is an instrument which has been so completely described in the Encyclopaedia, that it is introduced into this place merely to notice an ingenious suggestion of Mr Nicholson's for improving the achromatic telescope, by adding an artificial iris to the object glass. Suppose (says he) a brafs ring to surround the object end of the telescope, and upon this let eight or more triangular flaps of brass be fixed, so as to revolve on equidistant pins passing through each triangle near one of its corners. If the triangles be folded inwards upon each other, it may readily be apprehended that they will close the aperture; and if they be all made to revolve or slide backwards alike, it is clear that their edges will leave an octagonal aperture, greater or less according to circumstances. The equable motion of all the triangles may be produced either by pinions and one concave toothed wheel, or by what is called snail-work. Another kind of iris, more compact, may be made, by causing thin elastic flaps of brass to slide along parallel to the tube, and be conducted each through a slit in a brass cap which shall lead them across the aperture in a radial direction. It is probable also that the artist, who shall carry these hints into effect, may also think of several other methods.

This thought occurred to the author, from contemplating the contraction and dilatation of the iris of the eye, according as we look at an object more or less luminous. These variations are so great, that in the observable variations of the human eye, the aperture is thirty times as large at one time as at another, whilst in the cat the proportion is greater than a hundred to one.