an autobiography-第72节
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＇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 say that； having administered a dose of cement to Mrs。 Nasmyth's friend； Sir Fireside Brick of Green Lanes； he is now in a convalescent state。 The lecture is to be repeated in another fortnight。 With many thanks for your kind assistance； yours very sincerely；
〃EDWARD COWPER。〃
In the course of my astronomical inquiries I had occasion to consider the causes of the sun's light。 I observed the remarkable phenomena of the variable and some times transitory brightness of the stars。 In connection with geology； there was the evidence of an arctic or glacial climate in regions where such cannot now naturally exist； thus giving evidence of the existence of a condition of climate； for the explanation of which we look in vain for any at present known cause。 I wrote a paper on the subject； which I sent to the Astronomical Society。 It was read in May 1851。 In that paper I wrote as follows：
〃A course of observations on the solar spots； and on the remarkable features which from time to time appear on the sun's surface； which I have examined with considerable assiduity for several years； had in the first place led me to entertain the following conclusion： namely； that whatever be the nature of solar light； its main source appears to result from an action induced on the exterior surface of solar sphere； a conclusion in which I doubt not all who have attentively pursued observations on the structure of the sun's surface will agree。
〃Impressed with the correctness of this conclusion； I was led to consider whether we might not reasonably consider the true source of the latent element of light to reside； not in the solar orb； but in space itself； and that the grand function and duty of the sun was to act as an agent for bringing forth into vivid existence its due portion of the illuminating or luciferous element； which element I suppose to be diffused throughout the boundless regions of space； and which in that case must be exhaustless。
Assuming； therefore； that the sun's light is the result of some peculiar action by which it brings forth into visible existence the element of light； which I conceive to be latent in； and diffused throughout space； we have but to imagine the existence of a very probable condition； namely； the unequal diffusion of this light…yielding element； to catch a glimpse of a reason why our sun may； in common with his solar brotherhood； in some portions of his vast stellar orbit； have passed； and may yet have to pass； through regions of space； in which the light…yielding element may either abound or be deficient； and so cause him to beam forth with increased splendour； or fade in brilliancy； just in proportion to the richness or poverty of this supposed light…yielding element as may occur in those regions of space through which our sun； in common with every stellar orb； has passed； is now passing； or is destined to pass； in following up their mighty orbits。
〃Once admit that this light…yielding element resides in space； and that it is not equally diffused； we may then catch a glimpse of the cause of the variable and transitory brightness of stars；and more especially of those which have been known to beam forth with such extraordinary splendour； and have again so mysteriously faded away； many instances of which abound in historical record。
〃Finally； in reference to such a state of change having come over our sun； as indicated by the existence of a glacial period； as is now placed beyond doubt by geological research； it appears to me no very wild stretch of analogy to suppose that in such former periods of the earth's history our sun may have passed through portions of his stellar orbit in which the light…yielding element was deficient； and in which case his brilliancy would have suffered the while； and an arctic climate in consequence spread from the poles towards the equator； and thus leave the record of such a condition in glacial handwriting on the everlasting walls of our mountain ravines； of which there is such abundant an
