Part 5

These considerations will explain the occasional use of stops, and the expediency of having them ready to use when they can be of temporary advantage. There are nights in our climate when a 6 inch aperture may be the largest that can be used with satisfaction or advantage; indeed it used to be accepted as an incontrovertible fact that an aperture of G inches was the largest that could be used on _average_ nights in our country. But there are also not a few nights when large apertures, bring great gain to their fortunate possessors.

It is obvious that, _mutatis mutandis_, these conditions must affect refractors as well as reflectors, with perhaps this difference, that in the case of a reflector the rays pass down the tube and infringe on the speculum as a column of parallel rays; consequently all the external rays of the column travel near the surface of the tube for its entire length. This being so, it is obvious that until the internal and external air and the metal tube have time to equalize in temperature, it will be advantageous, in all work requiring high powers, such as the examination of difficult double stars, to “stop off” for a time the _extreme edge_ by a small diaphragm.

This will occur especially on frosty nights after a warm fine day. When the telescope is in the open air, ice will not infrequently form on the top of the tube, and the effect will be at once obvious in the irregularity of a star image at the edge corresponding to the top of the tube. The use of a small diaphragm will at once obviate this defect and give perfect images. On such nights a wooden tube is preferable to a metal one. It has been frequently my experience, when finally testing an instrument, that when I have been dissatisfied with its performance, and in order to trace the cause of the apparent defect, have removed the mirrors to my wooden testing tube, I have found every defect removed, and imperfect replaced by perfect images. The explanation is simple. Not only is the wooden tube much larger in diameter than the specula, but wood is of itself less sensible than metal to differences of temperature.

To make the tubes some 2 or 3 inches larger in diameter than the speculum would cure this evil, but it would involve various inconveniences and expenses in structure, which would more than outweigh the advantage of overcoming a hindrance which is after all only temporary and occasional.

The experiment of perforating or ventilating tubes has now been thoroughly and exhaustively tried, with the result that there is a fairly general consensus of experienced opinion against their use. The advantages are merely theoretical; the disadvantages are grave. Among them are the admission of cross reflections in every direction, the admission of dust, imperfect protection of the mirrors, and a decided loss of strength and rigidity to the tube.

CATALOGUE OF Silvered Glass Reflecting Telescopes, &c.,

AND THEIR ACCESSORIES,

OCTOBER, 1880.

G. Calver, while introducing the list of prices below, has the greatest confidence in calling the attention of the practical astronomer and the amateur to the very moderate prices charged, considering the principle and style of the mounting, and the firm and well-fitted arrangements in all the parts.

The principle of the mounting is that which is the most convenient to use, and with the greatest degree of steadiness with the easiest movements. Fig. 1 is the _best mounting ever applied to the Reflecting Telescope_.

He is also able to state that, with his processes and special facilities for working large specula, and substantially mounting them, he is prepared to construct automatic equatorials of large sizes, and with any special arrangements that may be required.

Fig. 1 is a very excellent mounting, and admirably suited for large instruments, especially when clock-power is applied; and as now manufactured by G. CALVER, is the most complete and reliable, its details of construction being such as to give the greatest freedom of motion and steadiness. For large sizes the top of polar axis works on friction rollers; and, as a superior German stand, its arrangements and means of astronomical as well as optical adjustments, are such as to make it an instrument of precision.

To ensure these qualities many well considered arrangements—entailing careful and expensive workmanship—must be provided; details that do not easily admit of description in an ordinary catalogue, or can be shown in an engraving, but are duly appreciated by the observer, who will find their value by practice.

The cradle bar is compound with fine screw movement for perfecting collimation, but to make this adjustment perfect, the telescope tube must be centred in a powerful lathe, and the cradle and solid metal rings in which the tube revolves (see illustration) has to be turned and fitted with true flanges or working bearings; the mechanical and optical centres will then coincide.

The hour circle, to be truly divided, must be truly made, and is a solid wheel of good substance, working on a secondary axis, and rotated by mechanical means, truly concentric, with a very strong polar axis.

The declination circle has fine tangent screw movement, with double action for setting the readings by milled-headed screw, and also long driving rod from the eye-piece to set and adjust the object in centre of field. There is slow hand motion, by a separate wheel in right ascension.

The clock-power is connected and disconnected instantly, by touching with the finger a small lever, conveniently placed at a small door in the clock case. The clock will go at exactly the same rate, whether it is driving the telescope or not, and the latter begins to move at the proper speed the instant the lever is moved, and the connection of the telescope and clock made, which is instantly effected by another lever.

£ s. d. 14, 15, to 16 inch Speculum mounted as (Fig. 1), with rotating hour circle reading to 5 seconds and decimation to 1 minute (the diameter of the circles never less than that of the Speculum, and often larger)

8 powers, from 50 to 700, with first-rate and very powerful driving clock, first-class instrument, and complete from £250 to 350 0 0

17, 18, to 20 inch Speculum, with 10 powers, from 50 to 800, including Kellners, Huyghenians, and Achromatics, from £360 to 500 0 0

22 to 24 inch Speculum from £600 to 700 0 0

30 inch Speculum, with 12 powers and position micrometer, and transit eye-pieces 1000 0 0

Prices will be forwarded for special arrangements in any of these large sizes.

£ s. d. Driving Clocks for 6 to 8 inch 35 0 0 〃 〃 12 〃 18 〃 40 0 0 〃 〃 20 〃 30 〃 50 0 0

These are fitted with governor, regulator, and self-adjusting break. They are made of gun metal and steel, and every wheel is cut. They drive with most excellent regularity, and are, when desired, made to beat seconds on a bell. The whole is enclosed in a mahogany frame, with glass panels.

Fig. 2 admits of equal completeness, &c., but the stand is not so convenient for clock-power.

SILVERED GLASS EQUATORIAL TELESCOPES, very substantially and well fitted as (Fig. 2). All these equatorials have revolving body.

£ s. d. 5-1/4 inch Speculum, from 4 to 6 feet focus, with 7 inch hour circles, reading to 5 seconds of time, and declination circle reading to 1 minute, 2 powers 30 0 0

6-1/2 inch Speculum, of from 5 to 6-1/2 feet focus, 10 inch rotating hour circle, reading to 5 seconds of time, and declination circle to 1 minute of arc, with 3 powers--100 to 500 45 0 0

8-1/2 inch Speculum, as above, with 10 inch circles, 4 powers 70 0 0

10 inch Speculum, as above, with 10 inch circles, 5 powers 90 0 0

12-1/2 inch Speculum, as above, with 10 inch circles, 5 powers 120 0 0

The EDUCATIONAL REFLECTOR is a plain and very steady and satisfactory instrument, mounted on (Fig. 2) stand, without circles, has revolving body, and made so as to be portable.

£ s. d. 8-1/2 inch Speculum as above, with 3 powers 40 0 0

The POPULAR REFLECTOR (Fig 3), with Angle-Block stand, with endless screw-motion to follow the stars with equatorial motion.

£ s. d. 5-1/4 inch Speculum, of from 4 to 6 feet focus, with 2 powers 15 15 0

These are also made with rotating body.

Reflecting Telescope on Alt-azimuth Stand, fitted with silvered glass Speculum, and provided with two eye-pieces.

£ s. d. 5-1/4 inch Speculum, 4 to 6 feet focus 19 10 0 6-1/2 〃 〃 5 〃 6-1/2 〃 25 0 0 8-1/2 〃 〃 〃 6-1/2 〃 33 10 0 10 〃 〃 〃 7 〃 50 0 0 12-1/2 〃 〃 〃 8 〃 70 10 0

Silvered-Glass Specula (_unmounted_).

THE FINEST QUALITY GUARANTEED.

£ s. d. 5-1/4 inch diameter, 4 to 6 feet focus 5 0 0 6-1/2 〃 4 〃 6-1/2 〃 6 5 0 8-1/2 〃 〃 6-1/2 〃 13 0 0 10 〃 〃 7 〃 25 0 0 12-1/2 〃 〃 8 〃 38 10 0 14 〃 〃 8 〃 55 0 0 16 〃 〃 10 〃 75 0 0 18 〃 〃 12 〃 100 0 0 20 〃 〃 12 〃 130 0 0 22 〃 〃 15 〃 165 0 0 24 〃 〃 18 〃 200 0 0 26 〃 〃 〃 250 0 0 30 〃 〃 〃 300 0 0 36 〃 〃 〃 400 0 0

Silvered-Glass Diagonal Mirrors (_unmounted_).

FINEST QUALITY GUARANTEED.

£ s. d. 1 inch in the minor axis, or narrowest diameter of the ellipse 1 0 0 1-1/2 ditto ditto ditto 1 10 0 2 ditto ditto ditto 2 0 0 2-1/2 ditto ditto ditto 2 10 0 3 ditto ditto ditto 3 3 0 3-1/2 ditto ditto ditto 4 4 0 4 ditto ditto ditto 5 5 0

Silvering and Polishing Specula.

£ s. d. 5 inch 0 6 0 6-1/2 〃 0 8 0 8-1/2 〃 0 10 6 10 〃 0 15 0 12-1/2 〃 1 1 0 Diagonal Planes 3s. to 0 5 0

ASTRONOMICAL EYE-PIECES.

OF BEST QUALITY.

Huyghenian Construction of the following magnifying powers on a 6-1/2 feet focus object-glass:—

£ s. d. 35, 60, 90, 130, 180, 200 each 0 15 0 320, 450 1 1 0 610 1 7 6

All fitted with Sunshades.

Improved Achromatic, Ramsden’s Construction.

£ s. d. 90, 130 each 1 0 0 189, 250 1 10 0 320, 450 1 12 6 610, 750 2 2 0 850 2 2 0

Kellner’s Construction.

£ s. d. 30, 58, and 80 each 1 5 0

Solar Eye-Pieces.

£ s. d. Solar Diagonal, with true plane 1 12 6 Barlow’s Lens 1 1 0 Day Erecting Eye-piece 1 5 0 Coloured Eye-piece cap for observing the Moon 0 4 0 Transit Eye-pieces, for use with Equatorials £1 1s. to 2 2 0 Adjusting Piece 0 2 6

_Good Instruments will be taken in exchange, and liberally allowed for._

GEORGE CALVER,

HILL HOUSE, WIDFORD,

CHELMSFORD, ESSEX.

LETTERS AND EXTRACTS FROM THE FOLLOWING GENTLEMEN.

_From_ W. E. PARKINSON, Esq.

11, WELLINGTON PARK TERRACE, BELFAST, _July 7th, 1876_.

DEAR SIR,—The night before last was a clear night, and I got the 6-1/2 inch out, and though the air was not good when using a 3-1/4 inch refractor, I was much pleased with the performance of the mirror.

Mr. W. came up about eleven o’clock and stayed till one; we got it on Saturn, and although rather low it was really a fine sight—Mr. W. was much pleased.

The moon being near to full we could not do much with faint points of light.

Yours sincerely, (_Signed_.)

Mr. G. CALVER.

* * * * *

_From_ F. G. LEMMAN, Esq.

74, HAGLEY ROAD, EDGBASTON, _November 27th, 1876_.

DEAR SIR,—I have now got the 5 inch speculum fairly into adjustment, and I am well pleased with it. Last night, the sky being clear for a short time, I turned it on the moon; the definition of minute craters was all that could be desired. I hope to have better opportunities.

I am, Yours truly, (_Signed_.)

Mr. G. CALVER.

* * * * *

(_From the same Gentleman._)

74, HAGLEY ROAD, EDGBASTON, _November 21st, 1876_.

DEAR SIR,—I have mounted the 5 inch speculum on a simple equatorial, and it works well. I am sure it will turn out a fine glass, and I shall not regret the time and pains I have spent over it. It is not quite in adjustment yet, when it is it will afford me a treat. I have just tried it once on the Orion Nebula and Trapezium, details of Nebula well seen, and the 5th star in Trapezium easy.

I am, Yours truly, (_Signed_.)

* * * * *

_From_ D. C. CARMICHAEL, Esq.

DOWLAIS, _December 27th, 1876_.

DEAR SIR,—I have thoroughly tested the speculum, and am pleased to tell you it stood its trials well.

Yours truly, (_Signed_.) Mr. G. CALVER.

* * * * *

ST. DENIES, SOUTHAMPTON, _August 14th, 1876_.

DEAR SIR,—With reference to your enquiry as to the performance of the 6-1/2 inch reflector, I have pleasure in stating that I am perfectly satisfied. It readily divides the test objects, and shows delta Cygni with as low a power as 160. The comes to Sirius may be considered an atmospheric test, perhaps, but I have repeatedly seen it. Your stand I find very steady and convenient; altogether I can fairly say that I consider that I have an instrument of considerable power at a comparatively small cost. I now find the attempt to observe with a refractor the reverse of pleasant.

The owner of a 3 inch refractor, after using my reflector, writes that he envies me its power, ease, definition, and comfort in observing.

I am, dear Sir, Yours truly, A. H. S.

* * * * *

_From_ W. L. LANCASTER, Esq.

25, HAMILTON TERRACE, ST. JOHN’S WOOD, _May 26th, 1877_.

MY DEAR SIR,—I have much pleasure in informing you that the very first time I saw Saturn through your 6-1/2 inch reflector, the definition of the planet was far superior to anything I had observed before, even with a good 4-1/4 inch refractor; and on another occasion I was greatly pleased with the clear and easy view of the “Comes” to ε Boötis. But even without including the stars, the views of Saturn and Jupiter through my 6-1/2 inch mirror do _alone_ (in my opinion) well repay the cost of the telescope.

Yours truly, (_Signed_.) Mr. G. CALVER.

* * * * *

From A. A. COMMON, Esq., F.R.A.S.

37, EATON RISE, EALING, _February 13th, 1877_.

DEAR SIR,—I like the 18 inch speculum, and I think it as good as it can be, and if the larger one is as good I shall be delighted with it.[1] It gives beautiful star images; I see Sirius as a brilliant dot, a glorious object without ray or flares of any kind. I never saw it so well before.

Yours truly, (_Signed_.) Mr. G. CALVER.

Footnote:

[1] This alludes to the 37 inch, then under consideration; and I may now add that Mr. Common is perfectly satisfied with it, and “consider it a _very fine_ mirror.”

* * * * *

(_From the same Gentleman._)

37, EATON RISE, EALING, _February 3rd, 1877_.

DEAR SIR,—I have tried the 18 inch speculum on some tests, and especially the satellites of Uranus, and it appears fine, very fine. What I have done in photography promises well.

Yours truly, (_Signed_.)

Mr. G. CALVER.

* * * * *

_From_ J. L. LANCASTER, Esq.

SOUTHEND-ON-SEA, _March 17th, 1877_.

MY DEAR SIR,—Circumstances have prevented me from making many observations of late, but I am more confirmed in my opinion that my telescope of your make is a very fine instrument (a 6-1/2 inch). I heartily wish you success, which I feel sure you will obtain, as you are so painstaking and turn out a thoroughly reliable article. I shall be curious to compare my brother’s telescope with mine.

Sincerely yours, (_Signed_.)

Mr. G. CALVER.

* * * * *

_From_ The REV. W. P. MATTHEWS.

GORLESTONE, _March 30th, 1875_.

DEAR SIR,—You have asked me for my opinion of the 10 inch speculum. I have tested it, and can speak in the highest terms of its performance. Definition in good air is as near perfection as it is possible to imagine. Such tests as ζ Cancri, γ^2 Andromedæ are well divided. The 6th star in the Trapezium stands out well clear of its brighter neighbour. On the moon any power may be applied, only lessening the light, but retaining sharpness of outline. I have tried a good many telescopes, but never yet saw these 10 inch mirrors surpassed. The image of a star disc expanded on each side of the focus is of as nearly the same appearance as possible.

Yours truly, (_Signed_.)

* * * * *

416, BRIXTON ROAD, _April 20th, 1875_.

DEAR SIR,—In compliance with your request, I send particulars of my observation on φ Draconis. It was about 12.45 this morning when I directed my 8-1/2 inch to this object, the air was very unsteady, in fact not nearly so good as some nights lately, and I well divided it with a power of 450. I then used a 6-1/2 inch stop, which I found very much increased the steadiness of definition.

Yours very respectfully, P. H.

* * * * *

_July 10th, 1874_.

DEAR SIR,—I tried the 6-1/4 inch mirror I had from you for the first time last night. The night was not a good one for definition, but I was very much pleased indeed with its performance.

Yours truly, C. H. W.

* * * * *

_From_ H. SADLER, Esq., F.R.A.S.

HONITON RECTORY, _December 23rd, 1874_.

DEAR SIR,—I have been wishing to tell you how pleased I am at the performance of your mirror. Many thanks for your kind offer to change the mirror if needful, but I think it could hardly be better than it is, its performance on different double stars is most excellent. I have examined nearly 100 of these since September with the 6-1/2 inch, most of them very difficult, but the mirror came out well under all tests. I send you a list of some of the objects I have examined.

Yours truly, H. SADLER.

* * * * *

π _Aquilæ._—Divided, power 80. 14-^m. Webb might have been rated 15-^m. [6-^m., 7-^m. 1″·5.]

A B C A-B A-C δ _Aquilæ._—[3-1/2, 16, 14, 96″, 194″.] Comites easy, 3 other excessively minute ones, not shown by the 11 inch refractor with which P. Smyth measured B.C. Found 6-1/2 in. Sept. 12th, est. ± 17 mag. 100″ to 110″.

ζ _Persei._—My friend “Linea” sees two minute stars with 4·28 in. Wray, not in Webb, and Wray himself sees an excessively difficult companion. I see these 3 stars easily with 6-1/2 in. (Wray says his companion is “very difficult with 7 in. refractor, really only a glimpse star, even with the best atmosphere”), and have added two more stars to the group.

β _Delphini._—Close double discovered by Burnham, est. 0″·7. In contact 6-1/2 in., power 430.

η _Coronæ Borealis._—Very low, long past meridian, in contact 164 power.

_a_ _b_ _c_ _n_ β _Equalei._—[5-1/2, 13, 14, 16: _a-b_ 35″, _a-c_ 50″ _b_-B 3″.] 16-^m. not seen, Webb 9-1/3 in. Just divided 164, easy 430 power. Two other companions (not seen by Smyth or Webb?)

μ _Andromedæ._—The 16-^m. comes, a very difficult test, easy; other comites (not seen by Smyth or Webb?) seen.

Companion to Vega pretty easy, Nov. 12th, 37m. after sunset.

_a_ _b_ _c_ B _a-b_ _b_-B _a-c_ _P._ 178 xx. _Delphini._—[7-1/2, 8, 16, 9: 14″·3, 0″.7, 20″.] Smyth 16 by evanescent glimpses; easy 164 power, 6-1/2 in. 8-^m. well elongated in direction of 230°, power 430.

* * * * *

_From_ H. BLYTH, Esq.

REGENT ROAD, GREAT YARMOUTH, _October 24th, 1874_.

DEAR SIR,—Doubtless you have anticipated hearing from me, relative to the 10 inch telescope, but the weather has been so indifferent for delicate astronomical work, that it has not yet had a fair trial.

In middling good air, when I could use a reduced aperture with satisfactory results, the star images reminded me of the 6-1/2 inch, which I considered so superb a mirror that nothing could surpass its fine performance either on stars or planets.

With the recollections of the truly splendid views I had of Jupiter last season, I count much to see him with the 10 inch.

Yours truly, (_Signed_.)

* * * * *

_From_ T. AYERS, Esq.

REGENT ROAD, GREAT YARMOUTH, _February 2nd, 1875_.

DEAR SIR,—In reply to yours, I have not had a really good night to test severely the defining powers of the 10 inch mirror, but judging from the few difficult objects I have seen, the mirror promises well, and I have no doubt of its excellence.

I had a fine view of the grand nebula in Orionis, and noticed that the 5th and 6th stars were quite plainly seen, when the aperture was reduced to 5-1/2 inches. I consider your stand a great improvement, being very convenient and steady.

Yours truly, (_Signed_.)

* * * * *

_From_ THE REV. A. P. ARNOTT.

AM STREET, EDINBURGH, 8-1/2″ ALT. AZIMUTH. _February 25th, 1878_.