114 



iVA TURE 



[June i, 1905 



fraction and deducing the well known expressions for the 

 intensity of the light reflected from or transmitted by a 

 transparent surface. 



Young, in his " Lectures on Natural Philosophy," illus- 

 trated in an admirable way the applications of optical theory 

 to instruments. Fresnel was an engineer by profession 

 attached to the service of the bridges and roads, and as 

 such was the inventor of the arrangements of lenses em- 

 ployed in the French lighthouses. 



The discoveries of these two men changed the whole of 

 the theory on which the construction of optical instruments 

 is based ; it is idle to attempt to explain the action of a 

 microscope, the resolution of a double star or of the fine 

 lines of the spectrum, to discuss the conditions for such 

 resolution, or, instead, to attempt the construction of 

 any of the more delicate of the beautiful apparatus about 

 us without clearly understanding the fu-idamental laws dis- 

 covered by these two, and verified with marvellous skill 

 by Fresnel in his country home in Xormandy, not by the 

 aid of modern apparatus, but by such means as his own 

 hands, aided by the skill of the village blacksmith, could 

 construct; and though it is true that only recently have 

 we appreciated the full importance of the wave theory in 

 its bearing on the construction of optical instruments, it 

 is the fact that without their labours and the work of 

 those who followed in their path few of the' modern dis- 

 coveries of the astronomer, few of the results which the 

 skilled optician of to-day has arrived at, would have been 

 possible. The object glass of a microscope, the lens of a 

 camera or a telescope, have reached their present perfec- 

 tion because men have been found who could apply to the 

 art of lens grinding the highest teaching of Young and of 

 Fresnel. 



In the earlier years of the last century Englishmen were 

 well to the fore in this work. In astronomy the labours 

 of the two Herschels are well known, and though, perhaps, 

 the success of the elder Herschel was due rather to his 

 mechanical skill than to a profound knowledge of optical 

 theory, Sir John Herschel advanced in no small measure 

 the application of theory to practice. 



At a somewhat earlier date Fraunhofer, of Munich 

 (1787-1826), a contemporary of Young and of Fresnel, had 

 realised the fact that the development of the achromatic 

 lens " depended on the exact determination of refractive 

 indices, and that the chief difficulty in that determination 

 lay in the difficulty of obtaining homogeneous radiations 

 to serve as standards " (Schuster, " Theory of Optics "). 



For these he used the dark lines of the solar spectrum, 

 originally observed by Wollaston, and in this we have an 

 example of the manner in which practical needs react to 

 assist in the advance of science, for from these observations 

 springs the whole of spectrum analysis and all that is 

 involved in that. 



Thus theory and practice progress together ; each alone 

 carries us but a short way, but the judicious use of hypo- 

 thesis and reason, supported by the verdict of experiment, 

 carries us on to new knowledge, and brings us nearer to 

 the truth. 



Until after the middle of last century we in Britain took 

 our full share in promoting this advance. We might add 

 to the names already mentioned those of Sir George .^iry 

 and of the distinguished men who, in the first half of the 

 century, adorned Trinity College, Dublin, notably Sir 

 William Hamilton. 



Sir George Airy gave, about 1802, an account of the 

 aberration of the lens of the camera obscura of the utmost 

 value to the early designers of the photographic lens, 

 while Sir William Hamilton's essay on the " Theory of 

 Systems of Rays " contains the essence of all that is 

 needed to calculate to a high degree of accuracy the 

 aberration of such a lens. 



But at that date photographic lenses were not thought 

 of, and when Daguerre announced his invention in 1839 

 the work of Airy and of Hamilton was forgotten. Thus to 

 quote, as I did lately in the Traill Taylor lecture, from the 

 recent work of Dr. M. von Rohr. 



" The important signification of Airy's writings for photo- 

 graphic optics does not seem to have been appreciated until 

 a later date. Although they exercised an influence on 

 English text-books, like that of Coddington, they seem 



NO. 1857, VOL. 72] 



unfortunately never to have become known in wider circles 

 on the Continent. It appears, then, that the theoretical 

 opticians of later years to whom his investigations into the 

 astigmatic deformations of oblique pencils would have been 

 of great interest did not base their work on that of Sir 

 G. B. .Airy," while Sir W. Hamilton's paper remained un- 

 noticed by the optician until Finsterwalder directed atten- 

 tion to it, and' another distinguished German, Profi 

 Thiessen, quite lately put his results into an accessible 

 form. 



There was a divorce between theory and practice in 

 England. The importance of Daguerre 's discovery was at 

 once realised, and English opticians set to work with 'no 

 small success to develop the lens and to make it perfect) 

 and splendidly in many ways they performed their task ( 

 but the work was empirical. A certain amount of progress 

 was possible, and was achieved, but without the guidance of 

 well founded theory the progress could not be for long.J ' 



The learned Transactions of the Cambridge Philosophieal 

 Society and of the Roval Society of Dublin were perhapi 

 the last places to which the practical optician would apply 

 for help, and so it came about that because the opticians 

 of another nation first recognised that a full knowledge of 

 the action of a lens on the light that traverses it was a 

 condition precedent to further truth, for some years past 

 the great improvements in the products of the optician's 

 skill which have taken place have had their origin mainly 

 in Germany. 



This brings me to our last example of the manner in 

 which science and practice may combine to produce effects 

 unattainable by either singly. But before dealing with 

 this I would mention one great advantage which, until a 

 few years ago, the English optician possessed in a special 

 degree, an advantage to which much of the progress of our 

 English lenses is undoubtedly due. The story of Gunand's 

 invention of optical glass is deeply interesting. .A poor 

 carpenter, and later a watch-case maker, of Brenetz, in 

 canton Neuchatel, he was born in 1740, and became at an 

 early age interested in telescopes. Prompted by the desire 

 to possess a pair of spectacles, he undertook to make the 

 glass for the lenses. A little later, through M. Droz, a 

 gentleman of the neighbourhood, he was allowed to ex- 

 amine one of Dolland's achromatic lenses, and learnt of 

 the difficulty of obtaining the flint glass required. This 

 he determined to make, and years of penury and un- 

 remitting toil followed, until at last he succeeded in cast- 

 ing discs sufficiently homogeneous to be used for optical 

 work. 



Fraunhofer persuaded him to migrate to Munich, but 

 the venture was not a success. He returned to Switzer- 

 land, and again started glass making. After his death 

 his son told the secret of the art to George Boutemps, a 

 Frenchman, who some years later was brought to England 

 by Messrs. Chance, and helped them to establish the 

 optical glass works which for so long were practically the 

 sole source of the supply of raw material for the optician. 



Our catalogue to-day bears witness to the progress in 

 glass manufacture that has taken place since Boutemps's 

 time, and it is right to recognise the influence that progress 

 has had on opticians' work. 



But to return to our main subject. An optical conven- 

 tion in 1905 would be incomplete without some reference 

 to the work of that master optician who a few months 

 ago was taken from us, the more so since the work of 

 Ernst Abbe aff'ords perhaps the most striking illustration 

 of the effects of the reasoned combination of theory and 

 practice. A comparison of the statistics of the optical 

 trade of Germany now and twenty years ago will suffice to 

 prove this. 



The story of the growth of the Jena industry has been 

 told frequently, still I will repeat it in barest outline. 

 Abbe, then a young man, had settled at Jena as a privat 

 docent in 1S63, and soon after Carl Zeiss, who then made 

 microscopes of the ordinary class, applied to him for help 

 in the development of the instrument. .Abbe's task was a 

 hard one ; the theory of the microscope was at that date 

 only partially understood, the corrections to the lenses 

 were made by a rough trial and error method, and the 

 results were doubtful. The first step was to solve a mathe- 

 matical problem of no small difficulty, to trace the paths 



