Object Description
For sale, a very rare early nineteenth Century French two draw “polyaldes” telescope by Robert-Aglae Cauchoix of Paris.
Comprised of a walnut barrel, the telescope extends to 66cms (or 72cms with the sunshade extended) and 32cms closed. The objective lens measures one inch in diameter with the lens cap in place or one and three quarter inches without.
The second draw is inscribed with the maker’s name “Cauchoix a Paris” and the first draw is engraved with a scale from 15 to 30.
Cauchoix developed this telescope in 1815 and it bears many similarities to David Brewster’s invention produced by William Harris in 1811 (see my website) although there is nothing to suggest that Cauchoix’s example involved any plagiarism.
Cauchoix released a pamphlet at the time to provide an introduction to his invention which is included in translation below. The original document is also included in the sale.
“All who have used telescopes have had occasion to notice that, in foggy times, these instruments give too dark images, and that, in calm times, do not magnify the objects as much as one would wish. They are also criticized for having little field, that is to say, for embracing too small a number of objects at the same glance. I have just imagined a construction which destroys these faults as much as possible, in that it allows the degree of magnification to be varied at will.
Hitherto, in order to produce this effect in telescopes, no other means have been known but to adapt several systems of eyepieces embedded in separate tubes which have to be substituted for each other, but this process which I myself have employed is defective for several reasons, such as the inconvenience of carrying with you spare parts, unscrewing and screwing in sometimes very awkward positions, finally the difficulty and sometimes even the absolute impossibility to choose among the various eyepieces which one possesses, the one which would be best suited to the circumstance in which one observes.
People who walk or who travel with glasses which they use frequently, officers of land and sea, and especially the latter, know what these changes in magnification are valuable in their object and inconvenient in their jobs. The means that I propose seems to me to combine all the advantages desirable for the goodness of effect and the convenience of use: but before I say what it consists of, allow me to give some details about the effect of lenses in general. My aim is to prevent and destroy some objections that could be made against this invention to mitigate the advantages it can offer by accusing it of not giving those which it is really impossible to collect.
The effect of all telescopes or telescopes is to direct the rays from distant objects, so as to form images of these objects at a certain point on the axis of the lenses which we call their focus; and as it would be impossible for the images to be clear if they were not limited, they are made to pass through a circular opening made in a copper plate which is placed in the focus itself and which is called the diaphragm; all that remains is to look at these images with a magnifying glass to see them clearly and magnify them further. It is the object of a last glass or of a system of glasses which is placed near the eye, and which is called by this reason ocular.
The magnification of a telescope therefore depends on two elements, namely, the absolute size of the images included in the diaphragm, and the force of the magnifying glass or the eyepiece with which these images are viewed. The magnifying glass becomes stronger, grows more, as one shortens its focus; this shortening would therefore be a means of increasing the total magnification of the telescope, but at the same time it would be necessary to reduce the aperture of the diaphragm, because it is advisable, for sharpness, that the diameter of this aperture be near the string of a thirty degree arc of which the middle of the magnifying glass would be the center. It does not seem easy to achieve this double goal by a successive movement and without changing the eyepiece. One succeeds better in making the images which are considered with the same magnifying glass grow in the diaphragm, and consequently by keeping the same diaphragm opening. In that case, the magnification of the instrument varies as the apparent size of the images. But at the same time we see that the field, that is to say, the number of objects seen at the same glance, must decrease as the magnification increases, since the same parts occupy more than surface in the diaphragm, the opening of which is constant. We would not be better able to preserve the extent of the field if, to increase the magnification, we made the magnifying glass stronger while leaving the images of the same size in the diaphragm, since it would then be necessary to reduce its aperture. We must therefore conclude that it is impossible not to reduce the field of a telescope when increasing its magnification.
There are two ways to operate the magnification of images in the diaphragm. We can make the objective of the telescope of two glasses or of two systems of glasses which vary distance between them: this construction, which also varies greatly the total length of the telescope, presents, in addition to this first disadvantage, that of placing in the total system one or two more glasses which reduce the clarity as much. The other way, the one I adopted, consists in varying the distance between the four glasses that make up the eyepiece system. This process does not increase the number of glasses required for instruments of this kind; it does not in any way detract from their goodness since each of these glasses could have been fixed in the different positional relations where they are placed successively, reports therefore some have even been frequently used by some practitioners; but no one had thought of bringing these varieties together in the same frame by the mobility of the lenses. It is in this and in the choice of the suitable dimensions of the glasses that my new construction consists, which gives my glasses the advantage of being polyaldes, that is to say, of bringing together various and variable magnifications at will. The elements which constitute the more or less good effect of the glasses, are: 1) the number of objects which they make available at a single glance, the field; 2) the apparent size under which they show these objects is their magnification; 3) the quantity of light they appear to be illuminated is their clarity. I have spoken of the field and the magnification, and we have seen that one was necessarily in inverse ratio to the other; I will show that the clarity is also sometimes attenuated by the increase in magnification; but it can still be modified by several parts of the instrument.
The clarity of a telescope depends on the number of glasses which compose it, their thickness, the purity and the piaphaneity of their material, the perfection of the work of their surfaces, the diameter of the objective glasses, the right proportion of a few parts of the frame; and when all these elements unite all the perfection of which they are susceptible, the magnification increases beyond certain limits, decreases the clarity of the images. When we put a telescope in the point of the focus on something a little distant, and we move it away from the eye of a few inches, we perceive, in the middle of the opening through which we were looking, a small bright circular space. The diameter of the objective divided by that of this cylinder of light is the measure of the magnification of the telescope. As long as this cylinder is larger than the aperture of the pupil, the apparent clarity of the images is about the same as if they were seen through flat glasses of the same thickness and in the same number as those whose telescope is composed. But when the magnification is increased and the circle of light becomes narrower than the pupil, the objects appear darker as the diameter of the light cylinder is smaller, compared to that of the pupil. There is no way to remedy this inconvenience.
It would seem at first glance that it is seldom interesting to have a telescope which magnifies little, and that it would be better to dispose it immediately for the greatest magnification it can withstand; but if we remember that by sacrificing part of the magnification we increase the field of view and the clarity, we will see that this clarity can be precious in dark and hazy times, and that the range of the field can also become useful when it is only a question of perceiving all that exists in a space that one has an interest in recognizing. We can then and immediately examine the details better by increasing the magnification, if the weather is favorable. At nightfall, when you can barely see, the wide field and the large light spread through the telescope reduced to low magnification make it very easy to see and distinguish very poorly lit objects.
Deprived of the means to make these variations easily, the opticians chooses a median between the weakest and the strongest magnifications: this is why their instruments are too strong and too dark in foggy weather and too weak in calm weather. Also, they often oblige the owners to use multiple eyepieces in order to better adapt to the various states of the atmosphere. We thus have two, three or four eyepiece tubes for the same telescope, and as many different effects.
The new procedure which I have adopted has the advantage of offering all the intermediaries between the smallest and the strongest magnifications, one of which can double the other; that is to say that a polyalde telescope can magnify from 20 to 40 times, from 30 to 60, etc. This inexpensive method can be applied to all existing telescopes, even the most portable, provided they are good. It is even remarkable that its application to a telescope becomes a guarantee of its quality, for bad lenses do not support it. No lenses are added; all the effect is produced by the change of position of one or two lenses among the four which usually form the eyepiece system of a terrestrial or marine telescope. A movable tube inside, which one moves easily with the fingers, operates in an instant the change which one wishes. It takes less time for this operation than to take the point of focus: there is no part to remove or replace, everything is inside the telescope. If we want a telescope thus arranged to give two or three, or a greater number of known focal magnifications, no other care is needed in order to achieve this goal than that of making a few lines placed on the movable draw tube coincide with a fixed line, marked on the tube within which it sits. However, all these movements must be done starting from the point where the inner tube is positioned for the least magnification, that is to say when it is brought towards the eye until it stops. Thus, to go from a lower magnification to a higher one, it is necessary to push the draw tube towards the object: one can, by repeatedly pushing this tube, successively increase the force of the telescope; but, to return from a stronger magnification to a lower one, it is first necessary to bring the same tube back to the nearest point to the eye, and then to push it to the position it must occupy to give the desired magnification. It is also necessary for each magnification to take the point of focus; it is with this maneouver, that all the skill necessary to enjoy this process is reduced, the results of which can be so multiplied.
This invention makes it possible to use a telescope to a degree that we never dared with fixed magnifications as the eyepiece apparatus would have been more cumbersome than useful, and the expense quite unnecessary. Here, the weak and medium magnifications are readily at the disposal of the observer and the strongest which is used very rarely, is however also favourably achieved without expense. Thus, in a telescope twenty inches long and sixteen lines (3.6cms) in diameter, one ordinarily employs a magnification of 18 to 20 times, and one does not go beyond 25. A polyalde telescope of this size can offer magnifications from 15 to 30 times. This equals the force which is ordinarily given to telescopes three feet long and twenty-five lines (5.6cms) in diameter. The difference in these diameters makes the sixteen-line (3.6cms) telescope darker, it is agreed that in order to be able to use an equal magnification in the telescope of smaller diameter, the object which is observed must be very bright.
A final advantage could still be added to the telescope with respect to the ease of focussing. In ordinary practice, there is no other way than to slide the tubes into each other. This process works well as long as they keep their polish; but as soon as they begin to change, they no longer slide except by jerks; it becomes quite difficult to take the point with precision.
I succeeded in removing this inconvenience; one of the bezel tubes is shaped on the outside from fine, triple-threaded screws. The rotation gives an easy and smooth way to take the point with the greatest precision. We first adjust roughly by pulling the draws as usual, well by turning the one that contains the eyepieces, we apply the same movement to the screw pipe. The lengthening or shortening of the frame is thus slowly operated, which is necessary to arrive at the point of focus. This very simple mechanism gives telescopes, without increasing their size, the advantage of movements which had heretofore been reserved for instruments on stands.
Several persons, who enjoy great esteem in the sciences and in the navy, have approved of these improvements, which they have regarded as important; the votes of the members of the advisory board, and the gratuity which His Excellency the Minister of the Interior has kindly honored me for this invention, embolden me to present it to the public with more confidence.”
Robert-Aglaé Cauchoix was born in 1776 in Cormeilles-en-Parisis just outside of central Paris. He studied at the colleges of Navarre and Lisieux and became an optician in 1792 establishing himself in 1803 at the Collège des Grassins, 27 quai Voltaire.
In 1804, the British scientist William Hyde Wollaston published his research on periscopic spectacle lenses (meniscus lenses) which provided an alternative to the standard double convex lens considered the most superior by London opticians. His work was intended (as per John Dollond before him) to find a solution to remove spherical aberration and was fiercely refuted by William Jones (of W&S Jones fame) but in 1813, the Frenchman Jean Baptiste Biot employed Cauchoix to replicate Wollaston’s work for evaluation. The outcome of Cauchoix’s work led to a paper being presented to the Academie on the subject.
In the same year, Cauchoix also obtained patents for the development of a telescope with variable magnification (lunettes polyaldes). Included above.
In 1819 he received a silver medal from the Society for the Encouragement of National Industry and a gold medal in 1824 for his work with Noel Jean Lerebours by which time he had built a 0.30 m aperture telescope which he sold in 1829 to the English amateur astronomer James South. In 1831 he sold a 0.33 m telescope to Edward Cooper of the Armagh Observatory in Ireland, the largest objective lenses available at the time.
His lenses earned him the Legion of Honor in 1834 after an exhibition of his work but he retired shortly after to Deuil where he became of mayor of the town from 1835 to 1844, ceding his business to his nephew, Rossin. He died on the 5th of February 1845.
Cauchoix was the first manufacturer to use flint glass produced in France, he also invented numerous development for telescope stands. He is recognised as one of the foremost instrument makers of the period, his work being used by the Paris Observatory to observe the rings of Saturn.
A rare and historic French telescope circa 1815.