TELESCOPE Early History.—The first binocular telescope, consisting of two telescopes placed side by side, was constructed in 16o8 by Johann Lepperhey, the inventor of the ordinary Dutch telescope, i.e., the combination of a collec tive lens as an objective and a dispersive lens as an eyepiece. The subject was next taken up by the monks. The Capuchin An tonius Maria Schyrlaeus de Rheita (A. M. Schyrl) 166o) described in 1645 the con struction of double terrestrial telescopes. Greater success at tended the efforts of the Ca puchin Cherubin d'Orleans who flourished a little later. He con structed large double telescopes of the Dutch type for use in war and small instruments of lower magnification. He introduced great improvements into these telescopes by providing them with adjustments to enable the interocular distance to be adapted to suit individual observers, and the object glass dis tance to suit far and near objects.
After these discoveries the subject received no more attention until the i9th century. The re-invention of the Dutch binocular telescope apparently dates from 1823, and is to be assigned to the skilful Viennese optician Johann Voigtlaender. He received a kind of patent for an instrument having two Dutch telescopes so arranged that the axes of both were parallel and in line with the eye-centres. Both telescopes had independent focussing ad justments for both eyes. A year and a half later another Viennese optician B. Weidholt obtained a patent for an arrangement of two Dutch telescopes with their axes parallel but adjustable for interocular distance, but the credit of having placed these struments on the market probably belongs to J. P. Lemiere, in Paris, who in 1825 took out a French patent for an improvement of the Dutch double telescope. Lemiere's instruments were nished with a common central focussing adjustment and the adaptation to the interocular tance was effected by turning the two parallel bodies round their common axis. During the next few decades very few improvements in this type of instrument are recorded. The last improvement was apparently made by P. G. Bardout, who combined two terrestrial tubes using Lemiere's method of mounting them parallel to a com mon axis with interocular adjustment and central focussing arrangement for both eyes. It was possible for him to achieve greater magnifications with his instruments.
The Prism Combination.—Ignazio Porro (1795-1$75) invented in 1851 a prism combination which was to play an im portant part in the future development of binocular instruments. The prism combina tion consists of two right-angle prisms placed with their hypotenuse faces adjacent, having the planes of total reflection at right-angles to one another (fig. I). An image viewed through such a prism will appear completely reversed. A. A. Boulanger was the first to utilize the Porro prism combination in his binocular telescope (fig. 2) for which he obtained a patent in France in the year 1859. The prism combination was mounted above the objective of the tele scope in such a manner that, by means of a right- and left-hand screw, the interocular distance could be adjusted to suit the ob servers. He overlooked, however, the possibility of mounting the Porro prisms in such a manner as to increase the distance be tween the objectives relatively to the eyepieces, thereby gaining en hanced stereoscopic effect. C. Nachet introduced this improve ment in 1875, but he also does not seem to have realized that any gain in stereoscopic effect would result therefrom as no mention of this is made in his patent. His instruments did not meet with much popularity.
E. Abbe took the matter up de novo in 1893 when he designed prism binoculars and telescopes. His constructions were the runners of the modern type of prism binocular. If the Porro prism combination (fig. I) is examined closely it will be seen that it contains two great advantages. Abbe made use of both by mounting them so that he obtained a greater distance between the objectives than between the eyepieces, and by separating the hypotenuse faces one from the other he shortened the telescope considerably in overall length. Abbe's binoculars had an object glass distance of 1-41 times to twice that of the eyepieces concur rently with great tube shortening, making the binocular very compact (fig. 3).
The type has undergone many and varied improvements. Be ginning with small objectives and eyepieces giving an angle of 36° at the eye, it has now been developed by many constructors into a high precision optical instrument with considerably larger ob jectives and eyepieces of double the original field of view, without destroying the compactness and handiness of the instrument.
A great number of other re versing prism combinations have been utilized, being de rived mostly from the form originally designed by Amici (fig. 4). It is unknown when this prism was designed by Amici, but it is known that he himself used it in such a way that the incoming and outgoing ray traversed the entrance and exit faces at right-angles to their planes, and became totally re flected on the roof, thereby causing a reversal of the image from right to left and by letting the incoming ray make an angle of 9o° with the outgoing ray, the reversal in the vertical direction was accomplished. This type of reversing prism has become known as the "roof prism." It has to be made with great accuracy, the two faces f orming the roof have to be at right-angles to one another within 2 seconds of arc and the line where the roof joins has to be free from imperfections. A number of roof prism com binations have been designed to obtain direct vision or at any rate parallelism of the entrance and exit beams, notably by Abbe, Sprenger, Daubresse and others (figs. 5, 6, 7 and 8). All these con structions have been used with more or less success in prism binoculars.
Fig. 5 shows a complete binocular with Abbe's direct-vision reversing prism. This type does not increase the stereoscopic power beyond that due to the magnification, since eyepieces and object-glasses are equal distance apart. Fig. 6 embodies two Sprenger prisms, the peculiar construction of which enables the object-glasses to be separated widely, giving correspondingly in creased stereoscopic effect. Fig. 7 shows a prism binocular with two modified Daubresse prisms, with very slightly increased object glass separations. Fig. 8 shows a binocular with two roof prisms, having 6 internal reflec tions, which for the sake of com pactness have been so arranged that the distance between the objectives is less than that of the eyepieces, the stereoscopic effect in this case being less than that due to the magnification alone. The Porro reversing prism, shown in fig. is capable of further modification in use if its compound parts are separated (fig. 9). If the object glass is placed in the entrance beam either in front or below the isolated upper prism, the distance between the eyepiece and objective can be increased at will. By combining two such instruments as a binocular (fig. 1o) the stereoscopic effect will be increased in accordance with the ratio of interocular distance to object glass distance. Furthermore these instruments are suitable for much higher magnifications than those previously described.
For measuring purposes instruments of the type shown in fig. 10 with a horizontal axis have been largely used. The measure ment is effected by using in con junction with the space or inter val to be measured some means of measurement; e.g., a movable pointer or a fixed scale. This in strument shows a transition to the stereoscope inasmuch as the scale or means of measurement is not directly observed, but to each eye a plane representation is offered just as in the stereoscope ; the space to be measured on the other hand is portrayed in exactly the same way as in a double telescope. The method of superposing the two spaces on one another was deduced by Sir David Brewster in 1856, but he does not appear to have dealt with the problem of measurements.
Hector de Grousillier in conjunction with E. Abbe and C. Pul frich designed the first stereoscopic rangefinder suitable for prac tical use. The power of perception of depth by the human eye is most accurate. This has been ascertained by the approximately equal keenness of vision of all normal-sighted people and by the interpupillary distance. The angle which serves as a measure for the keenness of vision is that subtended by two neighbouring points of an object space which are just seen by the single eye as a double point ; for smaller angles they merge into one. Ac cording to the older experiments of Helmholtz this angle is about one minute. When measured on the retina the keenness of vision is determined by the diameter of the nerve filaments situated in straight rows close to one another in the fovea. The diameter of these filaments is roughly o.005 mm., or, in angular measure, one minute. More recent experiments for keenness of vision and power of perception of depth have given considerably higher values; thus Pulfrich in 1899, when first introducing stereoscopic instruments for measuring distance, proved that as a rule persons with normal eyes have a power of separation of io seconds and sometimes even less.
There are two methods of extending the limits of stereoscopic vision and of increasing the accuracy of the perception of depth, (I) by augmenting the keenness of sight by the aid of telescope or microscope, and (2) by increasing the interpupillary by several reflections after the plan shown by Helmholtz in 1857 (fig. 11) which shows his telestereoscope but without telescopic magnifying power. By combining telescopic magnification with increased in terpupillary distance the type of instrument shown in fig. so is evolved. If there is a telescopic magnification of na times or a base magnification of n times, i.e., the distance between the objectives is n times the inter pupillary distance, then the radius of stereoscopic vision is increased m or n times with respect to that of the naked eye ; and, if both are active, m X n times. The action of telescopic magnification In and base magnification n are however fundamentally different from one another. In the first case the areal dimensions are diminished in the same proportion as the distances are lessened, whilst in the other case the real dimensions remain unchanged, with the result that three dimen sional images in the binocular vision space of an observer using such an instrument appear as a model proportionately diminished in all its dimensions and brought nearer to the observer, but pushed together to the front like the wings on the stage of a theatre. This effect is present in all stereoscopic binoculars and telescopes.