PERHAPS the most successful of all early single microscopes as

Transcription

1 208 XV.-THE WILSON MICROSCOPES. By R. S. CLAY, B.A., D.Sc., F.R.M.S., and THOMAS H. COURT. (Read li ebruaq 24, 1926.) ONE TEXT-FIGURE. PERHAPS the most successful of all early single microscopes as regards convenience, performance, and the extent to which it was made and used, was the screw-barrel microscope associated with the name of Wilson. Though certainly not the inventor of this form of microscope, Wilson so improved its optical performance, that its popularity and commercial success were undoubtedly due to his work. The originator of the screw-barrel microscope was the Italian Tortona, who described his microscope at a meeting of the Physics Mathematical Roman Academy held in the house of Lord Campini on Bug. 5th, The tube was 2 inches long and 2 scruples radius. It was composed of five parts, each joined by a screw to the succeeding part, and it could be divided into a microscope and an eye-glass. Tortona refused at the time to show its internal construction, but said that it contained five lenses, of which one was a non-spherical one. It was used to show the circulation of the blood. An account appears in Acta Eruditorum, 1685, p. 478 ; also in Campini Nuove Tnventioni di tubi ottici, 1686 ; and yet another in ( Miscellanea Curiosa, 1689, Norimberg, p. 444, where it is illustrated (fig. 39). It is there said that it was 4 inches long. In 1689 Joseph Campani and Marius Antonius Celi exhibited similar microscopes to the same Academy (fig. 41). In all these the focussing was effected by screwing the body of the microscope in and out of a ring, usually brass, to which the stage was fastened by three rods or similar connecting links. Also the instrument was intended to be used in the hand and held up to the sky, so that the light might pass through the (transparent) object. The body of the microscope was stated to be of wood. In all of them one or more illuminating lenses could be inserted in a wooden cell below the stage. In one case these illuminating lenses are shown as a pair of plano-convex lenses with the curved surfaces turned towards each other and practically touching one another. Nothing is said of their magnifying power. All these microscopes were cowpound ones, but no optical details are given, though a diagram (admittedly guesswork) in the ( Miscellanea Curiosa shows an objective, a field lens wd an eye-glass, all double convex. Two

2 The Wilso,b Microscopes. 209 microscopes of this type aiid of about this date were included in the Crisp sale. One, ellgraved ( Campani, was bought by the South Kensington Science Museum, and Mr. Court secured the other, which though not engraved is most probably by the same maker and may be the earlier of the two. The lenses made by Campani were well ground, and he consequently acquired great reputation as a telescope and microscope maker, But about this time a great many were using fused beads as objectives. The fused bead was first made by Torricelli of Florence. Tortma s microscope was also copied very closely by Bonanni (I1 Micrographia Curiosa, 1691, p. 26). Bonanni, however, made an important addition in the pair of brass plates between which the slider of objects was to be put ; one of these was fixed and the other pressed down towards it by a helical wire spring. All these microscopes of Tortona, Campani and Bonanni were as above stated compound microscopes. It was Nicolas Hwtsoeker who first applied this design to the single microscope. His microscope is shown in his ( Essay de Dioptrique, 1694, p. 1 75, where all the essential features of the screw-barrel microscope are included. Indeed in his microscope the condenser lens was carried in an additional focussing screw barrel, whereas in the later instruments this lens was held permanently at the end of the microscope barrel. Wilson has often been charged with plagiarizing Hartsoeker s microscope, and even Mayall in his Cantor Lectures, 1885, p. 32, refers to Hartsoeker s anticipation of Wilson s instrument in every essential point of design. A careful reading of the Preface to Wilson s original account (vide infra) of his microscopes shows however that he makes no claim to originality. There are fortunately at least two Wilson microscopes in existence with his signature, and we can, therefore, still see for ourselves the position which the single microscope had attained both optically and mechanically at the beginning of the 18th Century. James Wilson was in business ( At the Willow Tree in Cross Street, Hatton Garden. He makes all sorts of Dioptric and Catopric Glasses and Tellescopes (sic), Prospects, Camera.Obscuras, Magic Lanterns and Selleth the best of Spectacles and Reading Glasses,.etc. His sign is of interest as other opticians usually adopted a sign which bore some reference to optics, such as ( Golden Spectacles, Golden Prospect Glasses, Globes and Quadrants, Astrolobe, ( Newton s Head, etc. The date of Wilson s death is not known, but from the rarity of his microscopes and telescope8 he probably died very early in the 18th Century. Wilson has given three accounts of his microscopes which

3 210 Tyansactions of the Society. show progressive improvements. The first was in the Phil. Trans. for 1702, No. 281, under the title ( The Description and Manners of Using a Late Invented Set of Pocket Microscopes made by James Wilson, and the Preface to his pamphlet runs: (( The late Improvements made by Magnifying Glasses are not so much owing to the making them and composirig Microscopes, as the Methods of applying objects for the advantage of Light; in which I hope the Instruments before us will not be found inferior to any yet made, at least commonly Sold.... In this account he shows two independent instruments, a compass microscope and a screw-barrel one. The powers only are common LO the two. The screw barrel was of ivory. The two hrass plates, between which the slider containing the objects was placed, were held between a cork pad and a brass wire spring. There was an illuminating lens in the end cell. The objectives all screwed into the other end. The compass microscope was entirely of brass, and provided with a spike. The objective, which had a hole in the side, was attached to this spike and held in positioii by a spring. It is worthy of note that this method of attaching the magnifying object lens to a compass one by fixing it to a spike was the usual method on the continent of Europe, and perhaps in England, e.g. at this time the microscopes (single compass) of Musschenbroek and Cuno both had this form of attachment. There were eight objectives common to the two instruments, and a ninth of lower power for the compass microscope only. There is a letter from one Sir C. H. to the publishers in the ( Phil. Trans., Aug. 11, 1703, No. 284, in which the writer says he has compaxed Mellen s (cima 1680) microscope and considers Wilson s microscope to go far beyond any of Mellen s, although not 90 neatly set. The greater magnifier especially augments more and it shows an object more distinctly than his. He further states that he intends to fix Wilson s microscope to an arm to slide up and down on the square rod of his deep microscope (no doubt a Marshall) to be set by a screw to any settled position which will be necessary when (( an illuminating glass is added - probably referring to a mirror.* The second account was published by Wilson in a six-page pamphlet dated 1 706, with a plate showing the instruments. In this the words late invented are omitted, otherwise the wording is the same as in the first pamphlet. In this second form he had embodied several improvements. 1. He supplied a turned brass handle which could either be screwed to the outer framework of the screw-barrel microscope, or * (In the Court collection there is an early brass Wilson microscope without handle which has a screw thread at the end where the condensing lens is-no doubt to fit it to some such support).

4 The Wilson Microscopes. 211 be attached to the compass microscope, and the latter was much better made. 2. The spiral spring was of steel wire in place of the brass wire of the original. 3. The compass microscope had a ring into which the powers screwed (in place of the spike), and consequently the powers had no small hole provided for attachment to the spike. 4. The pad between the end of the screw barrel and the stage plates was of leather instead of cork, no doubt because of the bad Example of the third type, from the George 111. Collection, sold at the Crisp Sale: in the possession of Mr. Court. It is signed I. W. wearing quality of cork. A fine and complete example of this form is contained in the Orrery Collection at Oxford. This pamphlet is practically reproduced in Harris, (( Lexicon Technicum, vol. 11, 1710, including the plate. Smith, in his ( Optics, 1738, Vol. 11, p. 398, plate 58, fig. 64 7, figures and describes Wilson s microscope. He states that his account is taken from the Phil. Trans., but this is not so; he has probably copied Harris, whose account is from the pamphlet, and this instrument is not exactly the Same as described in 1702 to the Royal Society. Smith s description with its errors has been copied in nearly all Encylopaedias and Science Dictionaries right down to the middle of last century-an instance of the way

5 212 Transactions of the. Xociety. an initial error is perpetuated by those who rely on other writers instead of referring to original sources. The third account is contained in a pamphlet, of which Mr. Court also has a copy, on which is written ( Richard Oliver 1711, also a list of ( objects to be placed between the muscovy glasses to be seen by , and other writing by the same hand, thus proving that it must have been issued betweon 1706 and In printing this second pamphlet Wilson has used the original letterpress (which is thus still dated 1706) and the original copper-plate; but he has rubbed out one of the illustrations on the plate and substituted the picture of the opaque holder. He has also added a fly-leaf of reading matter explaining the new improvement. This fly-leaf is missing from the copy in the British Museum, and in its absence the account does not tally with the figure. The handle is of ivory and has a male screw as contrasted with the female screw of the second type. Wilson also provided a circular plate which screws into the end of the case of the screw barrel in place of the objective, and which carries a ring into which the objective itself screws. The stem of the spring forceps is passed through a hole in the leather, and both transparent and opaque objects can be held in them. Thus the compass microscope was uo longer necessary, and could be omitted. We have thus shown how Wilson introduced this popular form into England in 1702, improved it in 1706, and still again between 1706 and 1711, and left it in all essential details in the form in which it remained for at least 100 years. The hole in the leather for the stage forceps was not satisfactory, and led to the introduction of the forceps plate, probably by Culpeper about 1720 ; at a later date the hole for the forceps was restored, but now in a brass plate at its junction with the screw barrel, thus reverting to Wilson s original form. It was made in this form by Cuff, Mann and Ayscough, Adams, Nairne and others ; Culpeper and Scarlet, Cuff and other makers added a compound body which screwed to the case at the objective end. Each of the two first-named also added a stand, Culpeper s having a flat folding foot, and Scarlet s a tripod like that of a telescope. Another modification was the substitution of a rack-and-pinion to move the barrel in its case in place of the long screw on its sudace.