The improvement of the telescope was justly regarded as a sine qua non for the advancement of astronomical knowledge.
Yerkes telescope, is shown in figs.
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.'
The cutter employed to rule these lines removes the silver in fine lines from the surface of the glass, Thus, if a photographic plate, before it is exposed in the telescope, is placed with its sensitive surface nearly in contact with the silvered surface of this reseau, and if parallel light, normal to the surface of the plate, is allowed to fall on the silvered film through the glass on which the film has been deposited, that light will pass through the fine lines in the silver film where the silver has been removed by the cutter, but will otherwise be intercepted by the silver film.
Here stands the Royal Observatory, in which the great Dunecht telescope was erected in 1896.
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.
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 eye, unaided or armed with a telescope, is able to see, as points of light, stars subtending no sentsible angle.
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.
Degree surpassing the usual limits, without loss of definition, when the telescope is focused so as to secure the best effect.
At a moment when the eye, or object-glass of a telescope, occupies a dark position, the star vanishes.
Equatorial telescope, is the first position micrometer constructed capable of measuring position angles to 1' of arc.
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."
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 (8), A function of the telescope is in fact to allow the use of a wider, and therefore more easily measurable, aperture.
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.
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.
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.
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.
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.
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.'
In 1757 he presented a telescope to the king, so accurately driven by clockwork that it would follow a star all night long.
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.
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.
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.
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.
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.
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.
Observing through a telescope with light perpendicularly incident, he showed that the position of any ray was dependent only upon the grating interval, viz.
Hence a wide beam demands treatment with further apparatus (usually a telescope) of high magnifying power.
- 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.
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.
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.