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Rh the supposition that the plates are parallel and equally thick, the paths pursued by these two rays are equal. P represents a thin plate of glass interposed in the path of one ray, by which the bands are shifted.

In Jamin’s apparatus the two rays which produce interference are separated by a distance proportional to the thickness of the mirrors, and since there is a practical limit to this thickness, it is not possible to separate the two rays very far. In A. A. Michelson’s interferometer there is no such restriction. “The light starts from source S (fig. 8) and separates at the rear of plate A, part of it being reflected to the plane mirror C, returning exactly, on its path through A, to O, where it may be observed by a telescope or received upon a screen. The other part of the ray goes through the glass plate A, passes through B, and is reflected by the plane mirror D, returns on its path to the starting point A, where it is reflected so as nearly to coincide with the first ray. The plane parallel glass B is introduced to compensate for the extra thickness of glass which the first ray has traversed in passing twice through the plate A. Without it the two paths would not be optically identical, because the first would contain more glass than the second. Some light is reflected from the front surface of the plate A, but its effect may be rendered insignificant by covering the rear surface of A with a coating of silver of such thickness that about equal portions of the incident light are reflected and transmitted. The plane parallel plates A and B are worked originally in one piece, which is afterwards cut in two. The two pieces are placed parallel to one another, thus ensuring exact equality in the two optical paths AC and AD” (see Michelson, Light-Waves and their Uses, Chicago, 1903).

The adjustments of this apparatus are very delicate. Of the fully silvered mirrors C, D, the latter must be accurately parallel to the image of the former. For many purposes one of the mirrors, C, must be capable of movement parallel to itself, usually requiring the use of very truly constructed ways. An escape from this difficulty may be found in the employment of a layer of mercury, standing on copper, the surface of which automatically assumes the horizontal position.

Michelson’s apparatus, employed to view an extended field of homogeneous light, exhibits Haidinger’s rings, and if all is in good order the dark parts are sensibly black. As the order of interference increases, greater and greater demand is made upon the homogeneity of the light. Thus, if the illumination be from a sodium flame, the rings are at first distinct, but as the difference of path increases the duplicity of the bright sodium line begins to produce complications. After 500 rings, the bright parts of one system coincide with the dark parts of the other (Fizeau), and if the two systems were equally bright all trace of rings would disappear. A little later the rings would again manifest themselves and, after 1000 had gone by, would be nearly or quite as distinct as at first. And these alternations of distinctness and indistinctness would persist until the point was reached at which even a single sodium line was insufficiently homogeneous. Conversely, the changes of visibility of the rings as the difference of path increases give evidence as to the duplicity of the line. In this way Michelson obtained important information as to the constitution of the approximately homogeneous lines obtained from electrical discharge through attenuated metallic vapours. Especially valuable is the vacuum tube containing cadmium. The red line proved itself to be single and narrow in a high degree, and the green line was not far behind.

But although in Michelson’s hands the apparatus has done excellent spectroscopic work, it is not without its weak points. A good deal of labour is required to interpret the visibility curves, and in some cases the indications are actually ambiguous. For instance, it is usually impossible to tell on which side of the principal component a feebler companion lies. It would seem that for spectroscopic purposes this apparatus must yield to that of Fabry and Pérot, in which multiple reflections are utilized; this is a spectroscope in the literal sense, inasmuch as the constitution of a spectrum line is seen by simple inspection.

INTERIM, originally a Latin word for “in the meantime.” The word was hence applied to certain edicts and decrees passed by the emperor and the diets during the reformation in Germany with the object of temporarily settling a controversy. These “interims” regulated points of religious and ecclesiastical difference until they could be decided by a general council. The best example of such a modus vivendi is the Augsburg Interim of 1548, drawn up by Michael Helding, Julius von Pflug and John Agricola (a medievalist, an Erasmian, and a conservative Lutheran) at the bidding of Charles V., and accepted by the diet. It was an ambiguous document, teaching from the Roman Catholic side transubstantiation, the seven sacraments, adoration of the Virgin and saints, and papal headship, and from the Protestant, justification by faith, marriage of priests, the use of the cup by the laity. Maurice of Saxony was permitted to vary the interim for his dominions, and his edition was called the Leipzig Interim. An earlier interim was that of Regensburg, 1541.

 INTERLACED ARCHES, the term for a scheme of decoration employed in Romanesque and Gothic architecture, where arches are thrown from alternate piers, interlacing or intersecting one another. In the former case, the first arch mould is carried alternately over and under the second, in the latter the mouldings actually intersect and stop one another. An example of the former exists in St Peter’s in the East, Oxford, and of the latter in St Joseph’s chapel, Glastonbury, and in the cathedral of Bristol.

 INTERLAKEN, a Swiss town (1864 ft.) in the canton of Berne, situated on the flat plain (Bödeli) between the lakes of Brienz (E.) and of Thun (W.), and connected by steamer, as well as by railway (17 m.) with the town of Thun. It is built on the left bank of the Aar, and grew up around the religious house of Austin Canons, founded about 1130 and suppressed in 1528. In the surviving buildings of the convent religious services (Anglican, Scottish Presbyterian and French Protestant) are now held, while the more modern castle is occupied by offices of the Cantonal Government. The fine and well-shaded avenue called the Höheweg runs through the main portion of the town, and is lined on the north side by a succession of huge hotels and the large Kursaal. Interlaken is much frequented in summer, partly because of the glorious view of the Jungfrau (13,669 ft.) which it commands to the south, and partly because it is the best starting-point for many excursions, as to Schynige Platte, Lauterbrunnen and Grindelwald. The lines serving these places all start from the eastern railway station (that from Thun reaches the western or main railway station), whence steamers depart for the Giessbach Falls, Brienz and Meiringen, on the way to Lucerne or to the Grimsel Pass. In 1900 the population of Interlaken was 2962 (mainly Protestant and German-speaking). Opposite Interlaken, and on the right bank of the Aar is Unterseen (in 1900, 2607 inhabitants), which was built in 1280 by Berthold von Eschenbach.

See Fontes rerum Bernensium (original documents up to 1366) (8 vols., Berne, 1883–1903); Die Regesten des Klosters zu Interlaken (Coire, 1849); E. Tatarinoff, Die Entwickelung der Probstei Interlaken ''im XIII. Jahrhundert'' (Schaffhausen, 1892).

 INTERLOPER, one who interferes in affairs in which he has no concern. This word, with the verbal form “to interlope,” first appears at the end of the 16th and beginning of the 17th century in connexion with the interference of unauthorized persons in the trading monopoly of the Russia Company and later of the East India Company. The New English Dictionary quotes from H. Lane (1590), Hakluyt’s Voyages, “From those parts the Muscovites were furnished out of Dutchland by enterlopers with all arts and artificers and had few or none by us,” and also from the Minutes of the Court of the East India Company, 22nd of February 1615, “to examine all suspected personnes that intend interlopinge into the East Indies or Muscovy.” Edward Phillips (New World of Words, 1658) defines interlopers at common law as those “that without legal authority intercept the trade of a company, as it were Interleapers.” The word appears to be of English origin, for the Dutch enterlooper, smuggler, often given as the source, was taken from English, as was the French interlope. The word is a compound of inter, between, and lope, a dialectal variant of “leap.” A common word for a vagrant, or “straggler,” as it is defined, was till 1580 “landloper,” and the combination of “straggler” and “interloper” is found in Horsey’s Travels (Hakluyt Soc.), 1603–1627, “all interlopers and straglyng Englishmene lyving in that country.”

INTERNATIONAL, THE. The International Working Men’s Association, commonly called “The International,” was formed at London in 1864. It was a society of working men of all nations, somewhat like a cosmopolitan trades union, but