June i, 1905] 



NJLTUHE. 



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of the first reflecting telescope "designed to rrieeJ some of 

 these defects, and about this time t.wo men, whose worlc 

 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, ^^'^ 

 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 consider- 

 ation 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 dis- 

 covered 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, Covcnt Garden. 



In 1665 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, par- 

 ticularly the discovery of diffraction. 



Newton, who in the previous year had taken his B.A. 

 degree at Cambridge, purchased a prism at Stourbridge 

 Fair in 1666 " to try therewith the celebrated phenomena 

 of 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 

 already 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 

 says : — " 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 

 apple'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 ; 

 but, 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," 



NO 1857, VOL. 72] 



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 re- 

 searches 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 alter- 

 ation 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 in- 

 struments 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 com- 

 municating to it a parabolic figure could be found. In 

 1668 he thought of a delicate method of pohshing by 

 which he believed " the figure would bo 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, wrong y, 

 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. _ , cr . 



The Optical Convention aims at coordinating the etiorts 

 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 instru- 

 ments 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. r xi ^ > 



For nearly 100 years after the dale of ^ewtons 

 " Optics " progress was slow. The world was occupied 

 in assimilating what he had taught. English mathe- 

 maticians, overawed, perhaps, by his transcendent great- 

 ness 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 impossi- 

 bility of achromatism. 



But a change came with the new century. Ihomas 

 Young 1773-1829, was the first in his various papers 

 between 1801 and 181 1 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 estab- 

 lished 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 hini- 

 self the principle of interference and extended it to explain 

 diffraction, besides enunciating his theory of double re- 



