lloyal Astronomical Sociei^ §^^ 



positioo of, Mflr^^tilii* list includes many of those set down for a 

 similar purpose in the Nautical Almanac for 1845. ,j jj^j^ „^j^j 



loMay 9,— The following communications were read: — f,,^^^ -lofilni^ 

 ,iilr. Extract of a Letter from the President to the Secretary.. , ,.,. 

 fHf'I forward ypu a new epoch of y Virginis for this apparitipn^ 

 thinking it may be welcome to such of our Fellows as are following 

 up the orbit of that remarkable system, which i^romises, compara- 

 tively, to be to double stars what Halley's comet is among that clas3 

 of bodies. I made the observations in Dr. Lee's Observatory, s^t 

 Hartwell House, with my former instruments, which are always 

 obligingly kept in readiness for my immediate uae. The mean mi- 

 crometrical readings gave 174° 3 6'- 6 for the position, which, cor- 

 rected to the meridian, and brought to proportional parts, afford 



Position 185° 23'-3 (w. G); Distance 2"-10 (w. 4); Epoch 1845-34. 



"The measures made on the 2nd and 3rd inat. mferit considerable 

 confidence, for, previously to my placing the micrometer on the ob- 

 jects, it was steadily viewed in an unilluminated field with a negative 

 eye-piece magnifying 240 times, under which the minute companion 

 following nearly on the parallel was distinctly seen. On this occa- 

 sion, the wire-micrometer was under a positive eye-piece of 340 

 times during three sets of the measures, and once under one of 600'; 

 and the rock-crystal was applied with its power of 145, doubled hjr 

 the interposition of the achromatic lens described in my * Cycle.' '* 



IL On a new construction of the Divided Eye-glass Double- Image 

 Micrometer. By O. B. Airy, Esq., Astronomer Royal. 

 ,' Tlie author remarks, that the convenience and accuracy of the 

 *d'ouble-image micrometer are so great that it is highly desirable tb 

 removie, if possible, any of the inconveniences to which it is subject. 

 The form in which he has employed the instrument is that of a four- 

 glass eye-piece, the second lens (as measured from the object-glass) 

 being divided, and one or both segments being moveable. The Se- 

 cond lens must therefore be placed in such a position that every 

 pencil of rays coming from the object-glass is equally divided upon 

 the two segments, — a condition which is satisfied when the distance 

 of the second lens from the first is (sensibly) the same as the focal 

 length of the first lens. When this adjustment to focal length is 

 perfectly made, and when also the observer has the power of securing 

 the equal division of light upon the two segments for one pencil 

 (and therefore for all), the use of this eye-piece is extremely conve- 

 nient. Without any effort in placing the eye in a definite position, 

 p!. single image, in wl^ateyer pai^ of Sie field, is broken up into t\yo 

 of equal intensity. 



The most important defect of the micrometer is this, that, unless 

 certain conditions are respected, the images will be coloured in all 

 parts of the field of view except the centre. This colour is not pro- 

 duced, like the colour in an achromatic object-glass, from the want 

 of convergence of all the rays in one pencil to the same focal distance. 

 .It arises from a lateral chromatic disturbance of the focus of each 

 ijencil ; and it may be represented most conveniently by tracing the 



