XXVIII. On the construction of specula of six-feet aperture; and a selection from the observations of nebulæ made with them

Abstract

The period seems now to have arrived when it may be proper to lay before the Royal Society some further account of researches in sidereal astronomy, carried on with a Newtonian telescope of 6-feet clear aperture. The observations extend over a period of about seven years, during which few favour­able opportunities were lost; still in our climate, where there is so much cloudy weather, a year’s work, measured by the number of hours when nebulæ can be effectively observed, is not considerable. Here in winter the finest definition we have, and the blackest sky, is usually before eleven o’clock, after which the sky becomes luminous, and the fainter details of nebulæ disappear. In spring and autumn the change is neither so early nor so decided; but the nights are shorter. Guided by Sir John Herschel’s admirable Catalogue, we have examined almost all the brighter known nebulæ except a few in the neighbourhood of the pole, and a great proportion of the fainter nebulæ. No search has been made for new nebulæ; very many, however, have been found accidentally in the immediate neighbourhood of known nebulæ, but for the most part they were faint objects presenting no features of interest. In every case where any peculiarity was detected, as for instance the convolution of a spiral, dark lines, or dark spaces, a rough sketch was made, and the more remarkable objects were selected for examination on favourable nights, when the details were carefully filled in, sometimes with the aid of the micrometer. The very faint objects, and even the brighter, where there was a simple gradation of colour and no peculiarity of form, after having been examined on a tolerably good night, were rarely examined again. In our ever-varying climate, when we employ high powers and large apertures, vision is impeded more or less by the unsteadiness of the air; it is impeded also by haze; and in both respects the condition of the air varies immensely from night to night, and from hour to hour. The speculum also is not uniform in its action. With such sudden alternations of temperature, in a moist climate, it is frequently dewed, and gradually tarnishes. Artificially heating it would be a remedy; but it would be an objectionable one, and we have not employed it. From all these causes we can scarcely say that any one object has been examined under a combination of favourable circumstances; still it is not now probable that with the present instrument any remarkable additions will be made to the details of nebulæ already carefully sketched, except in very favourable states of the atmosphere. Occasion­ally the air is so transparent and so steady, that magnifying power may be pushed very far; and then, perhaps, something new comes out. Such opportunities, however, are rare; and the progress made is necessarily so very slow, that I think it would be inexpedient longer to keep back this paper in the distant hope of making it in some respects more complete.