A telescope is like a very strong eye.
This angle, therefore, divided by the magnifying power of the telescope gives the real angular distance of the centres of a double star.
2, looks as though composed of a number of segments, gradually decreasing in size from base to tip like the joints of a telescope, instead of tapering gradually and evenly from one extremity to the other.
' So far as we can ascertain, the first telescope of large size driven by clockwork was the 9-in.
The improvement of the telescope was justly regarded as a sine qua non for the advancement of astronomical knowledge.
He certainly describes a method of constructing a telescope, but not so as to lead one to conclude that he was in possession of that instrument.
To telescope the jars for the next blow coming up. A skilful driller never allows his jars to strike on the downstroke, they are only used to jar down when the tools stick on some obstruction in the well before reaching the bottom, and in fishing operations.
It has been found by Sir William Herschel and others that the definition of a telescope is often improved by stopping off a part of the central area of the object-glass; but the advantage to be obtained in this way is in no case great, and anything like a reduction of the aperture to a narrow annulus is attended by a development of the external luminous rings sufficient to outweigh any improvement due to the diminished diameter of the central area.'
Yerkes telescope, is shown in figs.
The planets in question appeared in the telescope as star-like objects which could be compared with the stars with much greater accuracy than a planetary disk like that of Mars, the apparent form of which was changed by its varying phase, due to the different directions of the sun's illumination.
Here stands the Royal Observatory, in which the great Dunecht telescope was erected in 1896.
He had equipped himself with a mental telescope and looked into remote space, where petty worldliness hiding itself in misty distance had seemed to him great and infinite merely because it was not clearly seen.
The eye, unaided or armed with a telescope, is able to see, as points of light, stars subtending no sentsible angle.
The only telescope erected in the establishment when he took it in charge was the transit instrument, and to this he vigorously devoted himself.
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.
Trans., 1802, p. 378), were independently discovered, and, by means of the telescope of a theodolite, between which and a distant slit admitting the light a prism was interposed, were for the first time carefully observed by Fraunhofer, and have on that account been designated "Fraunhofer's lines."
The spectra of the stars he obtained by using, outside the object-glass of his telescope, a large prism, through which the light passed to be brought to a focus in front of the eye-piece.
Since then, two elaborate series of observations made with the zenith telescope for the purpose of determining the variation of latitude and the constant of aberration have been carried on by Professor C. L.
The resolving power of a telescope with circular or rectangular aperture is easily investigated experimentally.
One of these, of width equal, say, to one-tenth of an inch, is inserted in front of the object-glass, and the telescope, carefully focused all the while, is drawn gradually back from the grating until the lines are no longer seen.
Merely to show the dependence of resolving power on aperture it is not necessary to use a telescope at all.
The (8), A function of the telescope is in fact to allow the use of a wider, and therefore more easily measurable, aperture.
The efficiency of a telescope is of course intimately connected with the size of the disk by which it represents a mathematical point.
4, AB represents the axis of an optical instrument (telescope or microscope), A being a point of the object and B a point of the image.
If the origin of light be treated as infinitely small, and be seen in focus, whether with the naked eye or with the aid of a telescope, the whole of the light in the absence of obstacles would be concentrated in the immediate neighbourhood of the focus.
As an application of this result, let us investigate what amount of temperature disturbance in the tube of a telescope may be expected to impair definition.
P. Langley has proposed to obviate such ill-effects by stirring the air included within a telescope tube.
Throughout the operation of increasing the focal length, the resolving power of the instrument, which depends only upon the aperture, remains unchanged; and we thus arrive at the rather startling conclusion that a telescope of any degree of resolving power might be constructed without an object-glass, if only there were no limit to the admissible focal length.
A rotation of this amount should therefore be easily visible, but the limits of resolving power are being approached; and the conclusion is independent of the focal length of the mirror, and of the employment of a telescope, provided of course that the reflected image is seen in focus, and that the full width of the mirror is utilized.
In 1757 he presented a telescope to the king, so accurately driven by clockwork that it would follow a star all night long.
On the same principle we may estimate the least visible displacement of the eye-piece of a telescope focused upon a distant object, a question of interest in connexion with range-finders.
Observing through a telescope with light perpendicularly incident, he showed that the position of any ray was dependent only upon the grating interval, viz.
The object-glass of a telescope, the Fresnel's zones are reduced to parallel and equidistant straight strips, which at certain angles coincide with the ruling.
Hence a wide beam demands treatment with further apparatus (usually a telescope) of high magnifying power.
Degree surpassing the usual limits, without loss of definition, when the telescope is focused so as to secure the best effect.
- These very remarkable bands are seen under certain conditions when a tolerably pure spectrum is regarded with the naked eye, or with a telescope, half the aperture being covered by a thin plate, e.g.
At a moment when the eye, or object-glass of a telescope, occupies a dark position, the star vanishes.
If a telescope be employed there is a distinction to be observed, according as the half-covered aperture is between the eye and the ocular, or in front of the object-glass.
In the former case the function of the telescope is simply to increase the dispersion, and the formation of the bands is of course independent of the particular manner in which the dispersion arises.
There appears to be no further advantage in the use of a telescope than the increased facility of accommodation, and for this of course a very low power suffices.
This produces an opening and closing of the edges symmetrically with respect to the telescope axis.
As the powers of the telescope were gradually developed, it was found that the finest hairs or filaments of silk, or the thinnest silver wires that could be drawn, were much too thick for the refined purposes of the astronomer, as p p they entirely obliterated the image of a star in the more powerful telescopes.
A still further facility was given to the use of the filar micrometer by the introduction of clockwork, which caused the telescope automatically to follow the diurnal motion of a star, and left the observer's hands entirely at liberty.'
Equatorial telescope, is the first position micrometer constructed capable of measuring position angles to 1' of arc.
To measure distances with the Fraunhofer micrometer, the position-circle is clamped at the true position-angle of the star, and the telescope is moved by its slow motions so that the component A of the star is bisected by the fixed wire; the other component B is then bisected by the web, which is moved by the graduated head S.
9 represents the same ' Professor Watson used to say, " After all the most important part of a telescope is the man at the small end."
Thus a latent image of the " reseau-lines " will be formed on the sensitive plate, and, when the latter has been exposed to the sky in the telescope, we obtain, on development, a negative of the images both of the stars and of the reseau-lines.
The microscope or viewing telescope is fitted with a spider-line micrometer having two screws at right angles to each other, by means of which readings can be made first on one reseau-line, then on the star, and finally on the opposite reseau-line in both co-ordinates.
Its object is a practical one, to determine by scientific considerations the shape of lens best adapted to improve the capabilities of the telescope, which had been invented not long before.
If 2R be the diameter of the objectglass and D the distance of the object, the angle subtended by AP is E/D, and the angular resolving power is given by X/2 D sin a = X/2 R (3) This method of derivation (substantially due to Helmholtz) makes it obvious that there is no essential difference of principle between the two cases, although the results are conveniently stated in different forms. In the case of the telescope we have to deal with a linear measure of aperture and an angular limit of resolution, whereas in the case of the microscope the limit of resolution is linear, and it is expressed in terms of angular aperture.