a beam of light from the first would be reflected directly back to it. By means of a transparent glass, fixed in the eye-piece at an angle of 45°, a beam of light was sent from the first telescope to the second, and, on its return through a total distance of 17 kilometres, could be seen as a star by an eye looking through the first. Alongside the eye-piece of the latter a revolving wheel, with 720 teeth cut upon its circumference, was fixed in such a way that the beam of light both in going and coming had to pass between the teeth. When the wheel was set so that the tooth was in the focus, the beam would be entirely cut off in its passage through the telescope. Changing the position of the wheel through half the space between the middles of two consecutive teeth, the light would go and come freely between the teeth. When the wheel was set in revolution a succession of flashes would be sent out. If, on the return of each flash, a tooth was interposed, it would be invisible to the eye looking through the telescope. Fizeau found that with a velocity of 12'6 turns per second each flash which went out was on its return cut off by the advancing tooth. With a velocity twice as great as this it was seen on its return through the opening next following that through which it went. With three times this velocity it was caught on the second tooth following, and so on.1

This experiment of Fizeau was soon followed by the application of the revolving mirror of Sir Charles Wheatstone. Shortly after measuring the duration of the electric spark this investigator called attention to the fact that the same system could be applied to determine the velocity of light, and especially to compare the velocities through air and through water. In 1838 the subject was taken up by Arago, who took pains to demonstrate that it was possible, by the use of the revolving mirror, to decide between the theory of emission and that of undulations by determining the relative velocities in air and in a refracting medium.2

The difficulties in the way of securing the necessary velocities of the mirror and of arranging the apparatus were such that Arago never personally succeeded in carrying out his experiments. This seems to have been done almost simultaneously by Foucault and Fizeau about the beginning of 1850. Both experimenters seem to have proceeded substantially on the same principle and to have reached the same result, namely, that the motion of light through water was slower than through air in the inverse proportion of the indices of refraction of the two media.3

An important and most necessary modification of Arago's plan was made by these experimenters. As originally proposed, the plan was to send an instantaneous flash of light through water and through the air, and to receive it on the revolving mirror and determine the relative deviations in the positions of the images produced by the two rays. This system would, however, be inapplicable to the measurement of the actual time of transmission, owing to the impossibility of making any comparison between the time at which the flash was transmitted, and that at which it was received on the mirror. This circumstance would, indeed, have rendered the actual realisation of Arago's project nearly impossible for the reason that the flashes of light, seen through the water, would have reached the mirror at every point of its revolution; and only an exceedingly small fraction of them could have been reflected to the eye of the observer. This difficulty was speedily overcome by Foucault and It is curious that the author's account of this remarkable experiment, which forms an epoch in the history of physical sc.ence, is conta.ned within the limits of two pages, and terminates without any definite discussion of the results. It is merely stated that the result is 70,948 leagues of 25 to the degree, but the velocity, in kilo netres, which must have been that first obtained, is not given, nor is it stated what length the degree was supposed to have in the computation.

2

Comptes rendus, 1838, vol. vii. p. 954; Eunes de François Arago, vol. vii. p. 569.

3 Comptes rendus, xxx. 1850, pp. 551 and 771.

one.

Fizeau by a most ingenious arrangement, of equal importance with the revolving mirror itself. Instead of sending independent flashes of light to be reflected from the mirror, a continuous beam was first reflected from the revolving mirror itself to a fixed mirror, and returned from the fixed mirror back on its own path to the revolving A succession of flashes was thus emitted as it were from the fixed mirror, but their correspondence with a definite position of the revolving mirror was rendered perfect. Moreover, by this means, the image was rendered optically continuous, since a flash was sent through and back with every revolution of the mirror, and after the velocity of the latter exceeded 30 turns per second, the successive flashes presented themselves to the eye as a perfectly continuous image.

It was not until 1862 that this system was put into operation by Foucault for the actual measurement of the velocity of light through the atmosphere. A new interest had in the meantime been added to the problem by the discovery that the long-accepted value of the solar parallax was too small, and that the measurement of the velocity of light afforded a method of fixing the value of that constant. The central idea of the method adopted by Foucault was that already applied in comparing velocities through different media. The element sought is made to depend upon the amount by which the revolving mirror rotates while a flash of light is passing from its surface to the distant reflector, and coming back again. As the details of Foucault's method will be best apprehended by a comparison of them with those adopted in the present investigation, a complete description of his apparatus will here be passed over. It may, however, be remarked, that what he sought to observe was not the simple deviation of a slit, but the deviation of the image of a reticule. The deviation actually measured was 07 millimetre, and the system adopted was to determine at what distance, with a definite velocity, this amount of deviation could be obtained. His result for the velocity of light was 298,000 kilometres per second.

The next measures of the element in question were those of Cornu. The method which he adopted was not that of the revolving mirror, but Fizeau's invention of the toothed wheel. His earlier measures, made in 1870, and communicated to the French Academy in 1871, led to a result nearly the same as that of Foucault.1 This result was, however, not so satisfactory that the author could record it as definitive. He, therefore, in 1874, repeated the determination on a much larger scale and with more perfect apparatus. The distance between the two stations was nearly 23 kilometres, and therefore much greater than any before employed. He was thus enabled to follow the successive appearances and extinctions of the reflected image to the thirtieth order; that is, to make fifteen teeth of his wheel pass before a flash returned from the distant reflector, and to have it stopped by the sixteenth tooth.

This method has a defect, the result of which is evident by an examination of Cornu's numbers. It is that the extinctions and reappearances of the light as the wheel changes its speed are not sudden phenomena, occurring at definite moments, but are so gradual that it is difficult to fix the precise moment at which they occur. Of this defect the able experimenter was fully conscious, and his discussion of the disturbing causes which come into play, and of the amount of error due both to the apparatus, the observer, and to the method of eliminating them, form altogether one of the most exhaustive discussions of a physical problem. But the uncertainties are not of a kind which admit of complete investigation, and it now appears that although his result was far superior in point of accuracy to that of Foucault, it was nevertheless in error by about 00015 of its whole amount. It was, in

fact, when reduced to a vacuum, 300,400 kilometres per second, while we may now regard it as well established that the true velocity is less than 300,000.

The next determination of the velocity of light was that of Michelson, whose result was 299,910 kilometres per second. His investigation being a part of the first volume of the present series need not be here discussed, but it is worth while to remark that his method seems far superior in reliability to any before applied.

An attempt has been made by Messrs. James Young and George Forbes to improve Fizeau's method, by diminishing the uncertainty arising from the gradual extinction of the visible image. By the method of these experimenters the result depends, not upon the moment when the image disappears, but when two images, side by side, are equal in brightness. This is effected by employing two reflectors, at unequal distances, but nearly in the same line from the telescope, to return the ray. Each reflector then forms its own image in the field of view of the sending telescope. With a regularly increasing velocity of the toothed wheel, each image goes independently through the same periodic series of changes as when only one mirror is used; but owing to the unequal distance the period is not the same. If the speed of the mirror be carried to such a point that the difference of phase in the two images is half a period, then one image will be increasing while the other is diminishing, and the stage at which the two images are equal would appear to admit of fairly accurate determination.

The distant reflectors were separated from the observing telescope by the Firth of Clyde. The distances were respectively 16,835 feet, and 18,212 feet. A study of the printed descriptions of their experiments gives the impression that the performance of the subsidiary parts of the apparatus was not such as to do justice to the method. The resulting velocity of light was 301,382 kilometres per second, and the difference between the extreme results of twelve separate determinations was kilometres.

4000

tion had been taken, the author, in April of that year, brought the subject before the National Academy of Sciences, with the view of eliciting from that body an expression of opinion upon the propriety of asking the Government to bear the expenses of the work. The subject was referred to a Select Committee, who, in January, 1879, made a favourable report on the subject, which was communicated to the Secretary of the Navy. On the recommendation of the Secretary, Hon. R. W. Thompson, Congress, in March following, made an appropriation of $5000 for the purpose, and the author was charged by the Department with the duty of carrying out the experi

ments.

In the meantime it became known that Mr. Michelson had made preparations for repeating Foucault's determination at his own expense, with the desirable improvement of placing the fixed reflector at a considerable distance. But before the reliability of Mr. Michelson's work had been established, the preparations for the present determination had been so far advanced that it was not deemed advisable to make any change in them on account of what Mr. Michelson had done. The ability shown by the latter was, however, such that, at the request of the writer, he was detailed to assist him in carrying out his own experiments, and acted in this capacity until September 1880, when he accepted the Professorship of Physics in the Case Institute, Cleveland, Ohio. After the departure of Mr. Michelson his place was taken by Ensign J. H. L. Holcombe, U.S.N., who assisted in every part of the work to the entire satisfaction of the projector until its close.

PANCLASTITE

DR. SPRENGEL has sent us a reprint of a note sent by him to the Chemical News on this subject. After showing that these new explosives, so named by Mr. Turpin, are not original, he continues:

"The 'beau idéal' of a detonating explosive is a mixture of 8 parts (88.9 per cent.) of liquid oxygen and 1 part (11'1 per cent.) of liquid hydrogen.

"In my paper of 1873 I say, p. 799:-' On referring to the foregoing table the reader will be reminded that peroxide of hydrogen is the highest oxygen compound known, while nitric anhydride is the compound which contains the largest amount of oxygen available for combustion (74 per cent.). But as this compound, as well as the next two, nitric peroxide (69.5 per cent. oxygen) and tetranitromethane (65.3 per cent. oxygen) are at present: on account of their nature and their difficult preparation, mere chemical curiosities, my attention naturally turned to the fourth, to nitric acid (63′5 per cent. oxygen), which is a cheap and common article of commerce.'

"Now, when Mr. Turpin's attention turned to the second oxidiser on my list—to nitric peroxide-he found that this substance does not corrode metals, such as iron, copper, and tin under 356° F. (180o C.); and further, that combustible liquids, such as petroleum, carbon bisulphide, and nitro-benzene are readily soluble in nitric peroxide without rise of temperature. These are valuable properties, first noticed by Mr. Turpin.

"What was formerly a chemical curiosity is now an article of commerce. Nitric peroxide may be bought to-day at eighteenpence the pound, and I see ways and means of producing it a great deal more cheaply. Nitric peroxide is a yellowish liquid, heavier than water (sp. gr. 1*451), and boils at 71° F. (22° C.), but may be kept like ether or similar volatile liquids. In France it is sent about in tinned-iron cans.

"Taking as a typical example a benzene-mixture170 C626 CO.

C6H6

71(NO2)

=

18:4

=

=

14 H 568 0

24.8 N

=

126 H2O

we see, that the 184 parts of benzene require 568 parts of oxygen for the oxidation of their carbon and hydrogen to carbonic acid and water. This oxidation or combustion takes place at the moment of explosion at the expense of the 568 parts of oxygen, contained in the rest of the mixture-the 816 parts of nitric peroxide. No other explosive now in use (including blasting gelatin) contains weight for weight a greater amount of combustible matter, and as an explosion of these bodies is simply a sudden combustion, I again beg to draw attention to the fact that the oxygen available for combustion in gun-cotton is most probably not more than 323 per cent. and in nitroglycerin 423 per cent., while in this case we have without a doubt 568 per cent. Hence no other explosive now in use can rival this and similar mixtures in power, as I published in 1873. They still remain the most powerful explosives known.

It hardly need be said that an explosive of this nature consists of two parts-an oxidising and a combustible agent- and that Mr. Turpin with the same naïveté lays claim not only to the first, but also to the latter half of the subject.

"None of my safety explosives are licensed in England, though many of them, when mixed, are much less sensitive to concussion than common gunpowder.

"In April 1884 the French military authorities were busy near Rochefort with shells of the 'système Turpin.' These shells, so my informant said, were made of such a size, and possessed such a prodigious power, that a ship struck by one of them would inevitably be sent to the bottom of the sea, even were she the strongest ironclad afloat. It is devoutly to be hoped that those whose office it is to provide for the defence of the British Navy will be ready in the hour of need to serve out shells, filled with an explosive of equal force or better still with something superior, approaching more closely the 'beau idéal.'"

MR. VERBEEK ON THE KRAKATÃO DUSTGLOWS

AS it appears from the letter of Mr. Douglas Archi

bald in NATURE of April 29 (p. 604) that some doubt exists as to the quantity of volcanic dust ejected during the Krakatao eruption in 1883, it may not be inopportune to give an abstract of what Mr. Verbeek-the best authority on the subject-says in the second part of his book. The mistake in the number of cubic kilometres-which Dr. Riggenbach or his critic magnified from 18 into 150-may possibly have arisen from the comparison Mr. Verbeek draws between the quantity of volcanic substances ejected by the Tambora in 1815 and that ejected by Krakatao.

was

Junghuhn estimated the quantity of ashes ejected by the Tambora in Sumbawa at 318 cubic kilometres, but Mr. Verbeek reduces it by calculation to about 150, though he adds that the data are insufficient to form a really correct estimate. It is certain, however, that the quantity was considerably larger than that ejected by Krakatao. To calculate this quantity Mr. Verbeek made observations everywhere on the islands and along the coasts of the Straits of Sunda; while the thickness of the ashes which fell into the sea computed according to the difference in the depths of the sea before and after the eruption, a difference which greatly varies, and amounts in some places to 40 metres, if not more. Wherever some doubt exists for want of previous accurate deep-sea soundings, Mr. Verbeek gives Of these, by the bye, only 38.8 per cent. can be utilised for want of fuel, as pointed out by me in my patent of 1871, and verified four years later by the force of Nobel's blasting gelatin, in which the excess of 3 52 per cent. oxygen is utilised by the dissolved gun-cotton, an explosive too rich in carbon. See Abbot's table, p. 17, in "The Hell-Gate Explosion near New York and so-called 'Rackarock, with a few words on so-called 'Panclastite," " by H. Sprengel. London: E. and F. N. Spon, 1886.

the lowest figures. These observations are all illustrated by maps. Mr. Verbeek estimates the quantity of ejected material which fell round the volcano at 18 cubic kilometres at least. The possible outside margin would, Of this however, not exceed 3 cubic kilometres. quantity, two-thirds, or 12 cubic kilometres, lies within a circle with a radius of 15 kilometres drawn round Krakatao, one-third, or 6 kilometres, outside it. Of the finer ashes a large quantity were already, during the first three days, blown into the sea, as appeared from observations made on ships; and Mr. Verbeek assumes that considerably less than I cubic kilometre remained floating in the upper regions of the atmosphere. This quantity would correspond to a layer of o'002 millimetre thickness divided over the whole surface of the earth, or of 0004 millimetre over the temperate zones only.

Such an infinitesimally thin layer could hardly have been the principal cause of the atmospheric phenomena. They must be accounted for in a great measure by the large volume of aqueous vapour ejected by Krakatao, the amount of which lies, unfortunately, beyond all calculation. We have to deal with two distinct phenomena, as Prof. Michie Smith also has shown by the two different spectra, and these phenomena had different causes: thus, the blue and green tints of sun and moon, which were specially observed during the first month after the eruption, and only in places close to the equator, must be principally ascribed to the solid particles in the volcanic ash cloud, as various observations have shown that these are the main cause of the special absorption of the rays of light by which the sun appeared blue and green; the aqueous vapour may have increased the phenomenon, for it is known that the sun can look bluish through mist. It cannot be said to be a proof to the contrary that Mr. Lockyer saw the sun green through the steam which escaped from the funnel of a steamer, for probably a quantity of ash and soot-particles escaped from the funnel at the same time, and it is possible that the sun appeared green from that very fact. The steam was thus in the identical condition of our volcanic cloud. It was only in the beginning after the eruption, before the ashes had spread very far, and when, therefore, their density was greater, that they were able within a limited space to give green tints to the sun. This phenomenon ceased when the ashes were dispersed further round the globe-in the northern hemisphere by the south-west, in the southern hemisphere by the north-west winds-and when probably also a portion of them fell gradually on the earth.

The crimson after-glows which soon followed the eruption were observed at the same time over a much larger area than that within which the blue and green sun was seen at successive periods, and they are believed by Mr. Verbeek to have been caused mainly by the masses of aqueous vapour thrown out by Krakatao, and which formed condensing and freezing in the higher and colder regions the greater part of the volcanic cloud. This vapour, after of the atmosphere, produced the remarkably beautiful sunsets, while the ashes may have intensified the phenomenon, besides serving as a centre of condensation for the vapour. The real cause of the crimson glows was therefore probably the same as that of the evening red, their intensity being a consequence of the extraordinary quantity of vapour in the upper regions emitted by

Krakatao.

for March 29, 1886, by Von Henri W. de Graaf, "Zur Anatomie und Entwickelung der Epiphyse bie Amphibien und Reptilien," wherein are described briefly (1) the development of the epiphysis, and (2) the structure of this part in the adult animal in certain amphibia and reptiles. An examination by means of sections at once revealed the fact that in Hatteria the epiphysis becomes modified in a manner more interesting than that found by Von Graaf to obtain in Anguis fragilis-the most modified form described by him.

The epiphysis apparently arises as a hollow outgrowth from the roof of the third ventricle (region of thalamencephalon), and in both amphibia and reptilia becomes divided into two parts-a proximal one remaining in connection with the brain, and a distal bladder-shaped

structure-the two becoming in most cases completely separated from each other. In Anguis fragilis Von Graaf finds that the distal part loses all connection with the brain, and develops into a structure resembling a highly organised invertebrate eye with, however, the important and curious exception that no nerve is present. In Hatteria a still more interesting modification takes place, the distal portion being, as in Anguis, modified to form an eye; but this, unlike that in the latter, is provided with a well-marked nerve.

Fig. 1 shows the structure of the eye. The whole is enclosed in a capsule of connective tissue (C); anteriorly a lens (L) is present, composed of cells whose nuclei are very distinct. The lens forms the anterior boundary of a vesicle, the walls of which are formed

from within outwards of the following layers: (1) a layer which is not well marked (x), and which may possibly be due to the shrinkage and clinging to the walls of the contents of the vesicle, fluid in life; (2) a layer of rods (R) embedded in dark brown pigment, the pigment being specially developed anteriorly at the part indicated by the letter K; (3) a double or even triple row (IN) of nuclei; (4) a clear layer (M) which scarcely takes stain, and may be called the molecular; and (5) an outer layer (EN) of nuclei two or three rows deep.

This structure will, so far, be seen to correspond closely with that of Anguis.

Posteriorly a nerve enters the eye, the fibres spreading round behind the vesicle; the rods may be observed giving off processes from their external ends, which in some cases appear to pass right through the layers (3),

(4), and (5), and in others to be connected in their passage with the nuclei of these layers.

However, I hope in a very short time to publish a detailed account of the histological structure of the organ.

The capsule containing the eye is filled posteriorly with connective tissue (CT), in which breaks up and ramifies a blood-vessel which enters along with the nerve (BV).

Fig. 2 represents somewhat diagrammatically a section transverse to the parietal foramen, showing that the eye is single and lies exactly in the median line. A depression of the skin of the head occurs immediately over the parietal foramen, but does not lead down into this, which is filled up by a plug of connective tissue (Fig. 1, T, Fig. 2, PT), specially dense (DT) around the eye capsule. The nerve