of the first reflecting telescope designed to meet some of these defects, and about this time two men, whose work has left indelible marks on the science, were led to study it in a great measure from their interest in astronomy— Christian Huyghens, who lived from 1629 to 1695, and Isaac Newton, 1642 to 1727.

Huyghens was the discoverer of the wave theory and of the law of double refraction, but he was also a skilled mechanic, and he worked himself at grinding his lenses and erecting his telescopes. He realised from a consideration of the theory that many of the most marked defects were due to the fact that the rays from a distant star traversing the various parts of the lens were not brought to a focus at the same point on the axis, and that for a lens of given aperture this axial aberration decreased rapidly as the focal length increased. The magnification of the telescope depends on the ratio of the focal length of the object glass to that of the eye-piece. Hence by keeping this ratio constant, and increasing both focal lengths in the same proportion, the magnification could be maintained and the spherical aberration decreased.

Thus he was led to make lenses of 120 feet focal length. Tubes for such instruments could not be produced, and they were mounted on the top of tall poles and moved from below by ropes. With one of these telescopes, which he afterwards presented to the Royal Society, he discovered Saturn's rings and its fourth satellite. In this case the desire to improve an instrument caused an appeal to theory, and theory led the optician to make a real advance. The advance, it is true, was an inconvenient one, and the defects, as we shall see, were not entirely due to spherical aberration, but the fact remains.

In another branch of instrument making Huyghens is famous for applying science to manufacture. His treatise Horologium Oscillatorium, " which discussed most ably many problems of motion, was long the standard work on clocks, and he was the first to bring into practical use, in 1657, the pendulum as a regulator for time measurements, though according to Sir E. Beckett the first pendulum clock actually made was constructed in 1621 by Harris, of London, for St. Paul's Church, Covent Garden.

In 100g a posthumous work of an Italian Jesuit, Francis Maris Grimaldi, entitled Physico Mathesis de Lumine, Coloribus et Iride aliisque annexis,' was published at Bologna. It contains some notable observations, partoularly the discovery of diffraction.

Newton, who in the previous year had taken his B.A. free at Cambridge, purchased a prism at Stourbridge Far in 1666 to try therewith the celebrated phenomena ef colours," and to repeat some of Grimaldi's experiments. During that year also he had applied himself to the grinding of "Optic Glasses of other figures than spherical.' He was already interested in astronomy, possibly had aladt made, but not confirmed, his great discovery. Writing to Halley in 1686 about some of the controversies which followed the publication of the "Principia," he savy." But for the duplicate proportion I gathered it from Kepler's theorem about twenty years ago." The celebrated apple is supposed to have fallen in his mother's garden at Woolsthorpe, in Lincolnshire, in 1665, where he was driven by the plague, and the story has some authority. It is stated to be the fact by Conduitt, the husband of Newton's favourite niece; it was told by Mrs. Conduitt to Voltaire, and the tree from which it was said to have fallen was seen by Sir David Brewster

in 1820.

Various suggestions have been made, for the reason why the discovery that the same cause which produced the ipple's fall also maintained the moon in her orbit was not published for many years; the true one is probably due to Dr. Glaisher, who pointed out that it was necessary to know the attraction not merely between two particles of matter, but between two spherical bodies of large size, and that this problem was not solved until much later; st, be this as it may, we are sure that in 1667 Newton was an astronomer, and realised the necessity for accurate astronomical observations, and all that the improvement of the telescope meant to astronomers.

Now his experiments with the prism in 1666 led to the discovery of the spectrum; little was known about colours at that time, and Dr. Barrow's " Treatise on Optics,

published with Newton's help in 1669, contains very erroneous views; but some time shortly after that date Newton was able to draw the important conclusion that white light is not homogeneous, but consists of rays some of which are more refrangible than others; the pictures of the spectrum, so familiar to us in numerous text-books, come from Newton's Optics," published first in 1704, though his discoveries as to the analysis of white light were laid before the Royal Society in various papers in 1671, and were given in lectures on optics as Lucasian professor in Cambridge in 1669, 1670, and 1671.

The bearing of all these physical experiments and researches on the practical manufacture of the telescope was at once obvious; the lenses behave like prisms, and decompose the light into its constituent colours. No alteration of shape will remove this entirely, and Newton was driven, too hastily as we know now, to the conclusion that the refracting telescope could not be greatly improved ; its defects were inherent in the refraction of light.

The defect, however, does not exist in images formed by reflection, and he came to the conclusion that optical instruments might be brought to any degree of perfection imaginable provided a reflecting surface could be found which would polish as freely as glass and reflect as much light as glass transmits, and provided a method of communicating to it a parabolic figure could be found. In 1668 he thought of a delicate method of polishing by which he believed "the figure would be corrected to the last, and the Newtonian reflecting telescope was the result. An instrument made with his own hands is now in the possession of the Royal Society, and the many noble instruments which have added so greatly to advance our knowledge of the stars are the direct outcome of Newton's experiments with the prism and the deductions he drew from them.

But these experiments convey another lesson, for Newton, misled by his observations on dispersion, decided, wrongly, as we know now, that achromatic lenses were impossible, and that the colour defects must always exist in reflecting instruments; and as a result attempts to improve these instruments were almost in abeyance for nearly ninety years. Two or three achromatic telescopes were made by Mr. Hall about 1730, but it was not until 1757 that Dollond re-invented this instrument and commenced the regular construction of such lenses.

Thus the discoveries of Huyghens and of Newton reacted powerfully on the instruments of their day. Indeed, in each of these two instances the discoverer and the instrument maker were the same person. Such a combination may be less possible now; still, there are mathematicians skilled in the theory of optics and opticians skilled in the practice of their art.

The Optical Convention aims at coordinating the efforts of the two. But if 200 years ago the progress of the telescope was determined by the advance of optical theory, theory itself was no less indebted to the interest in instruments and observations thus aroused for the progress that took place.

Huyghens was the founder of the wave theory, though the labours of Young and the genius of Fresnel were necessary before Newton's rival theory of emission was displaced.

For nearly 100 years after the date of Newton's Optics" progress was slow. The world was occupied in assimilating what he had taught. English mathematicians, overawed, perhaps, by his transcendent greatness, employed themselves in expounding his teaching. In England, at any rate, the emission theory was supreme, and few, if any, questioned his dicta as to the impossibility of achromatism.

But a change came with the new century. Thomas Young, 1773-1829, was the first in his various papers between 1801 and 1811 again to direct attention to Huyghens's work, and to place on a firmer basis the ground-work of the wave theory. He it was who established clearly the principle of the superposition of waves, and showed how interference may be explained by it.

Young's work, however, would have been incomplete without Fresnel (1788-1827), who re-discovered for himself the principle of interference and extended it to explain diffraction, besides enunciating his theory of double re

of the pencil through the object glass. Abbe soon realised the defects of the ordinary theory. He found it necessary to apply the principles of the wave theory, the teaching of Young and Fresnel, to the problem, and was led in 1870 to the theory of microscopic vision which bears his name. His work was the direct outcome of that of Fresnel.

He soon realised that it followed from the mathematical theory that with the glass then at the optician's disposal no great improvement in the microscope object glass could be expected. Certain relations between the dispersion and refraction in the various lenses were requisite to secure achromatism, and no glass having these relations existed. An inspection of the instruments in our loan exhibition at South Kensington in 1876 confirmed this view, and he published it in a report in 1878 on the results of the exhibition: The future of the Microscope as regards its future improvement in its dioptric qualities seems to be chiefly in the hands of the glass maker."

The investigations of Petzval and of von Seidel led to a similar result with regard to photographic lenses. Von Seidel's work dates back to 1856-7, but his main paper was not written until 1880, after the date of Abbe's report, and was not published in full until 1898.

It follows from these investigations that with the glass then on the market it was impossible to make the field of a photographic lens at once flat and achromatic.

Thus the theoretical work indicated a bar to future progress which could only be removed by the manufacture of new glasses having certain definite properties. It is fitting to say that at an earlier date this fact had been recognised by our countrymen Mr. Vernon Harcourt and Prof. Stokes, who for some eight years previous to 1870 had endeavoured, but with scant success, to make the glass required.

Abbe was more fortunate; his report fell into the hands of Dr. Otto Schott, a glass maker of Witten, in Westphalia, who realised its importance. In 1881 Schott communicated with Abbe, and the next year he removed to Jena, and the firm of Schott and Partners was born.

In the first catalogue of the Jena Glass Works they write: The industrial undertaking here first brought into public notice and which has arisen out of a scientific investigation into the dependence between the optical properties and the chemical composition of solid amorphous fluxes was undertaken by the undersigned (Schott and Abbe) in order to discover the chemicophysical foundations of the behaviour of optical glass." The inquiry was aided by large grants from the Prussian Minister of Education. The practical result is seen in the catalogue of the Jena firm and the enormous export of German optical goods.

Nor is this all, for in virtue of the distribution of profits settled by the scheme of the Carl Zeiss Stifting, drawn up by Abbe some years ago and ratified by the Bavarian Government, the University of Jena alone has received a sum approaching 100,000l. Abbe's work at Jena is perhaps the most striking illustration of the way in which progress depends on the cooperation of science and experience. One could give statistics to illustrate the truth of this and the important effect it has had on German trade and prosperity. They are hardly necessary; the facts are patent, and their cause well known to all who care to inquire. We can progress too if we follow the path laid down for us of old by Newton, Young, Herschel, Airy, and the others of whom I have spoken.

EXHIBITION OF OPTICAL AND SCIENTIFIC INSTRUMENTS.

The exhibition of optical and scientific instruments which is being held during the present week at the Northampton Institute, Clerkenwell, E.C., in connection with the optical convention, presents many features of interest, and all who have had any experience in the use of an optical instrument, from the wearing of a pair of spectacles to the handling of an accurate spectrometer, will find something to repay the trouble of a visit to Clerkenwell, still the centre of the optical industry. While the number of actual novelties offered is not, perhaps, very large, there are few classes of instruments unrepresented, and though the names of certain important firms are conspicuously absent from the list of exhibitors, the exhibition as a whole may be taken as well representative of the

activities of the British manufacturers of optical and other scientific instruments.

In the main of an optical character, the scope of the exhibition has been extended to cover such other scientific instruments as are usually manufactured by optical instrument makers. Meteorological instruments and thermometers, mathematical and drawing instruments and calculating machines, and laboratory apparatus generally, are thus included. Electrical measuring instruments, however, are not shown. It is for many reasons to be regretted that the exhibition has been confined to the work of British makers; a foreign section would have had much interest for the ordinary visitor, and would have been of great educational value both to the British manufacturer and his competitors; we understand, however, that the limitation was dictated by considerations as to space, and the necessity of restricting the magnitude of a somewhat novel undertaking.

In the catalogue which has been prepared in connection with the exhibition, the convention committee is to be congratulated on having produced a volume which should be of considerable value as well to the user of scientific instruments as to the firms whose instruments are there described. The volume is not confined to apparatus actually exhibited; the aim has been to provide a convenient work of reference generally descriptive of the productions of British firms, and in which particulars as to the types offered by different makers of any special instrument may be readily found. To this end the instruments have been arranged in classes, which are in many cases further subdivided, and in addition to a table of contents, an alphabetical list of exhibitors, with general information as to their manufactures, and an index of instruments have been provided. A short introduction to each class furnishes some particulars as to the instruments included thereunder, with notes as to recent advances in the mode of construction.

In class i., tools and materials, the most interesting exhibit is that of Messrs. Chance Bros., which includes some varieties of optical glass only quite recently produced by the firm, and not previously shown. Some special opal glass of low coefficient of expansion for speculum discs is also exhibited. Messrs. Jas. Powell and Sons, of the Whitefriars Glass Works, show specimens of glass for thermometers and other purposes. Tools for lens grinding, and exhibits illustrating processes of manufacture, are shown by Messrs. Geo. Culver and other firms.

Class ii., simple elements and instruments, includes some accurate glass work by Messrs. A. Hilger, while Lord Blythswood shows specimens of his diffraction gratings ruled on speculum metal, 14.400 lines to the inch, up to a length of 6 inches. Replicas of Rowland gratings, with spectroscopes of various forms in which they are employed, are shown by Mr. T. Thorp, of Manchester.

Class ., astronomical instruments, and class iv., nautical instruments, are by no means representative of the best English work, and it is to be regretted that the catalogue is here so meagre.

In class v., surveying instruments, on the other hand, few firms of importance are omitted, and some excellent work is shown. In particular may be mentioned the Wells theodolite of Messrs. Elliott Bros., which embodies several novel features; Messrs. Joseph Casartelli and Son, of Manchester, also show instruments of somewhat special pattern. Messrs. W. F. Stanley, J. J. Hicks, and E. R. Watts and Son are well represented. The chief characteristics of the more modern instruments are the use of larger and more powerful telescopes, and the increased accuracy of graduation.

Class vi. is devoted to range finders and heliographs, and the exhibits of most interest are the naval and field range finders of Messrs. Barr and Stroud, and the stereoscopic range finder of Prof. Geo. Forbes. Messrs. Ross, Ltd., show specimens of their new variable power gun sighting telescopes, in which by a simple device the power can be altered while the image remains always in focus on the cross wires.

Class vii. includes meteorological instruments and thermometers, and most of the well known makers have sent exhibits. In class viii., spectacles and eyeglasses, the exhibits are also sufficiently representative of the best English

work. An historical collection of no little interest is shown by Mr. M. W. Dunscombe, of Bristol.

In class ix., small telescopes and binoculars, are exhibited various patterns of prism binoculars by Messrs. Aitchison, Dallmeyer, Ross, Ltd., &c. Messrs. Aitchison show also a field glass of novel type with a body machined from a solid casting, focusing being effected by moving each object glass in its own tube.

In class x., microscopes and accessories, the catalogue furnishes a very complete account of the English microscope as produced by the best makers, including binocular microscopes and various forms of instrument for special purposes. Photomicrographic cameras are shown by Messrs. Beck, and Ross, Ltd. Information of interest with regard to different types of photographic lenses is given in class xi., though too much space is perhaps devoted in the catalogue to illustrations of camera bodies.

In the careful classification and selection of instruments to illustrate the various types, class xii., optical projection apparatus, appears to us to be the most successful in the catalogue. The class includes an exhibit by Messrs. Chance Bros. of a complete lighthouse optical apparatus of the fourth order. Other exhibits of interest are Mr. R. W. Paul's projector lamps, the triple rotating lantern of Messrs. Newton, and animatographs by Messrs. Paul, the Prestwich Manufacturing Co., and J. Wrench and Son. In class xiii., apparatus for optical measurement, some new optical benches are shown by Messrs. Aitchison and Beck, and there are interesting exhibits from the Cambridge Scientific Instrument Co. and Messrs. Hilger. A half-shadow polarimeter is shown by Prof. Poynting, the half-shadow field being produced by the tilting of two glass plates forming a V between the polariser and analyser.

Under photometric apparatus the Ediswan Co. show specimens of Prof. Fleming's large bulb standard lamps, and various forms of photometer are exhibited by Messrs. Alex. Wright. Class xv. is devoted to ophthalmic apparatus, and includes a novel form of ophthalmoscope of British design and construction. The Cambridge Scientific Instrument Co. and Messrs. Griffin show laboratory apparatus under class xvi. Under class xvii., mathematical and drawing instruments, some new forms of slide rule are shown, including one with additional slides by Messrs. Davis, of Derby, and an optical slide rule with reciprocal division for determination of conjugate foci, &c., by Mr. A. Salomon, of Huddersfield. An arithmometer of English make is exhibited by Mr. S. Tate, and an adding machine by the Burroughs Adding Machine Co.

UNIVERSITY AND EDUCATIONAL
INTELLIGENCE.

CAMBRIDGE.-Among the twelve distinguished men who will receive honorary degrees on June 14 only two are connected with scientific work. These are Commander

R. F. Scott, R.N., of the Discovery, and Colonel Sir Francis E. Younghusband, K.C.I.E. The latter has been appointed Rede lecturer, and has chosen as his subject "Our True Relationship with India." The lecture will be delivered in the Senate at 11.30 a.m. on Saturday, June 10. Mr. L. A. Borradaile, of Selwyn College, who is well known for his researches on the crustacea, has been appointed assistant secretary for lectures to the local examinations and lecture syndicate.

A university lectureship in mathematics will shortly be vacant owing to the resignation of Mr. G. B. Mathews, F.R.S., of St. John's College.

The special board for biology and geology has nominated Mr. J. J. Lister, Fellow of St. John's College, to occupy the university table at the laboratory of the Marine Biological Association at Plymouth for one month during the present year.

In spite of the efforts of the master of Pembroke, Prof. Ridgway and others to bring the work of the studies and examination syndicate to an end, the Senate decided by 112 votes to 99 that its deliberations should be continued. It seems evident that a majority of residents is in favour of some change.

The syndicate entrusted with the building of the new medical schools has exceeded the sum granted by Grace of the Senate by 2571. 15s. 6d. It is now asking for authority to pay this amount, and for 920l. for the completion and fitting of the Humphry Museum, and 38ol. for extra fittings and furniture in the departments of surgery, midwifery, medicine, pharmacology, and pathology,

THE Pioneer Mail states that a grant of 10,000 rupees has been made to the Victoria Diamond Jubilee Technical Institute of Lahore for buildings and appliances. A permanent grant of 100 rupees a month has also been made, and the assistance thus given will enable the governors to complete the equipment for the teaching of practical and applied chemistry.

Ar a meeting of the School Nature-Study Union held at the College of Preceptors on Friday, a paper was read on the training of teachers for nature-study by Miss R. Lulham. In it the necessity for a proper ground work was brought out, and during the discussion which followed a resolution was passed urging upon the London County Council the need of providing classes for those who have to teach nature-study, and suggesting that a wild garden for their benefit should be established in at least one of the London parks, in which the botanic gardens arranged for the students of systematic botany have already proved so useful.

Prof.

We have received the first number of the University Review, which is published by Messrs. Sherratt and Hughes at 6d. net. Dr. Bryce contributes an introductory note on the university movement, and among other articles dealing with many aspects of higher education may be mentioned one by Prof. Arthur Schuster, F.R.S., on Universities and Examinations," and another by Sir Oliver Lodge, F.R.S., on "Questions for Discussion." Schuster formulates briefly what the aims of an ideal university should be, and proceeds to divide its work into two parts. These are the acquisition of knowledge and the power of applying it. The second part of the work of the university is the higher, and is what is required for success in life. Prof. Schuster says that it can be taught, and therefore should be taught, in the university, but that this power of applying knowledge cannot be tested satisfactorily by examination. He then considers exhaustively the function of examinations, and shows what they are capable of doing and the qualities they are incompetent to gauge. He concludes by remarking that when a student has shown that he deserves a degree, it is right and proper that an opportunity shall be given him to develop his special powers and to distinguish himself." Prof. Schuster makes a proposal to secure this by giving a year which is absolutely at the student's disposal to be used under the guidance of his teachers as he thinks fit. Sir Oliver Lodge discusses the possibility of introducing a change in the "time of year when examinations should be held-whether candidates should be examined directly lectures cease, and before Session ends; or whether they should be given time for revision and digestion, and perhaps oblivion, and be exnew Session commences. amined just before a review also supplies full information of current events in British and foreign universities.

SOCIETIES AND ACADEMIES. LONDON.

The

Royal Society, March 30.-"The Determination of the Specific Heat of Superheated Steam by Throttling and other Experiments." By A. H. Peake. Communicated by Prof. Ewing, F.R.S.

This paper is an account of original investigations undertaken to determine the specific heat of superheated steam. Two methods have been followed (1) the throttling or wire-drawing of steam to obtain the law connecting the variation of temperature with pressure, for constant total heat; (2) the direct heating of a current of steam by electrical means.

An account of an investigation on the same lines as method (1), by Mr. J. H. Grindley, was published in the Philosophical Transactions of the Royal Society, A, vol.