18G5.] Astronomy. 651 



the importance of the examination at this special time) ; but conjointly 

 with that, in the interests, perhaps ultimately more important, of 

 geography and other sciences usually promoted by the Royal Geogra- 

 phical Society. 



A most valuable contribution to astronomical photography has 

 been made by Lewis M. Butherfurd to the American ' Journal of 

 Science.' He at first employed a reflecting telescope with a silvered 

 mirror, but after numerous trials he was obliged to abandon the 

 instrument, owing to several difficulties inherent in the employment 

 of this kind o£ telescope. He has now constructed an object-glass, not 

 achromatic as regards colour, but corrected solely with reference to 

 the photographic rays. The correction of this has been rendered 

 certain and tolerably simple by the employment of the spectroscope as 

 a means of examining its achromatic condition. 



The image of a star at the focus of a perfectly corrected objective 

 would be a point, the apex of all conceivable cones having the object 

 glass, or parts of it, as the bases. This point falling upon a prism 

 would be converted into a line, red at one end and violet at the other, 

 with the intermediate colours in their proper ^places. If, however, 

 the different coloured rays are not all brought to the same focus, the 

 spectrum will no longer be a line, but the uncorrected colours will be 

 expanded to a brush, the width of which will be the diameter of the 

 cone where intercepted by the prism. It will thus be seen that a 

 simple glance at a star spectrum will indicate at once what parts of 

 the spectrum are bounded by parallel lines and consequently converged 

 to one focal point, and what parts do not conform to this condition, 

 and also the amount of divergence. 



On applying this test, it was found that an objective of flint and 

 crown, in which the visual was united with the photographic focus (in 

 other words, where the instrument could be focalized on a plate of 

 ground glass by the eye, as in ordinary cameras), was a mere compro- 

 mise to convenience, in which both the visual and actinic qualities 

 were sacrificed. 



In order to bring the actinic portion of the spectrum between 

 parallel borders, i. e. to one focus, it is necessary that a given crown 

 lens should be combined wdth a flint which will produce a combined 

 focal length about one-tenth shorter than would be required to satisfy 

 the conditions of achromatism for the eye, and in this condition the 

 objective is entirely worthless for vision. 



Having obtained the achromatic correction, the most delicate task 

 was to produce the correction for figure, since the judgment of the eye 

 was useless unless entirely protected from the influence of all but 

 the actinic rays. A cell of glass, enclosing a sufficient thickness of 

 cupro-sulphate of ammonia, held between the eye and the eye-piece, 

 enabled the author to work for coarse corrections upon a Lyrse and 

 Sirius, but so darkened the expanded disc of a star in and out of focus 

 that all the final corrections were made upon tests by photography, 

 which gave permanent record of all the irregularities of surface to be 

 combated. Still, however, the process was long and tedious, dependent 

 vol. ii. 2 y 



