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CHAPTER XV.

DIAGONAL EYE-TUBES.

WHEN we wish to discern those delicate and minute objects, which are the most interesting exhibitions our Telescopes display to us, and with the finest Instruments are only discernible with the most favourable circumstances, we must be in a position of the greatest ease: no cramp or painful posture must distort the Body, or irritate the Mind;—the whole powers of which must be concentrated in the Eye.

Such is the sympathy between the various Organs of the human body, that we may as well attempt to think accurately on two Subjects at the same time, as to use two Senses at the same moment :— each must be used alone, if we wish to give it a fair chance of doing its utmost.-As Shakspeare has observed of Listening with such profound attention, that" each other Sense was lost in that of Hearing."

To avoid the painful position when observing Celestial objects when they are near the Zenith, I have tried 8 different kinds of Diagonal Eye-tubes. Several of them are so constructed that they will apply to, or will receive all the Eye-pieces, and render an

Achromatic Refracting Telescope, or a Gregorian as convenient as a Newtonian Reflector.

The diminution of light by the Diagonal is scarcely perceptible when observing the Moon or the fixed stars—such is the intense intrinsic brightness of the Fixed Stars, that the inferior degree of vividness of the pencil of rays is, I may say, imperceptible.

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a Lyra, I surmise, has Light enough to bear being magnified at least a hundred thousand times, with no more than 6 inches of aperture, provided we could have such a power, and other considerations would allow us to apply it."-See Sir Wм. HERSCHEL'S First Catalogue of Double Stars, in the Phil. Trans. for 1782.

The position is not only pleasanter, but the organ of sight is more perfect, when we look comfortably straight forwards-than it is when the head is bent back in the break-neck position required in observing objects in a high Altitude without such assistance.

It may be urged, that a Prism would bend up the rays with less loss of brightness than a Speculum can reflect them—but in the latter you have only One surface to get worked truly, and it is no easy thing to obtain that quite good*—in_the_former

* "I find more difficulty in correcting the figure of the little plain piece of metal next the Eye-glass than one would expect."See Sir ISAAC NEWTON'S Letter in the Phil. Trans. for 1672, vol. vii. p. 4032; and Sir WILLIAM HERSCHEL'S Obs. in Oct. 12,

Three, and the imperfections of the Glass to contend with into the bargain: the facilities of construction are thus at least three to one in favour of Specula: but if the Bull's Eye Prism be perfectly good, that is incomparably the best Diagonal Eye-piece that it can be made good, I know, because I have one which is as sharp, though not so bright, as a Single Lens.

1st. A Plane Speculum placed at an angle of 45 degrees between the Object-glass and the Eye-glasses. 2d. A Plane Speculum, fixed at the like angle, is placed next to the Eye, before the Eye-tube.

3d. A Plane Speculum is placed between the Eyeglass next to the Eye, and that next to the object. 4th. A Prism is placed between the Eye-glasses and the Object-glass.

5th. A Prism is placed between the Eye and the Eye-glasses.

6th. A Plane Prism is placed between the Eyeglass next to the Eye, and that next to the object.

7th. Another Eye-tube is composed of an Eye-glass and a Prism with one surface, ground to a certain degree of convexity, which acts as a Prism, and as one of the Eye-glasses also.

8th. The Bull's Eye Eye-tube, which is composed of a Hemisphere, and acts as a convex lens with a

1782, in p. 44 of the Phil. Trans. for 1795.- Messrs. WATSON and TULLEY have assured me that no figure is more difficult to make than a perfect Plane.

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prism; this is more simple in its construction, is more perfect and lighter than any other prism Eyepiece-but even this is darker than an Huygenian eye-piece of the same magnifying Power.

From various trials of Prisms and Specula, I am convinced that it will be much easier to obtain a good Speculum than a good Prism—though either are extremely difficult to get perfect—and however perfect they may be, the diagonal vision is never so light, and seldom so distinct as the direct vision is I say this, after a patient trial on the Planet Saturn, which suffered much more from the intervention of a Prism or Speculum than it did from using the Huygenian instead of a Single Lens Eye-piece: that is, the complexion of the Planet was diminished in brightness, and changed to a yellowish or greenish hue, and its features consequently became less striking on the Eye, and the Belts and Division in the Ring less marked; -the Observations I have quoted from Dr. Herschel respecting Single and Double Eye-pieces, apply very aptly to Diagonal and Direct Vision.-See Chapter V.

If a Diagonal Eye-tube, (No. 1.) made with a Speculum, is applied to an Achromatic Telescope of 5 feet focus, when an Huygenian Eye-tube magnifying 160 times is applied to it, the tube must be 2 inches shorter than is required with the same power with direct vision.

CHAPTER XVI.

MAGNIFYING POWER, AND HOW TO MEASURE IT.

AN easy way of judging of the relative magnifying powers of various Eye-tubes and Lenses-is to hold them up about 12 or 18 Inches from the Eye, and 3 or 4 Feet from a window-that in which the panes of glass appear least magnifies most ;— or place two Candles nearly together—and hold the lenses about a foot from them, that in which the Candles appear nearest together magnifies most.

TO ASCERTAIN THE MAGNIFYING POWER OF A

TELESCOPE,

measure the diameter of the aperture of the Objectglass, or Speculum, and that of the little image of it which is formed at the end of the Eye-piece, the number of times the latter is less than the former is the Magnifying power.

To measure the diameter of the pencil of rays with great ease and accuracy, Mr. RAMSDEN*, about the

* The highest praise is due to the merits of the late Mr. Jesse Ramsden, for his ingenuity, liberality, and persevering endeavours to invent and perfect the various instruments used in Astronomy,

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