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If the diameter of the lenses is reduced, not only is the size of the sphere on which a given number of them would lie reduced, but, since the definition of each lens decreases with the diameter, a less number of lenses will be required to give the maximum definition attainable under the changed circumstances. Thus the radius of the sphere proper for the surface of a composite eye decreases as the square of the defining power of the separate lenses of which it is composed.
Let A and B (fig. 2) be two adjacent lenses, C and D the sensitive spots of the retina. Let o be the angle between the axes of A and B, and the limit of definition of the lens. Then, if x = 0, the image of a distant object in the axis of A will just fall clear of the sensitive point D, but, if x>0, both C and D will be illuminated by light from the same object.
Supposing, however, x is less than 0, nothing will be gained in definition unless each lens has more than one sensitive point to operate on. If, then, we find that in actual composite eyes x
and o are nearly equal, that is, that the difference in the direction in which the adjacent lenses point is nearly equal to the defining power of the lens itself, it becomes almost certain that each lens has only one sensitive point behind it.
The following table contains measures, recently made by me, of the diameters and angles between the axes of the lenses of various insect eyes, and, although the measure of the angle of view was necessarily rather rough, the agreement of the results, in the larger number of cases, with the supposition above made seems to me sufficiently remarkable.
In estimating 0 there were two difficulties, one of which was that in many eyes the curvature of the surface was sharp at the margin and that the definition was probably bad there, and another that the line of sight of each lens was not always normal to the outer surface of the eye (fig. 3). Generally, I took the angle between the tangents
to the surface at the ends of a measured chord, choosing the chord so that the surface outside it should have fairly uniform curvature. The length of the chord wảs asually about three-quarters, or a little more, of that of the eye.
Taking the length of the chord as I, and r as radius of the sphere which best represents the surface of the eye, we have for the angle of view Ꮎ,
and 0 = d/r, where d is the diameter of the lens ; hence
The other columns of the table explain themselves.
On the whole, I think it must be concluded that Insects do not see well, at any rate as regards their power of defining distant objects, and their behaviour certainly favours this view ; but they have an advantage over simple-eyed animals in the fact that there is hardly any practical limit to the nearness of the objects they can examine. With the composite eye, indeed, the closer the object the better the sight, for the greater will be the number of lenses employed to produce the impression; whereas in the simple eye the focal length of the lens limits the distance at which a distinct view can be obtained.
The best of the eyes mentioned in the table would give a picture about as good as if executed in rather coarse wool-work and viewed at a distance of a foot; and, although a distant landscape could only be indifferently represented on such a coarse-grained structure, it would do very well for things near enough to occupy a considerable part of the field of view.
II. “The Action of Heat upon Ethylene.” By VIVIAN B. LEWES.
Professor of Chemistry in the Royal Naval College, Greenwich. Communicated by Professor THORPE, F.R.S.
Received December 6, 1893. The decompositions of the simpler forms of hydrocarbons at an elevated temperature have always been recognised as a question of the greatest importance, as upon them is dependent a true conception of inany of the actions taking place in the manufacture of coal gas and other kindred processes of destructive distillation.
Ethylene has in most cases been chosen as the hydrocarbon which would lend itself most readily to experimental researches upon this point, as, besides being one of the simplest, it is easily prepared, and is moreover found as one of the products in nearly all cases where organic compounds are subjected to distillation at high temperatures.
No sooner had the difference between ethylene and methane been recognised, than experiments were made by Deimann, Van Troostwyk, Lauwerenburg, and Bondt* to ascertain the action of heat upon the newly-formed compound, and the conclusions which they came to were that on heating no contraction in volume was observed, but that the tubes in which the decomposition was effected became coated with a black deposit, and drops of an oily body were formed, the gas
Annales de Chimie et de Physique,' 1st series, vol. 21, p. 48.
at the same time losing its property of forming an oily liquid with chlorine.
These experiments were afterwards repeated by Fourcroy, Hecht, and Vauquelin, * who showed that when heated, ethylene yields hydrogen with deposition of carbon, whilst in 1805 William Henryt showed that ethylene was formed during the destructive distillation of organic bodies, and that on further heating the gas, other changes were observed, and the gas was eventually converted into carbon and hydrogen. The deposition of carbon was also noticed later by Quet, I who on passing sparks through ethylene found that carbon was deposited, and formed a bridge between the poles used for the discharge, whilst Dalton showed by the continuous action of the electric spark that ethylene yielded double its own volume of hydrogen, carbon being deposited.
Marchand came to the conclusion that at a red heat this gas splits up into methane and carbon, but at a white heat into carbon and nearly pure hydrogen, whilst Magnus, in 1847, made the important observation that on leading ethylene through a red-hot tube a contraction in volume followed ; the residual gas consisted of methane, hydrogen, and unchanged ethylene, whilst carbon was deposited, and fluid and even solid hydrocarbons were obtained.
In 1860 H. Buff and A. W. Hofmann | published a paper on the “ Dissociation of Gaseous Compounds on Heating by Electricity.”
They found that when a platinum spiral is heated by the galvanic current in pure ethylene there is at once a visible separation of carbon, which covers the sides of the tubes with a black deposit, whilst hardly any expansion in the volume of the gas takes place, from which they assume that the ethylene has split up into methane and carbon.
If the action on the gas, due to the incandescent platinum wire, is allowed to continue, then an increased amount of the gas undergoes dissociation, and soon after the separation of carbon commenced, they observed an expansion which is rapid at first, and in ten minutes reaches a maximum. Similar phenomena were noticed with the spark current; at first the spark had a pale reddish tint which gradually turned to violet, immediate separation of carbon taking place, the spark being frequently stopped by scales of carbon which formed a bridge between the poles. They found that under these conditions the volume of gas expands very rapidly at first but afterwards more slowly, and that after twenty to twenty-five minutes, the point of maximum expansion is reached, so that 7 c.c. of dry ethylene gave, after decomposition, 12:25 c.c. They noted also that the residual hydrogen had an unpleasant smell, and burnt with a slightly luminous flame.
* Gilbert's ' Annalen,' vol. 2, p. 210. + Nicholson's Journal,' 1805. I 'Comptes Rendus,' vol. 42, p. 903. § 'J. für prakt. Chem.,' vol. 26, p. 478. || Liebig's ' Annalen der Chemie,' vol. 113, p. 119.
Berthelot,* in 1869, claims that ethylene breaks up under the influence of heat into acetylene and hydrogen, as expressed by the eqaation
C,H, = C2H2 + H2, and shows that the acetylene then polymerises into benzene, styrene, and other liquid products of higher boiling points. Naphthalene was also formed by the direct condensation of styrene and acetylene. He also points out that during the heating of ethylene a large proportion of ethane was formed, and his final conclusion is that the heating of ethylene results in the splitting up of 2 mols. of ethylene into acetylene and ethane, and that the formation of solid and liquid products is due to the subsequent condensation of the acetylene.
In 18867 Day made a number of experiments in order to determine the lowest point of temperature at which the constitution of ethylene undergoes alteration, and the nature of the changes taking place at that temperature. In order to do this, he devised an ingenious apparatus in which the ethylene could be heated for very long periods in a hard-glass tube. From these experiments he concluded that when the action is continued over a long period the gas undergoes change at much lower temperatures than had been previously observed. The alteration in constitution commences at about 350° C., at which temperature the change is one of condensation without the formation of members of any series of hydrocarbons having a percentage of hydrogen and carbon different from ethylene, whilst if ethylene is maintained at 400° for a sufficient length of time, it is entirely decomposed, marsh gas, ethane, and liquid products being obtained.
In the same year Messrs. Morton and Noyess made an elaborate investigation with the object of determining whether crotonylene, C.H., which is present in small quantities in illuminating gas and other products of the distillation of organic matter, is formed as a primary product of decomposition by heat, or as a secondary product of the action of heat upon ethylene.
Coal gas was passed slowly through a hard glass tube 15 mm. in diameter, which was maintained at a low red heat for a distance of 60 cm. The products issuing from this tube were first passed through a series of U-tubes surrounded by a freezing mixture; the products which were not condensed were passed through an ammoniacal solution of cuprous chloride, to absorb hydrocarbons of the
* 'Annales de Chimie et de Physique,' 4th series, vol. 16, p. 144. of American Chemical Journal,' vol. 8, p. 153. | Ibid., vol. 8, p. 362.