scholarly journals An account of a series of experiments made with a view to the construction of an achromatic telescope with a fluid concave lens, in stead of the usual lens of flint glass. In a letter addressed to Davies Gilbert, Esq. M. P. President of the Royal Society. By Peter Barlow, Esq. F. R. S. &c

1833 ◽  
Vol 2 ◽  
pp. 333-334
Keyword(s):  
Focal Length ◽  
Flint Glass ◽  
Double Ring ◽  
Proper Distance ◽  
Concave Lens ◽  
Satellites Of Jupiter ◽  
Series Of Experiments ◽  
Focal Power ◽  
Dispersive Power ◽  
William Herschel

The idea of constructing achromatic telescopes with fluid lenses was first suggested to the author by the attempt of Messrs. Gilbert to apply to practice the principles and rules for the construction of aplanatic object-glasses, laid down by Mr. Herschel in the Philosophical Transactions for 1821. In following these suggestions, the author became sensible of the difficulty of obtaining flint glass of sufficient size and purity for astronomical telescopes; and was thence led to consider the possibility of substituting some fluid in place of flint glass. Dr. Blair had, many years ago, succeeded in making very perfect telescopes of this description, but he still retained the use of flint glass. Among the various fluids adapted to this optical purpose, the author gave a decided preference to the sulphuret of carbon, which combines properties of perfect transparency and freedom from colour, with a refractive index nearly equal to that of flint glass, and with a dispersive power more than double, properties which it appears to retain under all the temperatures to which it is likely to be exposed in an astronomical telescope. After several trials, Mr. Barlow determined the best method of confining this fluid, but was at first unsuccessful in his attempts to construct with it a telescope of 6 inches aperture and 7 feet in length. He afterwards undertook a smaller one of 3 inches aperture, which he at length accomplished, and in the very first trials with it was able to separate a great number of double stars of the class which Sir William Herschel has pointed out as tests of a good 3½-inch refractor. Encouraged by his success, he again attempted a 6-inch object-glass, with a different manner of adjusting and securing the lenses, and considers the result of his endeavour as proving at least the practicability of the construction. This instrument, with a power of 143, shows the small star in Polaris so distinct and brilliant, that its transit might be taken with the utmost certainty; it exhibits distinctly the small stars in α Lyræ, Aldebaran, Rigel, &c. and decidedly separates Castor, γ Leonis, and ϵ Bootis. The belts and double ring of Saturn are well exhibited with a power of 150; and the belts and satellites of Jupiter are tolerably defined with the same power, but will not bear a higher power than about 200. In the usual construction of achromatic telescopes, the two or the three lenses composing the object-glass are brought into immediate contact. But the high dispersive power of the sulphuret of carbon enables Mr. Barlow to place the fluid correcting lens at a distance from the plate object lens equal to half its focal length. By this means the fluid lens, which is the most difficult part of the construction, is reduced to one half or less of the size of the plate lens. This construction, therefore, renders us independent of flint glass, enables us to increase the aperture of the telescope to a considerable extent; and gives us all the light, field, and focal power of a telescope of one and a half time the length of the tube. The author investigates analytically the formulæ for calculating the proper distance of the lenses on this construction, and expresses a hope that further experiments will enable us to determine the precise distance which shall reduce what has been termed the secondary spectrum, inseparable from the ordinary construction, either to zero, or to an inconsiderable amount.

scholarly journals II. On the spectra of some of the fixed stars

1864 ◽  
Vol 13 ◽  
pp. 242-244 ◽  
Keyword(s):  
Focal Length ◽  
Cylindrical Lens ◽  
Narrow Line ◽  
Stellar Spectra ◽  
Flint Glass ◽  
Small Telescope ◽  
Dispersive Power ◽  
Different Parts ◽  
General Method ◽  
Collimating Lens

After a few introductory remarks, the authors describe the apparatu which they employ, and their general method of observing the spectra the fixed stars and planets. The spectroscope contrived for these inqu ries was attached to the eye end of a refracting telescope of 10 feet foe length, with an 8-incli achromatic object-glass, the whole mounted equa torially and carried by a clock-movement. In the construction of th spectroscope, a plano-convex cylindrical lens, of 14 inches focal length, wa employed to convert the image of the star into a narrow line of light which was made to fall upon a very fine slit, behind which was placed an achromatic collimating lens. The dispersing portion of the arrangement consisted of two dense flint-glass prisms; and the spectrum was viewe through a small achromatic telescope with a magnifying power of between 5 and 6 diameters. Angular measures of the different parts of the spectrum were obtained by means of a micrometric screw, by which the position of the small telescope was regulated. A reflecting prism was placed over one half of the slit of the spectroscope, and by means of a mirror, suitably adjusted, the spectra of comparison were viewed simultaneously with the stellar spectra. This light was usually obtained from the in­duction spark taken between electrodes of different metals. The dispersive power of the apparatus was sufficient to enable the observer to see the line Ni of Kirchhoff between the two solar lines D ; and the three constituents of the magnesium group at b are divided still more evidently*. Minute details of the methods adopted for testing the exact coincidence of the corresponding metallic lines with those of the solar and lunar spectrum, are given, and the authors then proceed to give the results of their obser­vations.

scholarly journals Experiments relative to the effect of temperature on the refractive index and dispersive power of expansible fluids, and on the influence of these changes in a telescope with a fluid lens

1833 ◽  
Vol 2 ◽  
pp. 345-346
Keyword(s):  
Refractive Index ◽  
Focal Length ◽  
Effect Of Temperature ◽  
Focal Power ◽  
Dispersive Power

In a paper lately read to the Society, the author stated that he had not perceived any change in the focal length of the telescope, induced by changes of temperature; but he has since ascertained that in order to produce the brightest and most perfect image, the distance of the object-glass requires a minute adjustment, amounting to 0·134 of an inch, corresponding to an elevation of temperature from 57° to 84°, or a depression from 57° to 31°. In order to introduce greater clearness and precision, the author proceeds to define certain terms which he finds it necessary to employ. By the length of the telescope , he would be understood to mean the distance between the object-glass and the focus; by the fluid focus , that between the fluid lens and the focus; and by the focal power of the telescope, he means the focal length of a telescope of the usual construction, which gives the same convergency to the rays, or produces an image of the same size: but he also employs the term focal length of the telescope , as synonymous with the first; that of fluid focal length as synonymous with the second; and that of equivalent focal length as synonymous with the last of these terms.

VII. An account of a series of experiments made with a view to the construction of an achromatic telescope with a fluid concave lens, instead of the usual lens of flint glass. In a letter addressed to Davies Gilbert, Esq. M. P. President of the Royal Society. By Peter Barlow, Esq. F. R. S. &c

1828 ◽  
Vol 118 ◽  
pp. 105-112 ◽  
Keyword(s):  
Royal Society ◽  
The Other ◽  
Great Pleasure ◽  
Flint Glass ◽  
Concave Lens ◽  
Series Of Experiments ◽  
The One

You are aware that I have been for some time engaged in a set of experiments directed to the construction of achromatic fluid telescopes, and that I have succeeded in constructing, by the aid of Messrs. Gilbert, two instruments of that description, the one of 3 inches aperture and the other of 6 inches. You are aware also that it was my intention to have laid these before the members of the Board of Longitude; and if the construction had met with their approbation, I hoped they might have been disposed to have ordered a like instrument (but upon a scale much exceeding anything yet attempted), the construction of which it would have given me great pleasure to have superintended. It is, however, doubtful whether I shall be able at present to pursue the experiments*; and I wish therefore to place on record the progress I have made, the results which have been obtained, and the ultimate object I had in view; and I am in hopes this communication may not be thought undeserving a place in the Philosophical Transactions.

scholarly journals Eclipses of the satellites of Jupiter, observed by John Goldingham, Esq. F. R. S. and under his superintendance, at Madras, in the East Indies

1832 ◽  
Vol 1 ◽  
pp. 312-313
Keyword(s):  
Focal Length ◽  
The Sun ◽  
Short Account ◽  
East Indies ◽  
Satellites Of Jupiter

Mr. Goldingham’s observations are prefaced by a short account of the instruments employed, and some general remarks upon the circumstances necessary to be attended to, in drawing inferences from them. Two telescopes were employed, both made by Dollond, of 3½ feet focal length, with a magnifying power between 70 and 80; and the time observed by a good clock, with gridiron pendulum, was deduced from the transit of the sun nearest to the eclipse, and verified by the next preceding or following transit. The longitude of the place of observation is given, as determined from numerous observations of various kinds, that it may be compared with that deduced from each eclipse by means of the time given in the ephemeris.

scholarly journals Experiments for investigating the cause of the coloured concentric rings, discovered by Sir Isaac Newton , between two object-glasses laid upon one anther

1832 ◽  
Vol 1 ◽  
pp. 264-268
Keyword(s):  
Focal Length ◽  
Considerable Extent ◽  
The Other ◽  
Sir Isaac Newton ◽  
Isaac Newton ◽  
Series Of Experiments ◽  
Made In

The account, given by Sir Isaac Newton, of these coloured arcs, appeared to Dr. Herschel highly interesting, but he was not satisfied with the explanation of them. Sir Isaac Newton accounts for the production of the rings, by ascribing to the rays of light certain fits of easy transmission and alternate reflection; but this hypothesis seemed not easily to be reconciled with the minuteness and extreme velocity of the particles of light. With the view of inquiring further into the cause of these phenomena, Dr. Herschel, so long since as the year 1792, borrowed of this Society the two object-glasses of Huygens, one of 122, and the other of 170 feet focal length. Notwithstanding various interruptions, the series of experiments, made in the course of this time, has been carried to a considerable extent; and Dr. Herschel thinks the conclusions that may be drawn from them, sufficiently well supported to point out several modifications of light that have been totally overlooked, and others that have not been properly discriminated.

scholarly journals Experiments for ascertaining how far telescopes will enable us to determine very small angles, and to distinguish the real from the spurious diameters of celestial and terrestrial objects: with an application of the result of these experiments to a series of observations on the nature and magnitude of Mr. Harding’s lately discovered star

1832 ◽  
Vol 1 ◽  
pp. 166-171
Keyword(s):  
Celestial Body ◽  
Focal Length ◽  
Similar Question ◽  
The Real ◽  
Series Of Experiments

Dr. Herschel commences his paper by stating, that, being desirous of ascertaining the magnitude of the moving celestial body lately discovered by Mr. Harding, and intending, for that purpose, to make use of a ten-feet reflector, it appeared to him a desideratum highly worthy of investigation, to determine how small a diameter of an object might be seen with that instrument. He had, he says, in April 1774, determined a similar question relating to the natural eye; and found that a square area could not be distinguished from an equal circular one till the diameter of the latter came to subtend an angle of 2′ 17″; but, as he did not think it right to apply the same conclusions to a telescopic view of an object, he, in order to determine the first-mentioned question, made a series of experiments, of which the following is a summary account. Dr. Herschel’s first experiment was made upon the heads of pins, the size of the largest of which was ·1375, and that of the smallest ·0425 of an inch. These pins were placed at the distance of 2407·85 inches from the centre of the object-mirror of the author’s ten-feet telescope; the focal length of the mirror of which, on Arcturus, is 119·64 inches, but on the objects above mentioned 125·9.

scholarly journals XXI. Eclipses of the satellites of Jupiter, observed by John Goldingham, Esq. F. R. S. and under his superintendance, at Madras, in the East Indies

1808 ◽  
Vol 98 ◽  
pp. 322-332
Keyword(s):  
Focal Length ◽  
The Sun ◽  
East Indies ◽  
Astronomical Clock ◽  
Satellites Of Jupiter ◽  
The One

The following eclipses of the satellites of Jupiter, were observed with achromatic telescopes, by Dollond, of three and half feet focal length, and magnifying power between 70 and 80; having been constructed more immediately for this purpose, for which they were exceedingly well calculated in all respects. An astronomical clock, with gridiron pendulum, and dead beat, regulated by transits of the sun and stars, was used for the time; which was deduced from the transit of the sun nearest the eclipse, and verified by the one immediately preceding or following.

scholarly journals II. On the performance of fluid refracting telescopes, and on the applica­bility of this principle of construction to very large instruments

1831 ◽  
Vol 121 ◽  
pp. 9-15 ◽  
Keyword(s):  
Short Account ◽  
Flint Glass ◽  
Direct Observations ◽  
Series Of Experiments

In the Philosophical Transactions for 1827, a paper of mine was published containing an account of a series of experiments which I had carried on with Messrs. W. and T. Gilbert on the curvature of object-glasses for telescopes. In the course of these experiments, I saw so much the difficulty which opticians experience in obtaining large pieces of good flint-glass, that I turned my attention to supplying this material by a fluid. Having, after several attempts, at length found an admirable substitute in sulphuret of carbon, I wrote a short account of my intended construction, addressed to His present Majestv, at that time Lord High Admiral, and, as such, President of the Board of Longitude, soliciting from that Board assistance in carrying forward my ex­periments. Having obtained this aid, the result of my first trial was the con­struction of an eight-inch fluid telescope, at that time the largest refractor in this country. A description of this instrument is given in the Philosophical Transactions for 1829, and some objects are pointed out which had been se­lected as tests of its performance. I have however since had more time and better means of testing the instrument; first, through the kindness of Mr. Herschel, who pointed out to me several objects that he had observed with his new twenty-inch speculum; and secondly, by direct observations on the same objects in Sir James South’s new twenty-feet refractor, and in my own telescope. A few of these, which serve to mark distinctly the progress I have made, are given below; but I will first state two or three of my own observations, which, I conceive, tend also to the same object.

Experimental Study on Correction of Spherical Aberration in Electro-Magnetic Round Lens

1990 ◽  
Vol 48 (1) ◽  
pp. 234-235
Author(s):  
Ergang Chen ◽  
Chongjun Mu
Keyword(s):  
Electron Beam ◽  
Spherical Aberration ◽  
Focal Length ◽  
Image Point ◽  
Electron Trajectory ◽  
Optical Instruments ◽  
Concave Lens ◽  
In Series ◽  
Electron Microscopes ◽  
Electro Magnetic

As we know that the spherical aberration in a round electromagnetic lens can not be eliminated. Therefor,the correcting of such aberration is an important subject to improve the ultimate resolution of electron microscopes and the performance of other electron optical instruments, such as electron beam manufactureing machines, electron lithographic machine etc.A combination of quadrupoles and octupoles which was proposed by Scherzer is a reasonable way to correct this aberration, but it has been proved practically unsuccessful. Crewe suggested that the sextu-pole elements could be used as a device to correct the 3rd order aberration of a round lens. Later he has showed that a system of two sextupoles with a round lens placed in the middle of it can acturely act as an electromagnetical concave lens if the focal length of this middle lens be settled to satisfy certain condition. Instead of taking terms of z7 in series solution of motion equation of electron in sextu-pole which Crewe had done, we took it up to z20to compute the amplitude r and slope r' of electron trajectory at each specific point z1 , z2 , z3, and z4along the optical axis. The results show that the action of sex-tupole-lens-sextupole system does act as an optical concave lens. It produces a negative spherical aberration. Differing from optical concave lens, in this system the negative aberration can be varied in a wide rang by adjusting the strength of sextupole k. Fig.2 shows a series of computed performances of electron beam near Gaussian image point while the strength of sextupole k=0 incase(a) and k1< k2in case (b) and (c). Evidently, this system does produce a negative spherical aberration while k is not zero.

scholarly journals I. A new formula for a microscope object-glass

1873 ◽  
Vol 21 (139-147) ◽  
pp. 111-118
Keyword(s):  
Flint Glass ◽  
Concave Lens ◽  
The Subject ◽  
New Form ◽  
New Formula

A pencil of rays exceeding an angle of 40° from a luminous point cannot be secured with less than three superposed lenses of increasing focus and diameter, by the use of which combination rays beyond this angle are transmitted, with successive refractions in their course, towards the posterior conjugate focus: until quite recently, each of these separate lenses has been partly achromatized by its own concave lens of flint glass, the surfaces in contact with the crown glass being of the same radius, united with Canada balsam; the front lens has been made a triple, the middle a double, and the back again a triple achromatic. This combination therefore consists of eight lenses, and the rays in their passage are subject to errors arising from sixteen surfaces of glass. In the new form there are but ten surfaces, and only one concave lens of dense flint is employed for correcting four convex lenses of crown glass: as this might at first sight be considered inconsistent with theory, a brief retrospect of the early improvements of the microscope object-glass will help to define the conditions. The knowledge of its construction has been entirely in the hands of working opticians; and the information published on the subject being scanty, this has probably prevented the scientific analyst from giving that aid which might have been expected.

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