The large reflector, B E, receives the rays, a c, 6 d, from the distant object, and reflects them to its focus, e, where they form the inverted image, or where they cross each other, and then fall diverging ly upon the small reflector, x y, whose focus is at f; viz. a little further than the focus, e, of the large reflector : hence the rays are reflected back upon the lens, x, not in a parallel, but in a converging manner ; and that convergency is increas ed by the action of that lens, so as to come to a focus, or to form a second image, It S, much larger than the former, and erect like the object. Lastly, this image is. viewed through the eye lens, n o ; or, in other words, the rays from every single point of the object, after this 'second crossing, fall divergingly upon the eye lens, which sends them nearly paral lel to the eye at P, through a very small bole. Sometimes the eye lens, no, is dour ble, viz. it, consists of two lenses, which perform the office of a single lens.
If the first lens, a, x, were removed, the image would be formed somewhat larger at hut the area or field of view would be smaller and less pleasant. At the place of the image, ft S, there is situated a cir cular piece of' brass, called a diaphragm, with a hole of a proper size to circum scribe the image, and to cut off all super fluous or extraneous light, in order that the object may appear as distinct as pos sible.
The magnifying power of this telescope is computed in the following manner : If this telescope consisted of the two reflec tors only, and these were situated so that e were the focus of each reflector ; then the rays which came parallel from the dis tant object to the large reflector, and di. vergingly from that to the small reflector, would, - affer the second reflection, go parallel to the eye at P, and of course the object would appear magnified in the pro portion of the focal distance of the large reflector to the focal distance of the small reflector ; so that if the .focal distance of the former be to that of the latter as she to one, then the object would be magni fied aix times in diameter. But since the first image 'is magnified into a second image much larger, which is viewed through the eye lens, therefore the whole magnifying power is in a proportion com pounded of deto ex, and of zx, to z 0. If the termer proportion be as six to one, and the latter as eight to one, then the object will appear forty-eight (viz. six by eight) times larger in diameter through the tele scope than to the naked eye.
The fourth species of reflecting tele scope goes under the name of" Cassegrai nian Telescope." It differs from the pre ceding, in having the small reflector con vex, instead of concave ; in consequence of which the small reflector must be plac ed nearer to the large reflector than the thcus of the latter ; then the rays from the large reflector fall convergingly upon the convex small reflector, and are by it sent back convergingly upon the lens, x, &c. The chief difference be
tween this and the preceding telescope is, that in this the object appears inverted, because in it there is no image formed, or the rays do not cross each other, between the two reflectors. Also with the same magnifying power, &c. this telescope is shorter than the Gregorian, by twice the focal length of the small speculum.
To both those telescopes a long wire is fixed all along the outside of the tube, at the end of which there is a screw which works into an external projection, g, of the internal arm, H, and serves to move that arm with the small speculum nearer to or further from the large speculum, in order to adjust the focus of the instru ment, according to the distance of the ob ject. The action of this wire is easily un derstood; for it passes through a hole at F, where it is prevented going forwards or backwards by two shoulders, which are indicated by the figure : hence, when the observer looks through the hole, P, he turns with his hand the wire by the nut, Q, which screws the projection, g, of the arm nearer or further, &c. until the object appears very distinct.
The largest reflecting telescope now existing, was constructed by. that excel lent astronomer, Dr. Herschel. Is is a tele scope in which the observer looks through an eye lens down upon the larg- reflector, whose polished surface is ches in diameter. Its focal length is about forty feet. • There are however two useful• appen. dages to telescopes, which deserve to he briefly described. A finder, viz. a short telescope,- A, fig. 3, is generally affixed to the tube of a large telescope, for the pur pose of finding out an object expeditious ly. This finder does not magnify the ob ject more than four, six, or eight times ; hut it has a great field of view, so that through it a great part of the heavens may be seen at once. In the inside of -its tube, and exactly at the thew; of the eye glass, there are two slender wires, which cross each other in the axis of the tele scope. Now the finder is adjusted by means of screws upon the tube of the great telescope, in such a manner as that when an object, seen through the finder, appears to be near the crossing of the above-mentioned wires, it is at the same time visible through the great telescope : hence, when the observer wishes to view a small distant object, as a star, a planet, &c. he moves the instrument to one side or the other, until, by looking through the finder, he brings the object nearly to coincide with the crossing of the wires ; and when that takes place, he immediately looks through the large telescope, etc.