Elements of natural philosophy (Volume 2-3) . heoptical centre of the small reflector being sensibly equalto the radius of curvature. This gives A it A! Its value (83). DTNAMETER. Dynameter § 86. If any telescope, properly adjusted to view dis-tant objects, be directed towards the heavens, the fieldlens may be regarded as a luminous object whose im- Distance ofage will be formed by the eye lens. The distance of object; 17 258 NATURAL PHILOSOPHY. the object in this case will be the sum of the princi-pal focal distances or Fu + (i^;/), and this being sub-stituted for /, in Equation (57), we get,
Elements of natural philosophy (Volume 2-3) . heoptical centre of the small reflector being sensibly equalto the radius of curvature. This gives A it A! Its value (83). DTNAMETER. Dynameter § 86. If any telescope, properly adjusted to view dis-tant objects, be directed towards the heavens, the fieldlens may be regarded as a luminous object whose im- Distance ofage will be formed by the eye lens. The distance of object; 17 258 NATURAL PHILOSOPHY. the object in this case will be the sum of the princi-pal focal distances or Fu + (i^;/), and this being sub-stituted for /, in Equation (57), we get, by invertingand reducing, Relation betweenobject and image; F u _ » • • • • • m Rule. hence, any linear dimension of the object glass of a tele-scope, divided by the corresponding linear dimension ofits image, as formed by the eye glass, is equal to themagnifying power of the telescope. This is the princi-ple of the Dynameter, a beautiful little instrumentused to measure the magnifying power of telescopes. Fig. 53. Illustration;. Construction ofthe dynameterilained; To understand its construction, let us suppose two lu-minous circular disks, a tenth of an inch in diameter,to be placed one exactly over the other in the principal siTosedtobo ^ocus m °f a lens -^ an(l with their planes at right an-moved tangent to gles to its axis; an image of the common centre of thedisks will be formed on the retina of an eye viewingthem through the lens, at m. If one of the disks bomoved to the position m, so that its circumference betangent to that of the other, the image of its centrewill be at mf, determined by drawing from 0, theoptical centre of the eye, a line parallel to that joining ELEMENTS OF OFTICS. 259 the optical centre of the lens and the centre of theTake 0De disk -itt *- i mi i l. and suppose the movable disk, article (73); the images will be tangent len8 dMded. to each other, and the movable disk will have passed over a distance equal to its diameter, viz.: one te
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