In the centre of the solid floor, levelled to the utmost perfection, was left a circular pillar supporting the polar axis of an instrument widely differing from our telescopes, especially in the fact that it had no opaque tube connecting the essential lenses which we call the eye-piece and the object-glass, names not applicable to their Martial substitutes.

It was said that coming to America he brought with him the object-glass of a telescope, at a time when scientific apparatus paid a high duty.

The aperture of your object-glass is sufficient for doing much useful work, but, if I may hazard an opinion to you, do not attempt too much, for it is quality rather than quantity which is now desirable.

"In Jan. 1824, at Playford, I was working on the effects of separating the two lenses of an object-glass, and on the kind of eye-piece which would be necessary: also on spherical aberrations and Saturn's figure.

On my quires at Cambridge I was working on the effects of separating the object-glass lenses, with the view of correcting the secondary spectrum: and on Jan. 31st I received some numbers (indices of refraction) from Mr Herschel, and reference to Fraunhofer's numbers.

I also wrote a draft of my Paper on Achromatic Eye-pieces, and on May 17th, 1824, it was read at the Philosophical Society under the title of 'On the Principles and Construction of the Achromatic Eye-pieces of Telescopes, and on the Achromatism of Microscopes,' including also the effects of separating the lenses of the object-glass.

I applied to the Royal Society for the loan of Huyghens's object-glass, but they declined to lend it.

I was at this time pressing Tulley, the optician, about an object-glass for the Mural Circle.

In this December I had a quartz object-glass by Cauchaix mounted by Dollond, and presented it to the Observatory.

On Aug. 15th Herschel wrote to me, communicating an offer of the Duke of Northumberland to present to the Cambridge Observatory an object-glass of about 12 inches aperture by Cauchaix.

In fact the observations of 1842 were sent to press on Mar. 1st, 1843.About this year the Annual Dinner at the Visitation began to be more important, principally under the management of Capt. W.H. Smyth, R.N.In November I was enquiring about an 8-inch object-glass.

"In April I received authority to purchase of Simms an 8-inch object-glass for the new Transit Circle for £300.

The principle of it is as follows: Let the micrometer be placed close to the object-glass, the frame of the micrometer being firmly connected with the object-glass cell, and a reflecting eye-piece being used with no material tube passing over the object-glass: and let a basin of quicksilver be placed below the object-glass, but in no mechanical connection with it, at a distance equal to half the focal length of the object-glass.

The principle of it is as follows: Let the micrometer be placed close to the object-glass, the frame of the micrometer being firmly connected with the object-glass cell, and a reflecting eye-piece being used with no material tube passing over the object-glass: and let a basin of quicksilver be placed below the object-glass, but in no mechanical connection with it, at a distance equal to half the focal length of the object-glass.

The principle of it is as follows: Let the micrometer be placed close to the object-glass, the frame of the micrometer being firmly connected with the object-glass cell, and a reflecting eye-piece being used with no material tube passing over the object-glass: and let a basin of quicksilver be placed below the object-glass, but in no mechanical connection with it, at a distance equal to half the focal length of the object-glass.

The principle of it is as follows: Let the micrometer be placed close to the object-glass, the frame of the micrometer being firmly connected with the object-glass cell, and a reflecting eye-piece being used with no material tube passing over the object-glass: and let a basin of quicksilver be placed below the object-glass, but in no mechanical connection with it, at a distance equal to half the focal length of the object-glass.

'Regarding the new Equatoreal the Report states that 'For the new South-East Equatoreal, the object-glass was furnished by Messrs Merz and Son in the summer of last year, and I made various trials of it in a temporary tube carried by the temporary mounting which I had provided, and finally I was well satisfied with it.

From what I have seen, I have no doubt of its proving a first-rate object-glass.'On

The reflector was cleaned, but in vain; at last it was discovered that one of the lenses (the convex lens) of the combination which forms the object-glass of a Reversed Telescope in the interior of the Transit-axis, and through which all illuminating light must pass, had become so corroded as to be almost opaque.

and Calculations of the Observations. 1834 Nov. 24 On the Diffraction of an Object-Glass Camb.

Our old friend Time has this year illustrated his march, or object-glass, with a host of images or spectrathat is, woodcuts of head and tail piecesto suit all tastesfrom the mouldering cloister of other days to the last balloon ascent.

Newton surmounted this difficulty by laying a double convex lens, the radius of the curvature of each side of which was fifty feet, upon the flat surface of a plano-convex object-glass, and in the way he obtained a plate of air, or of space, varying from the thinnest possible edge at the centre of the object-glass where it touched the plane surface to a considerable thickness at the circumference of the lens.

Newton surmounted this difficulty by laying a double convex lens, the radius of the curvature of each side of which was fifty feet, upon the flat surface of a plano-convex object-glass, and in the way he obtained a plate of air, or of space, varying from the thinnest possible edge at the centre of the object-glass where it touched the plane surface to a considerable thickness at the circumference of the lens.

When the light was allowed to fall upon the object-glass, every different thickness of the plate of air between the object-glasses gave different colours, so that the point where the two object-glasses touched one another was the centre of a number of concentric coloured rings.

Now, as the curvature of the object-glass was known, it was easy to calculate the thickness of the plate of air at which any particular colour appeared, and thus to determine the law of the phenomena.