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In a paper lately read to the Society, the author stated that he had not perceived any change in the focal length of the telescope, induced by changes of temperature; but he has since ascertained that in order to produce the brightest and most perfect image, the distance of the object-glass requires a minute adjustment, amounting to 0·134 of an inch, corresponding to an elevation of temperature from 57° to 84°, or a depression from 57° to 31°. In order to introduce greater clearness and precision, the author proceeds to define certain terms which he finds it necessary to employ. By thelength of the telescope , he would be understood to mean the distance between the object-glass and the focus; by thefluid focus , that between the fluid lens and the focus; and by thefocal power of the telescope, he means the focal length of a telescope of the usual construction, which gives the same convergency to the rays, or produces an image of the same size: but he also employs the termfocal length of the telescope , as synonymous with the first; that offluid focal length as synonymous with the second; and that ofequivalent focal length as synonymous with the last of these terms.

abstracts of the papers printed in the philosophical transactions of the royal society of london

Experiments relative to the effect of temperature on the refractive index and dispersive power of expansible fluids, and on the influence of these changes in a telescope with a fluid lens