Book i. chap. vii. page 59, published in 1813, has adverted to this

circumstance, in his description of an ‘Eye-piece wire micrometer,’ and complains of Mr. Ezekiel Walker, having in the ‘Philosophical Magazine’ for August, 1811, described such an instrument as an invention of his own. Dr. Kitchener some years afterwards, described what he called a Pancratic or omnipotent eye-piece, and got one made by Dollond, with a few modifications different from that suggested by Brewster and Walker, which were little else than cutting the single tube into several parts, and giving it the _appearance_ of a new invention. In fact, none of these gentlemen had a right to claim it as his peculiar invention, as the principle was known and recognised long before. I had increased the magnifying powers of telescopes, on the same principle, several years before any of these gentlemen communicated their views on the subject, although I never formally constructed a scale of powers. Mr. B. Martin, who died in 1782, proposed many years before, such a moveable interior tube as that alluded to, for varying the magnifying power.

In order to give the reader a more specific idea of this contrivance, I shall present him with a figure and description of one of Dr. Kitchener’s Pancratic eye-pieces, copied from one lately in my possession. The following are the exact dimensions of this instrument, with the focal distances, &c. of the glasses, &c. of which it is composed.

In. Tenths. Length of the whole eye-piece, consisting of four tubes, when fully drawn out, or the distance from A to B. fig. 81. 14 4

Length of the three tubes on which the scale is engraved, from the commencement of the divisions at B to their termination at C. 9 15

Each division into tens is equal to 3-10ths of an inch.

When the three inner tubes are shut up to C, the length of the eye-piece is exactly 5 5

When these tubes are thus shut up, the magnifying power for a 3-1/2 feet achromatic is 100 times, which is the smallest power. When the inner tube is drawn out 1/3 of an inch, or to the first division, the power is 110, &c.

Focal distance of the lens next the object 1 0

Breadth of Ditto. 0 65

The plane side of this glass is next the object.

Focal distance of the second glass from the object 1 5

This glass is double and equally convex, Breadth 0 5

Distance between these two glasses 1 7

Focal distance of the third or field lens, which is plane on the side next the eye 1 1

Breadth of Ditto. 0 55

Focal distance of the lens next the eye 0 6

Breadth 0 43

This glass is plane on the side next the eye.

Distance between the third and fourth glasses. 1 1

From the figure and description, the reader will be at no loss to perceive how the magnifying power is ascertained by this eye-piece. If the lowest power for a 44 inch telescope be found to be 100, when the three sliding tubes are shut into the larger one, then by drawing out the tube next the eye 4 divisions, a power of 140 is produced; by drawing out the tube next the eye its whole length, and the second tube to the division marked 220, a power of 220 times is produced, and drawing out all the tubes to their utmost extent, as represented in the figure, a power of 400 is obtained. These powers are by far too high for such a telescope, as the powers between 300 and 400 can seldom or never be used. Were the scale to begin at 50, and terminate at 200, it would be much better adapted to a 3-1/2 feet telescope. Each alteration of the magnifying power requires a new adjustment of the eye-piece for distinct vision. As the magnifying power is increased, the distance between the eye-glass and the object-glass must be diminished. Dr. Kitchener says, that ‘the pancratic eye tube gives a better defined image of a fixed star, and shows double stars decidedly more distinct and perfectly separated than any other eye tube, and that such tubes will probably enable us to determine the distances of these objects from each other, in a more perfect manner than has been possible heretofore.’ These tubes are made by Dollond, London, and are sold for two guineas each. But I do not think they excel, in distinctness, those which are occasionally made by Mr. Tulley and other opticians.