GERMANY (continued) †Dr. Lamont. †Dr. Littrow. †Prof. Schwerd. †Dr. Weisse. CONTENTS. Observations of Small Planets, made at the Royal Observatory, Green- Letter to the President from M. le Comte G. de Pontécoulant, ac- companying a Memoir containing "Observations on the New Terms which Mr. Adams has proposed to introduce into the Ex- pression of the Coefficient of the Secular Equation of the Moon"... Sur les Réfractions Anormales dans les Eclipses de Soleil et la Détermi- Micrometrical Measures of the Triple Star Scorpii, by Capt. Noble ib. Note on the Occultation of Saturn, May 8, 1859, by Mr. Pollock Mr. Alvan Clark's New Micrometer for measuring large Distances Free List of the Institutions and Individuals to whom the Publications ROYAL ASTRONOMICAL SOCIETY. VOL. XIX. Supplemental Notice. No. 10. On the Determination of Personal Equation by Observations of the Projected Image of the Sun. (Letter from C. Ragoonaiha Charey to Captain W. S. Jacob.) "I beg to trouble you once more on the subject of the Observation of the Sun's Image. Prior to your departure to England, you were good enough to approve the observations of the above, taken by me with the transit instrument of the observatory, and you yourself continued to make a few observations, together with two or three observers simultaneously; and on my addressing you while here, on the 23rd April, 1858, (a copy of which is herein enclosed), you were pleased to apprise me that you find no deficiency in the mode of observation, that you will consult on the subject with some of the scientific friends in England, and will inform me the same. "Having had in view for personal conference on the subject after your arrival in India, I was hitherto unmindful of intruding upon your valuable time; but, to our greatest misfortune, we came to know that you gave up your appointment as astronomer at Madras. 66 Though I have a very poor knowledge in scientific matters, yet I presume to think from little experience that this mode of observing the sun is in no way adverse to accuracy, but it only adds a greater weight (than that we have hitherto obtained) to the result of observations, in proportion to the increase of the number of simultaneous observers; and, further, it affords ample opportunities to ascertain the personal equation of the observers easily. "I happened to see a few months ago in the introduction of the first volume of the Washington Observations, p. 49, where Lieutenant Maury, the Director of the Observatory, states as follows, 'I have made trials for a personal equation, but could not succeed in establishing any uniformity as to the difference of eye and ear among the observers.' "One of these experiments consisted in casting, by a proper adjustment of the eye-piece, a sharp image of the sun and wires at the time of transit, upon a screen placed at a little distance from the telescope, and to require each of the four officers to observe and record in silence the transit of the sun. The differ ence among them as to the mean of the seven wires was altogether insignificant. "From the above I see that the transit of the sun's image is only taken for ascertaining the personal equation, and that also for an experimental purpose, without recommending the mode as one that can be employed daily for the general use of the observatory. "I beg leave herewith to show the results of the personal equation deduced from the observations of the sun's image, with the transit instrument here, by yourself with the assistants of the observatory, together with the results derived from recent observations, made with the 5-feet achromatic telescope on the outside of the observatory, when the sun's altitude was about 50 degrees. "For observing the sun's image on the outside of the observatory, I selected a spot where the action of the wind may not shake the instrument, and a shade for protecting the chronometer and the observers from the sun, and made the following contrivance to obtain the observations. "I made a cylindrical cage of about ten inches in diameter, of bamboo slits and black cloth, covered its mouth with an oiled white paper, containing nine parallel lines, finely ruled in ink, at an interval of one inch in lieu of wires, introducing the same to the eye-end of the telescope, leaving a projection of about one foot and a half from it, and tieing up the other end on the body of the telescope, adjusted the eye-piece so as to cast a sharp image of the sun's limb on the paper, and thereby made a series of observations with a chronometer, for both the upper and lower limbs of the sun by myself, Sashoo Iyengar and Mootosamy Pillay. "The following are the results of personal equation, deduced from the observations made by the transit instrument, between the months of February and May 1858: 66 Solving the above (as treated in the introduction of the Greenwich Observations, by the Astronomer Royal) and assuming J to be zero, we get the final result as follows, "And the results of personal equation deduced from the ob servations made with the 5-feet telescope, on the 5th April, 1859, between 3 and 4 P.M., are as follows: The numerical coefficients are the number of observations of transit taken in each line of the cage. [ 30 (R-M) 20 (S-R) = S 8.50 = 9'20 19 (S-M) =11·60 "Solving the above, and assuming M to be zero, we get, "The differences between the former and the latter values of M-S and M-R, are, I think, not of great consequence, considering the small number of observations. "In conclusion, I beg to request that you will be pleased to take into consideration all the above my humble statements, and enlighten me as to the propriety and accuracy of this mode of observing the great luminary, and what you think proper on the subject. "Madras, April 25, 1859.” Note in reference to the foregoing Communication. By Capt. W. S. Jacob. "This method has for some time been in use at Madras, having been proposed by the writer of the letter and approved of by me after due trial; it has several advantages, one of the chief of which is that it enables any number of observers to make the same identical observation simultaneously, and thus furnish a correct value of the personal equation of each. The method seems to have been tried for this special purpose at Washington, but found not to answer; we, however, found it to succeed perfectly, the personal equations appearing to have the nearly same value with those determined in other modes, but showing much greater uniformity in the separate measures. As the ordinary mode of taking the sun's transit it has further the advantage (a very great one in a tropical climate) of not exposing the observer's eyes to be scorched, or breaking the coloured glasses, a circumstance of continual recurrence even when the aperture has been reduced to an inconvenient degree, and this with no sacrifice, but rather an increase in point of accuracy. "I am aware that the method does not possess any novelty, having been occasionally employed by myself more than 20 years ago, and probably by many others; being also the mode regularly employed by Mr. Carrington in fixing the places of the solar spots; but it is not, I believe, used in any other observatory besides that of Madras as the mode of observing the sun at noon, and therefore it may be useful to bring it to the notice of the directors of observatories." |