Mr J. R. Green, Trinity College, was balloted for and duly elected an associate of the Society.

Mr GLAISHER having taken the Chair, the following communications were made to the Society:

(1) On the construction of a negative eye-piece. By Dr J. B. PEARSON.

I wish to be allowed to offer a few remarks on the construction of the negative astronomical eye-piece: only as designed for the use of abnormally short-sighted or long-sighted persons. For good average sight, I have no doubt that the existing construction is all that is to be desired.

Sir E. Beckett (Astronomy without mathematics, p. 301) says: "All eye-pieces are made adjustable for different eyes by the eyeglass sliding nearer to the field-glass for short-sighted eyes, which require the rays of each pencil to diverge a little instead of being parallel." It is true this is the case with the sextant-circle telescopes which I use, the two German ones having positive eyepieces, and the French a negative one: but it is not so with the common astronomical negative eye-pieces, such as are sold in England. Now with powers up to say 70, and an aperture of about 3 inches, I have found I got very good results by focussing in the usual way; but with a power of 120, I found I could get no satisfactory definition; and as a fault, obviously the same, came out with different object-glasses, I was led to examine the construction of the eye-piece to see whether part at any rate of the fault might be there.

Theoretically, and in practice, for perfect eyes, the focus and diaphragm or stop of a negative eye-piece are placed between the two lenses: but if a short-sighted person adapts the focus of the telescope to his own eye, he moves the whole eye-piece by means of the large screw. This really means that he pushes forward the field-glass, i.e. the lens nearest the object-glass, so that it stands nearer to the object-glass: by this means the field-glass intercepts portions of the pencils coming through the object-glass, which on an average are more nearly parallel to their axis: the focus of the rays passing through the object-glass and field-glass is thus thrown farther from the field-glass than it was before; and, as the distance between the field- and eye-glass is unchanged, also nearer to the eye-glass they thus diverge after passing through the eye-glass, and are consequently fit for vision by a short-sighted eye.

This process answers very well in ordinary cases and with moderate magnifying powers, but if extreme precision is needed, it seems to me to be defective. The field-glass foreshortens slightly the focal length of the object-glass, but its special purpose is to

chromatise the rays, if I may use such a phrase, in order that they may be achromatic when they have passed through the eye-glass. The place of its focus however, as far as I can see, ought not to be changed: it is one easily ascertained: it is the focus for rays not parallel, but with the convergence given them by the object-glass at the point where the lens intercepts them to the greatest advantage. Knowing the focal length of the object-glass and of the field-glass, it seems to me that this point ought to be carefully ascertained by the maker, and indicated by the stop: my French sextant-telescope has wires at this point which I can get perfectly distinct by adjusting the eye-glass; and I think without distortion, though theoretically it seems this ought to be a result. Any how the eye-glass should be made moveable as I thought myself, or else should be altered in form as was suggested to me by a friend, i.e. made less convex for a short-sighted person, more so for one with long-sight; so that the person using the telescope, by first focussing on the stop, will have as perfect vision as possible. When I mentioned the difficulty, to a friend or to my maker, though I do not remember which, the remedy he suggested was, that I should focus on the stop: and this is no doubt correct, but I am not sure that this prescription is sufficient of itself. Mr Coddington says that the stop is to be placed half-way between the two lenses, the field-glass and the eye-glass: but this is not actually done in practice; it being placed much nearer to the eye-glass than to the other and also that the two lenses should be meniscus-shaped, the field-glass with the surfaces of radii 4 and 11, the convex side turned towards the object-glass: and the eye-glass with surfaces 6 and 1, making what is called a crossed lens, the convex side here also being turned towards the other glass: and both these principles, I may add, seem borrowed from the rules given in Sir Geo. Airy's paper on "Achromatic Eye-pieces" in Vol. III. of our Transactions, read in 1827. To my mind, the stop ought to be fixed by the relative focal-lengths of the object-glass and of the field-glass: and the eye-glass may vary in distance from it according to the nature of the observer's eye.

The following objection has been taken to making the eyeglass moveable: that the achromatism is injured by doing so but it seems to me that the great thing is definition, and that the eye itself may possibly rectify a slight defect in this respect. At the same time an alteration of the form of the eye-glass to suit a shortsighted eye seems a most proper thing to introduce: the focussing on the stop being reserved to determine finally its position. A positive eye-piece is, I believe, not achromatic.

To sum up: my principle is this. The field-glass, of a defined focal length, placed in front of an object-glass of defined focal length, produces the best defined image at a definite point. The

stop should be placed here. The eye-glass should be adapted, in form and position, to receive this image to the best advantage. Not till then should the large focussing screw be used to bring the stop, and practically the eye-piece, into its proper place.

I believe I am justified in thinking that achromatism may be, comparatively speaking, neglected: first, because it is allowed that positive eye-pieces are not achromatic: secondly, because it seems probable that chromatic aberration shews itself principally when the lenses are large, which those of the eye-piece are not. I may add that this kind of eye-piece seems to have been designed by its inventor, Huyghens, to correct distortion, and not chromatic aberration.

(2) On the circumstances producing the reversal of spectral lines of metals. By Professors LIVEING and DEWAR.

Our object in investigating the reversals of the lines of terrestrial elements has been to trace the parallel between the conditions of the elements as they exist in the sun, and those in which we can place them on earth; with a view to illustrate more fully the problems of solar chemistry. A knowledge of the reversible lines may also help us to distinguish those rays which are directly due to the vibrations communicated to the luminiferous ether by the freely moving molecules themselves from those which are produced by superposition of waves, or by some peculiarity in the mode in which the molecules are put in motion, or by some strain upon them, such as the violent Leyden jar discharge might give, which would not be an easily reversible action.

In a former communication we brought before the Society the results of our observation of the reversal of the lines of some of the more volatile metals when the continuous spectrum of the hot walls or end of an iron or porcelain tube was observed through vapour filling the tube. As the temperature of the tubes was only that of a crucible furnace (about 1500° C. at the outside), it was a very limited number of the lines even of the volatile metals which could be reversed in that way, because the relative strength of the emissive (and therefore of the absorptive) power of a vapour for particular rays often varies considerably when the temperature is changed. Thus the vapour of sodium in an ordinary flame emits little radiation except that of the yellow pair of lines and only absorbs the same; and it is only at higher temperatures such as that of the electric arc, or that of a flame locally increased by the introduction of an endothermic salt such as a chlorate, that the other pairs (or groups) of lines red, orange, green and blue are seen either bright or reversed. We want then a source of light which shall give a more or less continuous spectrum of an intensity greater than that of the lines to

be observed, and the metallic vapour not only in a layer of some thickness, but heated up till it is capable of freely emitting the light of those lines which we want to reverse. In the case of the less volatile metals such as iron and aluminium it would not be expected that these conditions should very easily be fulfilled.

After trying the oxy-hydrogen jet to ignite the bottom of a tube bored in a block of lime, which gave us reversals of the wellknown green and orange bands of lime, and of the blue line of calcium which was barely reversible in tubes heated in a crucible furnace, we turned our attention to the electric arc as a source of heat both to give the requisite bright background and to vapourize the metals examined, but enclosed it in a block of lime or magnesia by which we were able to maintain a considerable amount of

b. hole bored to the centre of block through which observations were made of the arc.

c. hole by which metals were dropped into the arc, usually covered with a piece of lime.

d.d. carbon rods forming electrodes of dynamo-electric machine.

refractory metal in a state of vapour, and from the tube-like form of the opening through which the arc was viewed could observe the absorption of a tolerable thickness of such vapour gradually diminishing in temperature as it receded from the arc. The greater part of our observations were made with some one or other of several modifications of this apparatus.

For the ultra violet part of the spectrum photography was used. Generally 15 photographs were taken on the same plate in order to vary the time of exposure, as good impressions of faint lines cannot be obtained without over exposing the strong lines: and also in order to catch different phases of effect as the metal introduced evaporates.

The reversals thus observed may be classed under several heads.

I. Reversals by the expansion of the line observed or as they may be called self-reversals. These are the reversals most generally known. The sodium lines are frequently so reversed, indeed always when a sufficiently volatile salt of sodium such as the carbonate or chloride is held in a gas flame. If sufficient sodium is in the flame the lines are widened, and the less dense and less hot sodium vapour outside the flame produces a narrow absorption line down the middle of the bright yellow band. In this way the metal itself gives the background against which the reversed line is seen; and in order to produce the result the line must be more expanded at the higher temperature than at the lower. That does not always happen, but it is so common that many reversals may be seen in this way. The magnesium line of wave length 2852 has great power of expansion and is always seen reversed in this way when the arc is taken in a magnesia crucible as in Plate I. fig. 1.

One set of the photographs exhibited are original negatives of the arc spectrum shewing reversals of this class in the case of lines of magnesium, thallium, indium, tin, aluminium, antimony, lead and zinc. They are only a selection out of a very large number taken by us but sufficient to shew the characters of this class of reversals.

Professor Hartley has lately (Proc. Roy. Soc. XXXIV. 84) called attention to the pseudo-reversals of this class which may be produced in the case of a strong line by over-exposure. It is well known that over exposure (solarization as we used to call it formerly) produces such an alteration in the sensitive preparation of the photographic plate that the over-exposed parts cease to be de

velopable, so that a very strong line may appear white in the negative where it ought to be black, but with a dark border, and so give the appearance of a reversed line. Professor Hartley finds it difficult to distinguish real reversals of the class we are now discussing from these pseudo-reversals. His difficulty has not occurred to us, first because we have always been in the habit of taking our photographs in series with varying exposure, in order to get impressions both of the feeble lines in some and of strong lines in others; and secondly because we almost always close part of the slit of the spectroscope with a shutter so that the