Accordingly, copies of the SCIENTIFIC AMERICAN containing the description of Messrs. Hanson and Rasmussen's telescopes were mailed to Captain Ainslie, and portions of his reply, written from The High House, Cocking, Midhurst, Sussex, England, are quoted herewith, and the old Herschel binocular, redrawn from Captain Ainslie's sketch, is shown below at the left.

"One reason," the Captain writes, "why this form of binocular never became popular must have been the difficulty of providing for variation in inter-ocular distance; people's eyes vary enormously in this respect. Also, if used for celestial objects, the eye-pieces would come very inconveniently for the eyes, at any rate for objects at any considerable altitude.

"WITH regard to the binocular reflectors made by your readers, perhaps I may offer one or two criticisms: The first is that, as shown in use by Mr. Hanson in your illustration, the observer's head is so close to the mouth of both tubes that the heat radiated from it would be certain to cause currents of unequally heated air just at the point where they can do most injury to the definition. Over on this side we always make our telescope tubes project a considerable distance beyond the observer's head, for this reason. To show how powerful this cause may be in spoiling definition, I may mention that when the new 24-inch Cassegrain which has been recently put up at Mill Hill, near London, is to be used visually, it is found necessary to put a screen of asbestos behind the mirror to cut off the heat of the observer's head, which is found to affect the figure of the mirror after a few minutes' observation if there is no screen. There would, however, be no difficulty in setting the tubes of the binocular farther apart, and carrying them up well beyond the observer's head, if thought desirable. Possibly with a low power, such as 65 diameters, the power mentioned in the account, the definition would not, after all, suffer very seriously from this cause; but with high powers I should expect serious deterioration.

"But, to my mind, a more serious objection to the plan adopted by Mr. Hanson and, apparently, by Mr. M. Rasmussen is that there are an odd number of reflections before the image is formed in the focus of the eyepiece. The result of this is that the image is reversed about a horizontal axis, but not about a vertical one: right remains right, but top becomes bottom. This appears to me to be a serious drawback, if the instrument is to be used for planetary or lunar observation.

"There is another point which has to be remembered by the designer of any binocuIar telescope; that is, that one must be quite certain that the stereoscopic effect is preserved, and not reversed so as to become pseudoscopic. An example of what I mean is to be found in some binoculars which consist of two ordinary astronomical telescopes side by side: I have a pair of this sort, and the reversal of the relief is ludicrous on some objects. I have, for example, seen Venus apparently hanging in front of the distant landscape like a suspended lamp. The general rule is that, if the image is completely reversed (i.e., both horizontally and vertically) then the left-hand telescope must supply the right eye, and vice versa; and the same applies if there is reversal about a vertical axis but not about a horizontal. Thus Mr. Hanson's telescope, if furnished with erecting eyepieces, would be pseudoscopic; right and left are, in each image, in their proper positions, but the right eye receives the image formed by the left-hand telescope (looking toward the object) and vice versa. To be truly stereoscopic, the images should r be further reversed right and left. If completely re-erected, the result will be still pseudoscopic, for the reason just quoted.

"Another very noticeable thing about the American instruments is that most of them have metal tubes. Most of our telescope constructors have long abandoned the metal tube for wood, the general experience being that the performance of a reflector in a wooden tube is distinctly better than one in a metal tube. I had at one time a 9-inch in a square wooden tube, side by side with an 8-1/2 inch in an iron tube, the optical quality being the same in both instruments but there was no mistaking the superiority of the 9-inch. The wooden tube, too, was a great protection against dewing of the flat; this took place incessantly in the iron tube, but never in the wooden. It was not uncommon, on a rise of temperature, for the speculum to become dewed in the iron tube: this never happened in the wooden.

"THE advantages of using no tube at all, but merely a frame, have long been recognized over here. Such old-time workers as Lassell, De La Rue, Lord Rosse (in the case of his 36-inch), and others, showed long ago that the freedom from tube currents thus obtained was most striking although one gets nearly the same effect with a square wooden tube, owing to the space at the corners which is clear of the entering light.

"A very striking instance of the advantages of doing away with the tube has recently been afforded here in the case of an 18-inch reflector belonging to the 'B. A. A.' and now at the observatory of the Rev. T. E. R. Phillips, whose name will be familiar to you as that of a very well-known planetary observer, and a leading authority on Jupiter. He has had for many years an 8-inch Cooke refractor and a 12-1/4 inch Calver reflector, both under domes and in iron tubes. He found that he hardly ever used the reflector on Jupiter or Mars, because the 8-inch refractor invariably gave a sharper and far steadier image; and when he first mounted the 18-inch reflector in its wooden tube out of doors, although its performance was pretty good the 8-inch was still better.

"However, he removed the 12-1/4 inch reflector from its mounting, and mounted the 18-inch in its place, cutting away the sides of the tube so as to leave virtually a mere framework. The effect, to use his own expression, was absolutely 'startling'. The 18-inch at once surpassed the 8-inch, and is now his regular working instrument under all conditions.

"Over here we suffer, in most places, from dewing of object glasses and mirrors to an extent from which you may be happily free, at least I hope so, for your sake!

"It is, as I remarked above, a matter for sincere congratulation that your amateur telescope makers show such energy. Is their output of practical work in the form of recorded observations correspondingly large?" [No.-Ed.] "I ask this, because so often a man who has made a mirror goes on to make another and then another and becomes an amateur optician rather than an amateur astronomer. We have had many cases of this over our side.

"Again congratulating your amateur workers on their results, and wishing the cause of amateur astronomy the greatest success over your side, I remain with many thanks, Yours very truly, Maurice A. Ainslie, Instructor-Captain, R. N., Past-President of the 'British Astronomical Association."'

PREVIOUS to its present publication Captain Ainslie's sketch of Herschel's binocular was shown to an amateur who was known to be interested in unusual mountings, Mr. James C. Critchett, a retired mining-engineer who lives on the edge of the Imperial Desert in California (address Banner, via Julian) and this is what he writes:

"Good for Capt. Ainslie! But my Yankee blood, originating in New England, says that we must whip the English again, so I send on my single-barreled repeater." [See right-hand sketch-Ed.] "Fine-grind mirror, embed strongly in some kind of cement, let cement dry thoroughly, then saw mirror in halves and varnish the cement which holds the halves together, to keep out moisture. Polish and figure and remove cement. May have to follow the sawing wire down with cement which sets and hardens before finishing cut. The difficulty of making this mirror makes me believe that it would be better to put it up to some one with better equipment than I have, here in the 'sticks.' Years ago I hit upon a plaster which might do: mix plaster of Paris in a solution of copper sulphate and water. It gets very hard and strong, but must be handled quickly."

Now then, who will elect himself the "goat" to try out this interesting suggestion; and who will recreate the old Herschel binocular described by Captain Ainslie? And will there be some more wooden telescope tubes?

SPEAKING of recreating old telescopes, here is a suggestion which someone may be interested in carrying out: recreate the original telescope of Galileo, also Newton's first reflector. Fairly good data for both are available; a real, honest-to-goodness working model to look through, not merely at, is suggested.

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