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

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.