An Autobiography [160]
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
"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