tance between the Eye-glass and the Objectglass must be diminished.

"It has long been known, that the Magnifying powers of Telescopes could be augmented by increasing the distance between the two glasses next to the Eye, and the two that are next to the Object-glass, to almost double the power of the Eye-tube in its usual form, i. e. from 30 to 55. This is the utmost that Opticians have heretofore accomplished;· yet this variation is so desirable, that I think it only requires to be generally known, to be generally desired, both for Convenience and Cheapness*.

"A few months ago, I saw an Eye-tube, made by MR. CAUCHOIX, with a scale of magnifying powers from 25 to 73; but, upon trial, I found that the vision was perfectly good only between 35 and 45.

"My attention was strongly excited by the idea of ONE Eye-tube effecting the whole busi

* Before Mr. JESSE RAMSDEN invented, about 1785, The Pipe-drawer for the Terrestrial Eye-tube- and changed the Magnifying power, by changing the two Glasses next to the Eye - for which half Eye-tube he charged 10s. 6d. for every change of Magnifying power, there was the incumbrance and expense of another Eyetube, costing 11. 1s.

ness of Magnifying; and after several experiments, with the assistance of Mr. S. PIERCE, I combined lenses of such proportions that they admitted of being separated from each other so as to Magnify at one extremity, more than double what they did at the other, the vision continuing uniformly distinct.

Having now done more than had been previously effected, I brought it to You. The approbation You expressed of what I had done, so encouraged me, that I applied unceasingly, determined to perfect the object in view, which I have now accomplished.

"I beg to present to you the following accurate measurement of the powers, and faithful account of the performance of

"The PANCRATIC EYE-TUBE, which I think gives a better defined image of a fixed Star, and shews Double Stars decidedly more distinct*, and perfectly separated, than any other Eye-tube, and I hope will enable us to determine the distances of these objects

* Especially in Achromatic Telescopes, which are, what is termed, a little over-corrected, and the purple rays predominate: that is, when the focal length of the Conver Lens, is formed rather too long for the Concave.

from each other, in a more perfect manner than has been possible heretofore.*

"This Eye-tube, when accurately made, applied to an Achromatic of 44 inches focus, produces, in the most perfect manner, every intermediate degree of Magnifying power between 100 and 400, either for Celestial or Terrestrial uses the Field of Vision continuing uniformly distinct.

"Therefore it is presumed, that the advantage of my PANCRATIC Eye-tube over the usual common Eye-tube, in variety of Magnifying power, convenience, cheapness,and portability, is as 300 to 1.- The cost of a common Eye-tube is One Pound; of the Pancratic only Two Pounds, Two Shillings.

"The Tubes are graduated; every 10 degrees, thus, 100, 110, 120, up to 400.

*This may, perhaps, be accounted for, from the greater degree of the aberration arising from the extreme sphericity of the lenses in other Eye-pieces which magnify so highly.

+ It may be said that Common Eye-tubes have rather a larger field but of what use is that part of the field in which objects appear distorted and fringed with Colour.

That, can only be considered the actual and useful field of view, the Margin of which, is as perfectly distinct as the middle of the field, when the Telescope is adjusted at an object seen in the middle of the field.

"The change from one Power to another may be made instantaneously, with the utmost facility and certainty, and the Observer always knows exactly what Power he is using."

[The above is an extract from Dr. Kitchiner's Letter to Sir JOSEPH BANKS, P.R.S., which was read at the meeting of the R.S. on the 20th of April, 1820.]

Another is made, which is adapted for TERRESTRIAL purposes, Magnifying with an Achromatic of 44 inches focus, from 55 to 200 times and with a 30 inch from 40 to 160.

The power it will give to a Telescope, is according to the focal length thereof, and is easily found by the Rule of Three; for instanceif the Telescope be of 18 inches focusIf a Telescope of? 44 Inches gives

55

18

S what will one of 18
Inches?

from 22 times, up to 88-being as low a power as is requisite for Land Objects-and as high as is requisite to shew the Ring of Saturn, the Belts and Satellites of Jupiter and several Double Stars.

If the Pancratic be applied to a Telescope with sliding tubes, it is desirable, on account of the great power it produces, that it should have an Adjusting Screw with a tooth and pinion; these are made separate from the Telescope, and introduced between the first and second sliding tubes.

The Advantage-of having ALL Powers in ONE Eye-tube is sufficiently obvious.

In very Clear Days the Air is so transparent, that we can use a Power of 100 for Land Objects, and on objects well illuminated sometimes 150, as well as in some other days we can a power of 50. Telescopes act best when used in the same direction that the Sun shines.— I have known good Telescopes condemned by trying them upon objects situated towards the East in the morning, or the West in the evening.

To have Perfect Vision every Day, and every Object, must have its appropriate de