In a recent number we published a letter from Ellison, co-author of "Amateur Telescope Making," in which the following statement was made: "The real difficulty about 16- and 21-inch jobs is in the mounting and using. I know from my experience with an 18 inch that observing with a 21 inch is no picnic."

The Reverend Mr. Ellison is of course, referring to his own seeing conditions in the climate of Northern Ireland, and these conditions will differ from those in some parts of America, particularly in the west. Nevertheless, some of the considerations having to do with seeing and its principles, touched on below, are likely to prove of value. They may explain why it is all but futile in certain parts of our country to attempt to "manufacture" better seeing than can be had, simply by using large mirrors and crowding on higher magnification; also why it is that experienced observers keep pointing out that the practicability of larger sizes than ten inch, except for photographic work, is illusory in many eases.

Discussion by Mr. A. A. C. Elliot Merlin, under the heading "Fifty Years at the Telescope," in English and Amateur Mechanics (London) May 24 and June 7,1929, quoted by kind permission of the publisher, bears directly on this point.

"A small instrument or five- or six-inch aperture is far more likely to reveal quickly its latent good qualities than one of eight or nine inches, for the reason that our earth's atmosphere is more frequently in a sufficiently tranquil state to allow the smaller apertures to attain their full defining limits on what must be classed as nights of exceptionally good seeing in this country. This is the secret of the popularity that the five-inch or six-inch refractor has long enjoyed. Telescopes of that size are large enough to afford sufficient light grasp when used with magnifications of 200 to 300 diameters, or even more, and at the same time encounter no excessive air hindrance, thus enabling them frequently to furnish sharp stellar images, whose spurious disks of 0.91" for a five inch and 0.76" for a six inch, will appear cleanly and steadily imaged on any tolerably favorable night; one which would quickly reveal its shortcomings with an instrument of perhaps only slightly larger aperture.

"The lack of appreciation of the most telling fact that air hindrance must necessarily increase as the square of the aperture, or ignorance of it, has led to misapprehension, chiefly directed against reflectors, for the reason that they have been, as a rule, of much larger aperture than the refractors with which their performance has been compared. The swamping of the defining. quality of, say, an 18-inch aperture by what, to it, is a tempestuous atmospheric sea, is conspicuously observable; while an eight-inch aperture, reflector or refractor, placed alongside the big telescope, may be found to do sufficiently well in what is, to it, a moderate atmospheric disturbance.

"The obvious cause," Mr. Merlin continues, "is that the 18-inch instrument has an area of 254.5 square inches on the entire surface on which parallel light rays impinge, each of which has encountered equal air disturbance in its passage to the telescope, while the little eight inch has only 50.3 square inches of area, so can only collect on its surface five times fewer air-disturbed light rays; hence the atmospheric handicap at any one time is five times greater for the 18-inch telescope which, if used through no air at all, suppose on the airless moon, could only afford a defining power of a little over twice that of its small competitor, the eight-inch instrument."

In the last statement Mr. Merlin is referring to the fact that the resolving or refining power increases in direct proportion, not to the mirror's area, but only to its diameter. If we divide 4.56 seconds of arc by the diameter or aperture of our mirror or objective lens we get what is called the "Dawes Limit." (See Bell, The Telescope," Chapter XI). For example, consider a six-inch mirror. Performing this simple feat of arithmetic we ascertain that this size ought theoretically to resolve or separate two stars not closer than 0.76" apart-although closer pairs can sometimes be seen elongated, their diffraction disks overlapping.

Separating a close double is essentially and optically the same thing as defining minute detail on, say, the moon. Resolving power, and therefore aperture, is here the decisive factor. But as we shall see later there is a fly in the ointment: much of what we gain thus we lose in lack of contrast.

Professor A. E. Douglass, now Director of the Observatory at the University of Arizona but then on the staff of Lowell Observatory, published in Popular Astronomy, June, 1897, a long article entitled "Atmosphere, Telescope, and Observer," laying down certain basic principles which are equally valid in 1929 as in 1897, or any time. We shall quote salient parts of Prof. Douglass' article in a later number.

The Journal of the Royal Astronomical Society of Canada for October contains an interesting list of the reflecting and refracting telescopes of the world, compiled by W. E. Harper. In the list of reflectors we find two instruments made from the instructions in the book "Amateur Telescope Making," one being that of G. H. Hamilton, of Jamaica, B. W. I., a 21 inch described in our issue of last May, and the other that made by Steber and Thurn, of Warren, Pa., described last June.

These two lists, as complete as the compiler could make them, show the world's telescopes above 15 inch distributed among sizes as follows: 100 to 50 inches, eight reflectors; 50 to 40 inches, five reflectors, two refractors; 40 to 30 inches, 24 reflectors six refractors; 30 to 20 inches, 20 reflectors 29 refractors, 20 to 15 inches, 15 reflectors, 56 refractors. No telescopes under 15 inches are listed-which lets out most of us who "roll our own." If what Mr. Merlin writes is correct, we amateurs are a wiser lot than we supposed.-A. G. I., Tel. Ed.

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