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ose； and we wended our way towards the city。
Let me； however； before concluding； say a few words in reference to my dear departed friend David Oswald Hill。 His name calls up many recollections of happy hours spent in his company。 He was； in all respects； the incarnation of geniality。 His lively sense of humour； combined with a romantic and poetic constitution of mind； and his fine sense of the beautiful in Nature and art； together with his kindly and genial feeling； made him； all in all； a most agreeable friend and companion。 〃D。 O。 Hill；〃 as he was generally called； was much attached to my father。 He was a very frequent visitor at our Edinburgh fireside； and was ever ready to join in our extemporised walks and jaunts； when he would overflow with his kindly sympathy and humour。 He was a skilful draughtsman； and possessed a truly poetic feeling for art。 His designs for pictures were always attractive； from the fine feeling exhibited in their composition and arrangement。 But somehow； when he came to handle the brush； the result was not always satisfactorya defect not uncommon with artists。 Altogether； he was a delightful companion and a staunch friend； and his death made a sad blank in the artistic society of Edinburgh。
CHAPTER 19。 More about Astronomy。
Astronomy； instead of merely being an amusement； became my chief study。 It occupied many of my leisure hours。 Desirous of having the advantage of a Reflecting Telescope of large aperture； I constructed one of twenty…inches diameter。 In order to avoid the personal risk and inconvenience of having to mount to the eye…piece by a ladder； I furnished the telescope tube with trunnions； like a cannon； with one of the trunnions hollow so as to admit of the eye…piece。 Opposite to it a plain diagonal mirror was placed； to transmit the image to the eye。 The whole was mounted on a turn…table； having a seat opposite to the eye…piece； as will be seen in the engraving on the other side。
＇Image＇ 〃Trunnion Vision〃 Reflecting telescope of 20…inch diameter mounted on a turn…table。
The observer； when seated； could direct the telescope to any part of the heavens without moving from his seat。 Although this arrangement occasioned some loss of light； that objection was more than compensated by the great convenience which it afforded for the prosecution of the special class of observations in which I was engaged namely； that of the Sun； Moon； and Planets。
I wrote to my old friend Sir David Brewster； then living at St。 Andrews； in 1849； about this improvement and he duly congratulated me upon my devotion to astronomical science。 In his letter to me he brought to mind many precious memories。
〃I recollect；〃 he said； 〃with much pleasure the many happy hours that I spent in your father's house； and ever since I first saw you in your little workshop at Edinburgh；then laying the foundation of your future fortunes；I have felt a deep interest in your success； and rejoiced at your progress to wealth and reputation。
〃I have perused with much pleasure the account you have sent me of your plan of shortening and moving large telescopes； and I shall state to you the opinion which I have formed of it。 If you will look into the article 'Optics' in the Edinburgh Encyclopedia （vol。 xv。 p。 643）； you will find an account of what has been previously done to reduce by one…half the length of reflecting telescopes。 The advantage of substituting； as you propose； a convex for a plane mirror arises from two causes that a spherical surface is more easily executed than a plane one； and that the spherical aberration of the larger speculum； if it be spherical； will be diminished by the opposite aberration of the convex one。 This advantage； however； will disappear if the plane mirror of the old construction is accurately plane； and in your case； if the large speculum is parabolic and the small one elliptical in their curvature。
〃The only objection to your construction is the loss of light； first of one…fourth of the whole incident light by obstruction； and then one…half of the remainder by reflection from the convex mirror； thus reducing 100 rays of incident light to 37 1/2 before the pencil is thrown out of the tube by a prism or a third reflector。 This loss of light； it is true； may be compensated by an additional inch or two to the margin of the large speculum； but still it is the best part of the large speculum that is made unproductive by the eclipse of it by the convex speculum。 〃With regard to the mechanical contrivance which you propose for working the instrument； I think it is singularly ingenious and beautiful； and will compensate for any imperfection in the optical arrangements which are rendered necessary for its adoption。 The application of the railway turn…table is very happy； and not less so is the extraction of the image through the hollow trunnions。
〃I am much obliged to you for the beautiful drawing of the apparatus for grinding and polishing specula； invented by Mr。 Lassell and constructed by yourself。 I shall be glad to hear of your further progress in the construction of your telescope； and I trust that I shall have the pleasure of meeting you and Mr。 Lassell at the Birmingham meeting of the British Association。
In the course of the same year （1849） I sent a model of my Trunnion turn…table telescope for exhibition at a lecture at the Royal Institution； given by my old friend Edward Cowper。 In the model I had placed a neat little figure of the observer； but the head had unfortunately been broken off during its carriage to London。 Mrs。 Nasmyth had made the wearing apparel； but Edward Cowper wrote to her； before the lecture； that he had put 〃Sir Fireside Brick〃 all to rights in respect of his garb。 His letter after the lecture was quite characteristic。
〃The lecture；〃 he said； 〃went off very well last night。 All the models performed their duty； and were duly applauded for doing so。 My new equatorial was approved of by astronomers and by instrument…makers。 The last gun I fired was a howitzer； but mounted swivel…gun fashion； on a sort of revolving platform； or something like a turn…table proper the gunner at the side of the carriage。 Do you know anything of the kind？ Bang！ Invented by one Nasmyth。 Bang！ The observer is sitting at ease； the stars are brought down to you instead of your creeping up a scaffolding after the stars。 Well； the folks came to the table after the lecture； and 'The Nasmyth Telescope' kept banging away for a quarter of an hour； and was admired by everybody。 The loss of light was not much insisted on； but it was said that you ran the risk of error of form in three surfaces instead of two。 I see that Sir J。 South states that Lord Rosse would increase the light of his telescope from five to seven by adopting Herschel's plan。
〃De La Rue was quite delighted。 He said； 'Well； I congratulate you on a most splendid lectureI cannot call it anything else。' My father； who takes very little interest in these things； said； 'Well； Edward has made me understand more about telescopes than I ever did in my life。' The theatre was full； gallery and all。 They were very attentive； and I never felt more comfortable in a lecture。 I am happy to s