SPECTACLES. 423 SPECTKOMETER. spy). A device for the purpose of aiding the sight when impaired by age or otherwise. (!See Sight, DEFEt"r.s of.) They were invented during the thirteenth century. The credit is attributed by some to Alessandro di Spina, a Florentine monk, and by others to Roger i3ac«in. The lenses are nearly always made of the best optical glass, and by the best makers are ground with extreme care. Lenses may be mounted in either eye-glass or spectacle frames, care being taken that the centre of the lens is opposite the pupil and that the glasses are tilted at a proper angle for read- ing or distance as desired. When separate lenses are required for these purposes the so-called bifocal glasses may be used. They are made by cementing a small strong lens upon the lower part of the other, so that when the eyes are low- ered, as in reading, the line of vision passes through that portion. Lenses are now numbered, with one of a focus of one meter as a unit, known as one diopter. One with four times that strength has a focal distance of one-quarter of a meter and is known as 4 D. (diopters). Con- vex spherical lenses are used in hypermetropia, presbyopia, and after removal of the crystalline lens for cataract. Concave spherical lenses are employed in myopia. If astigmatism exists a cylindrical lens, either concave or convex as re- quired, is used. As the astigmatism may be in only one meridian, while that at right angles to it is normal, or as there may be either hyperopic or myopic astigmatism in both, or hyperopic in one and myopic in the other, it is sometimes necessary to employ cross-cylinder lenses, a com- bination of two cylinders with their axes at right angles to each other. Prismatic glasses are used in cases with weakness of ocular muscles, one being chosen which will make the images seen by the two eyes coincide. Combinations of cylindrical and spherical lenses and prisms are needed when errors of refraction or accommoda- tion are combined with astigmatism and muscu,- lar weakness, which cause double vision. SPECTATOR, The. ( 1 ) A famous periodical published daily in London from March 1, 1711, to December 6, 1712, to which Addison and Steele were the principal contributors. Ad- dison wrote 274 papers, Steele 230. Addison's essays, almost perfect in form, fixed a new standard in manners, morals, and taste, whose influence lasted for years. The Spectator was revived in 1714, when 80 numbers were issued, of which Addison contributed 24. (2) A London weekly periodical, founded in 1828, devoted at first solely to literature and social questions; politics were, however, intro- duced -and it became a strong advocate of the Reform Bill. The first editor was Robert S. Rintoul, who continued in office till his death in 1858. John Stuart Mill was a member of the staff. From 1861 to 1897 it was edited by Richard Holt Hutton, who wielded great influ- ence in literature and theology, while his partner, Meredith Townsend, managed politics, which were liberal until 1886. He was succeeded as editor and owner by .Jolvi St. Loe Strachey, the polit- ical attitude of the paper being Liberal-Unionist. SPECTRAL BAT. See Bat and Colored Plate of Mammalia. SPECTRE (from Lat. spectrum, appearance, image, apparition). A fanciful book-name for one of the small lemurine animals of the Malayan region, the tarsier, on account of its gaunt form, enormous eyes and nocturnal appearance. Many superstitious notions attach to these and related animals (see LoRis) among the natives of the East Indies, causing them to be regarded with more dread than curiosity. Their nocturnal and irregular appearance have caused certain bats and small owls also to be called 'spectral.' SPECTROGRAPH (from Lat. spectrum, ap- pearance, image, apparition, from specere, to see -+- Gk. ypd(p€ii>, graphein, to write). A particular form of spectroscope (q.v.) in which the appa- ratus to record the radiations is a photographic plate. The apparatus, then, consists of a colli- mator, a prism or a grating, and a telescope in which the eyepiece is replaced by a photographic camera. SPECTROMETER (from Lat. spectrum, ap- pearance, image, apparition -f- Gk. fi4Tpov, metron, measure). An instrument especially de- signed to determine the index of refraction of transparent bodies. The ordinary type of instru- ment is illustrated in the cut, and, as shown, con- sists essentially of two arms which are movable around an axis carrying a platform on which rests a prism of the substance to be investigated. One of the arms carries a telescope. A, while the other supports what is known as the 'colli- mator,' B. This consists of a tube at one end of which is a slit made by two movable parallel edges, and at its other end is a lens so placed that the slit is in its principal focus. If a .source of light, for instance a sodium flame, is placed in front of the slit, a beam of parallel light will proceed from the collimator lens, fall upon the prism, suffer deviation, enter the telescope and be focused by the objective on the cross-hairs of the eyepiece. The angles made between the tele- scope and collimator may be read on a scale at- tached to the platform of the instrument. SPECTROMETER. To determine the index of refraction of a sub- stance in the form of a prism, for radiations of a given wave length, it is necessary to know: First, the angle of the prism, that is, the angle made between the two plane faces of the prism; second, the angle of minimum deviation for the given radiations. Both these quantities can be meas- ured with the spectrometer, care being taken to have the axes of the telescope and collimator per- pendicular to the axis of the instrument, and the edge of the prism parallel to it. The following formula then gives the index of refraction, in which A represents the angle of the prism and D the angle of minimum deviation : A + D Inde.x of refraction n sm 2
