APERTURE, in OPTICS, from ajierire, to open, is the open part of an object glass, through which the rays of light are transmitted from the object, in order to form an image of it in the interior of the tube. It is also em ployed to signify any hole or opening through which light is transmitted for other optical purposes.
A series of ten apertures, formed in circular plates of brass, and decreasing gradually from half an inch to of an inch, were made for the editor of this work for a photometrieal telescope, by that celebrated and unrivalled artist, Mr Edward Troughton, who has fa voured us with the following account of the method by which these delicate perforations were made : A micrometer microscope, similar to those used for reading off the angles upon astronomical instruments, was fixed with its axis of vision parallel to the plane of a piece of wood. In the focus of this and perpendicular to the axis, was fixed a cock, having therein a cell, into which the pieces of metal could readily be placed. The microscope had an adjustment for distinct vision. The
value of the parts of the micrometer were previously ascertained, or which there were 30684 in an inch, and of course the 10 pieces have their apertures measured by the number of those parts, as inserted below. The cen tre of each plate was first indented by the point of a common drill, or borer of metals. This made an im pression of an obtuse conical form, whose vertex reach ed very near the opposite surface. The smallest aper ture being the most difficult, was first attempted ; for, should it have been made too large, it would still do for the next size, kc. A fine pointril, whose angle of sharp ness was about 12°, served to make and enlarge the holes. In doing this, as soon as light was observed to pass through, the plate was applied to the cock, and measured by the wire of the micrometer ; if it was found too small, the pointril was again introduced and turned round in the holes ; thus, by repeated trials bringing it to its exact measure.