This picture of Mr. Porter, which was taken by Dr. Clyde Fisher, president of the Amateur Astronomer's Association (American Museum of Natural History New York), is a much better likeness than the one on page 19 of "Amateur Telescope Making." Mr. Porter is still at Pasadena co-operating on various pieces of work, optical, mechanical, and architectural, preliminary to the final construction of the 200-inch reflecting telescope.

THIS month we shall discuss the merits of the open or skeleton tube reflector in contradistinction to the more common closed or solid-sided tube, and two open tube telescopes of this type are shown here. Of these the one below is a strictly professional job. The mirror was made some years ago by Ritchey, who supervised the construction of the mounting. The figure of the mirror is very perfect, and extremely fine photographs of nebulae have been obtained with it, according to Director E. B. Frost of Yerkes Observatory, mainly by Ritchey, when the latter was connected with this Williams Bay, Wisconsin, institution. A detailed description of this reflector will be found in Volume 14 of the Astrophysical Journal, and a large photograph for those who care to study its structural detail may be had from the Observatory, for 50 cents.

The other skeleton tube telescope is one recently built by Professor Dinsmore Alter of the University of Kansas and Mr. William Pitt, retired manufacturer. As several requests for information have failed to bring forth much satisfaction (the photograph itself having been purchased from a news photograph agency), no details can be stated here, except that it has been known among astronomers for several years that Professor Alter was engaged making the instrument. The 27-inch mirror was made from a 27-inch disk Pyrex four inches thick.

OPEN-TUBE telescopes of the kind just mentioned are strongly advocated the Reverend T. E. R. Phillips, M.A. F.R.A.S., and F. J. Hargreaves, F.R.A.S. in comment reproduced by kind permission from the Journal of the British Astronomical Association (London). The Reverend Phillips is Director of the Section on Jupiter, and Mr. Hargreaves of the section on Photography, of that organization They write:

"There is a widely held view that reflectors are much inferior to refractors defining power, assuming equal optical quality, and there can be little doubt that in most eases this is so, especially when the reflector tube is of metal and closed on all sides. Experience over a long period at Headley with an 8-inch refractor and 12-1/4 inch reflector in an iron tube shows that the number of nights when the image in the reflector is as good as that in the refractor is very small.

"A third instrument at Headley, the Association's 18-inch reflector, formerly the property of the late N. E. Green, has recently been remounted in the dome, formerly occupied by the 12-1/4 inch, and its performance since remounting is worthy of record insofar as it affects the question of the relative merits of the two types of instrument.

"Before the remounting, the 18-in mirror was carried by a square wooden tube, completely closed on all sides, and the image usually had the well-known reflector characteristics-unsteadiness, lack of crispness, and inability to bear high magnification.

"The sides of this wooden tube were made after the manner of a door, with stiles, rails, and panels. The panels having for the most part rotted, they were removed prior to the re-mounting of the instrument and it was decided not to close in at once the large openings left by their removal, in order to observe the effect of allowing the air to circulate freely through the tube. In brief, the effect is to remove completely the characteristic defects of the reflector, so far as planetary images are concerned.

"The performance of this instrument has surpassed all expectations-so much so that in spite of the great disparity of aperture it is now rare to find, on occasions of poor seeing, that the refractor image is steadier or better-defined than that of the reflector."It should be understood that no claim iis made for novelty in the idea of the skeleton tube, which, of course, is used for all the very large reflectors; it is only desired to point out the very great benefits to be obtained by adopting this type of tube for the smaller instruments, for which it has not hitherto been generally used.

"Experiments have shown that when the external temperature is falling, as of course is the general rule at night, it is impossible to keep the temperature of the air inside a "closed" tube uniform and equal to that of the external air. An electric blower sending a current of air up the tube is a help in the smaller sizes, but the trouble begins again as soon as the blower is stopped. In the case of a metal tube, the upper wall becomes colder than the lower wall by radiation, thus setting up convection currents within the tube. If the tube is lagged with felt, or if it is of wood, this trouble is avoided in part, but the tub and the air within it will then remain warmer than the external air during the whole time that the temperature is falling, and for some time after.

"The presence of this mass of relatively warm air can be made visible readily when the conditions are suitable. The telescope should be directed to a bright star and the eye-piece racked out considerably to give a large out-of-focus disk, which will be seen to be irregularly in motion. If an assistant then opens the door near the bottom of the tube and flaps a piece of cardboard vigorously in front of the opening, the out-of-focus disk will "boil, owing to the inrush of the cooler air. The air within the tube can be readily set in rotation by this means, and the rotation is visible in the eyepiece; but no amount o flapping will bring about uniform conditions."

So much for one side of the question In a long article entitled "Reflectors versus Refractors," in the March number Popular Astronomy (Northfield, Minn. )Prof. W. H. Pickering discusses both sides.

STILL another telescope, a 19-innh reflector having an open tube, is described in the January, 1930, number of the Journal of the Royal Astronomical Society of Canada, by its maker, Professor R. K. Young the Department of Astronomy at the University of Toronto. The illustrated article is detailed (16 pages) and ought to be obtained by every amateur who aspires to make a fairly large telescope at some future date. The address is Journal of the R. A. S. C., Mr. H. W. Baker, Treasurer, 198 College Street, Toronto, and the price is 2S cents.

Another current scientific journal which those who are especially interested in the Hale spectrohelioscope should not miss is the December, 1929, number of the Astrophysical Journal (6750 Ellis Avenue, Chicago; 75 cents), containing a 46-page article on that instrument written by Dr. Hale and strikingly illustrated by B. W. Porter. Sine the publication of "Amateur Telescope Making" with its description of the spectrohelioscope, numerous alterations in design have been made.

Professor Charles W. Woodworth of the University of California, is making a mosaic type of telescope containing 400 mirrors, each l5 inches in diameter, and all attached to a single rigid backing. Each is to be adjusted, if possible, to the position called for by theory. Dr. Woodworth submitted his idea to this journal a year or more ago and the opinion given through one of its corresponding editors was that the theory was erred but in practice the difficulties would probably prove insuperable (see "A. T. M.", page 242, near bottom). He is now trying it out, and this is the correct procedure in cases where there is any doubt. Dr. Woodworth is an entomologist who has specialized on microscope theory with which telescope theory has much in common. The amateur fraternity will wish him success.-A. G. 1., Tel. Ed.

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