom in sinking to a lower
atomic weight casts out with enormous velocity an atom of helium.
It thus loses a definite portion of its mass and of its energy.
Helium which is chemically one of the most inert of the elements,
is, when possessed of such great kinetic energy, able to
penetrate and ionise the atoms which it meets in its path. It
spends its energy in the act of ionising them, coming to rest,
when it moves in air, in a few centimetres. ”

“ Weight for weight, it emits during its change an amount of energy at least one million times greater than any molecular compound known (see section 249) . In addition, it must be supposed that the rate of breaking up of the helium compound is independent of great ranges of temperature—a result never before observed in any molecular change. The helium compound in its breaking up must give rise to the peculiar radiations and also pass through the successive radio-active changes observed in radium. ”

“ A fact which may have a significance which we cannot at present see is that the emanation from radium gradually and spontaneously changes into helium, an alchemistical feat of nature that has opened many curious vistas to speculative thinkers. The eruptive prominences, which do not spread horizontally like the others, but ascend with marvelous velocity to elevations of half a million miles or more, are apparently composed largely of metallic vapors—i.e. metals which are usually solid on the earth, but which at solar temperatures are kept in a volatilized state. ”

“ These effects could be explained and the difficulties attendant on the view that the helium comes from an impurity of the ordinary kind avoided by some such view as the following. The evolution of helium is not due solely to the bombardment, by the cathode rays. In the atoms of the ordinary elements and especially in those of the alkali metals a process is at work analogous to that which causes the expulsion of a particles from the atoms of the ordinary radioactive elements ”

“ The spectrograph has great advantages over the telescope in discovering and observing double stars whose components are very close together, by virtue of the facts that the spectrograph measures, velocities of approach and recession in absolute units—so many kilometers per second—and that the speeds of rotation in binary systems are higher the closer together the two components are. The observations of the brighter helium stars, especially those made at the Yerkes Observatory by Frost and Adams, have shown that one helium star in every two and one half on the average is a very close double. ”