Before astronomical telescopes were mounted parallactically, the measurement of position angles was seldom attempted.
When equatorial mountings for telescopes became more general, no filar micrometer was considered complete which was not fitted with a position circle.'
1893) for large telescopes is shown in fig.
The great reflecting telescope at Dorpat was manufactured by him, and so great was the skill he attained in the making of lenses for achromatic telescopes that, in a letter to Sir David Brewster, he expressed his willingness to furnish an achromatic glass of 18 in.
It had been supposed that, with the greatly improved telescopes of modern times, contact observations could be made with much greater precision than in 1761 and 1769, yet, for some reason which it is not easy to explain completely, the modern observations were but little better than the older ones.
The resolving power of telescopes was investigated also by J.
But although the argument from gratings is instructive and convenient in some respects, its use has tended to obscure the essential unity of the principle of the limit of resolution whether applied to telescopes or microscopes.
C and d are telescopes for reading the position circle p, e the handle for quick motion in position angle, f the slow motion in position angle, g the handle for changing the separation of the segments by acting on the bevel-wheel g' (fig.
Sighting telescopes were also intro FIG.
From 1827 he devoted himself to the improvement of reflecting telescopes; in 1839 he mounted a telescope of 3 ft.
The first constructor of reflecting telescopes on a large scale, William Herschel, never published anything about his methods of casting and polishing specula, and he does not appear to have been very successful beyond specula of 18 in.
In 1877 he decided to devote one of the telescopes of the observatory to stellar photometry, and after an exhaustive trial of various forms of photometers, he devised the meridian photometer (see Photometry, Stellar), which seemed to be free from most of the sources of error.
With Professor Pickering's usual comprehensiveness, the inquiry was so arranged as to cover the whole sky; and with four telescopes - two at Cambridge for the northern hemisphere, and two at Arequipa in Peru for the southern - to which a fine 24-in.
His name is best known for the improvements he effected in the mirrors of reflecting telescopes and especially in the construction of the microscope.
At the same time, on land, the new necessities imposed on field artillery by the growing use of covered positions led to the development of scissors-telescopes (see Rangefinders) and panorama-telescopic sights (see Sights), in which the optical system was arranged with the tube of the telescope vertical and the object-glass and eyepiece systems at right angles to the axis of the tube.
In the best telescopes, whether for theodolite or level, the diaphragm on which the image is formed is made of glass, and the cross hairs are engraved thereon.
One of the most important applications of the heliostat is as an adjunct to the newer forms of ' horizontal telescopes (q.v.) and in conjunction with spectroscopic telescopes in observations of eclipses.
Foucault invented in 1857 the polarizer which bears his name, and in the succeeding year devised a method of giving to the speculum of reflecting telescopes the form of a spheroid or a paraboloid of revolution.
It has been pointed out by Dr Robert Smith, in his Complete System of Opticks, that Bacon imagines some effects of telescopes which cannot be performed by them, and his conclusion is that Bacon never actually looked through a telescope.
All the original Dutch telescopes were composed of a convex and a concave lens, and telescopes so constructed do not invert.
Telescopes seem to have been made in Holland in considerable numbers soon after the date of their invention, and rapidly found their way over Europe.
Knowing the theory of his instrument, and possessed of much practical skill, coupled with unwearied patience, he conquered the difficulties of grinding and polishing the lenses, and soon succeeded in producing telescopes of greatly increased power.
The first powerful telescopes of this construction were made by Huygens, after much labour, in which he was assisted by his brother.
Cassini discovered Saturn's fifth satellite (Rhea) in 1672 with a telescope of 35 ft., and the third and fourth satellites in 1684 with telescopes made by Campani of looand 136-ft.
1691) and others are said to have made telescopes of from 300 to 600 ft.
Huygens contrived some ingenious arrangements for directing such telescopes towards any object visible in the heavens - the focal adjustment and centring of the eyepiece being preserved by a braced rod connecting the objectglass and eye-piece.
Telescopes of such great length were naturally difficult to.
Until Newton's discovery of the different refrangibility of light of different colours, it was generally supposed that object-glasses of telescopes were subject to no other errors than those which arose from the spherical figure of their surfaces, and the efforts of opticians were chiefly directed to the construction of lenses of other forms of curvature.
He was well aware of the failures of all attempts to perfect telescopes by employing lenses of various forms of curvature, and accordingly proposed the form of reflecting telescope which bears his name.
Bradley and Molyneux, having been instructed by Hadley in his methods of polishing specula, succeeded in producing some telescopes of considerable power, one of which had a focal length of 8 ft.; and, Molyneux having communicated these methods to Scarlet and Hearn, two London opticians, the manufacture of telescopes as a matter of business was commenced by them (Smith's Opticks, bk.
In Short's first telescopes the specula were of glass, as suggested by Gregory, but he afterwards used metallic specula only, and succeeded in giving to them true parabolic and elliptic figures.
All Short's telescopes were of the Gregorian form, and some of them retain even to the present day their original high polish and sharp definition.
The first person who succeeded in making achromatic refracting telescopes seems to have been Chester Moor Hall, a gentleman of Essex.
After devoting some time to the inquiry he found that by combining lenses formed of different kinds of glass the effect of the unequal refrangibility of light was corrected, and in 1733 he succeeded in constructing telescopes which exhibited objects free from colour.
It was known that, about seven years after the patent for making achromatic object-glasses was granted to Dollond, his claim to the invention was disputed by other instrument-makers, amongst them by a Mr Champness, an instrument-maker of Cornhill, who began to infringe the patent, alleging that John Dollond was not the real inventor, and that such telescopes had been made twentyfive years before the granting of his patent by Mr Moor Hall.
The whole history of his researches proves how fully he was aware of the conditions necessary for the attainment of achromatism in refracting telescopes, and he may be well excused if he so long placed implicit reliance on the accuracy of experiments made by so illustrious a philosopher as Newton.
The triple object-glass, consisting of a combination of two convex lenses of crown glass with a concave flint lens between them, was introduced in 1765 by Peter, son of John Dollond, and many excellent telescopes of this kind were made by him.
We proceed to give an account of the methods and principles of construction of the various kinds of telescopes, and 1 Ayscough was an optician in Ludgate Hill, London.
In diameter when viewing faint objects, we obtain the rule that the minimum magnifying power which can be efficiently employed is five times the diameter of the object-glass expressed in inches.`'- The defects of the Galilean and Kepler telescopes are due to the chromatic and spherical aberration of the simple lenses of which they are composed.
Of the magnifying power of telescopes, for the rays emerging from the eye-piece when adjusted for distinct vision to be parallel.
This want of proportion in the dispersion for different regions of the spectrum is called the "irrationality of dispersion"; and it is as a direct consequence of this irrationality, that there exists a secondary spectrum or residual colour dispersion, showing itself at the focus of all such telescopes, and roughly in proportion to their size.
The following are the various forms of reflecting telescopes: The Gregorian telescope is represented in fig.
The practical difficulty of constructing Gregorian telescopes of good defining quality is very considerable, because if spherical mirrors are employed their aberrations tend to increase each other, and it is extremely difficult to give a true elliptic figure to the necessarily deep concavity of the small speculum.
Gregorian telescopes attained great celebrity.
Xliv., February 1857), proposed to employ glass for the specula of telescopes, the reflecting surface of the glass speculum to be covered with silver by Liebig's process.
Those silver-on-glass specula are now the rivals of the achromatic telescope, and it is not probable that many telescopes with metal specula will be made in the future.
The chromatic aberration of the object-glass of one of these telescopes is corrected for photographic rays, and the image formed by it is received on a highly sensitive photographic plate.
The peculiar form of the tube is eminently suited for rigid preservation of the relative parallelism of the axes of the two telescopes, so that,;i the image of a certain selected star is retained on the intersection of two wires of the micrometer, by means of the driving clock, aided by small corrections given by the observer in right ascension and declination (required on account of irregularity in the clock movement, error in astronomical adjustment of the polar axis, or changes in the star's apparent place produced by refraction), the image of a star will continue on the same spot of the photographic film during the whole time of exposure.
In these telescopes the photographic objectglass has an aperture of 13 in.
Type C. - Many more telescopes have been made of type C than of any other, and this form of mounting is still most generally employed for the mounting of modern refractors.