144

The refleß-ing telescope.

Fig. 7.

Of Optics .

through the middle of the glafs, or at least at a good distance from itsedges, and pass through the hole in the middle of the plate. Butthis circumscribes the image, and leflens the field of view, which wouldbe much larger if the plate could be dispensed with.

The great inconvenience attending the management os long tele-scopes of this kind, has brought them much into disufe ever since therefieSling telescope was invented. For one of this fort, six feet in length,magnifies as much as one of the other an hundred. It was inventedby Sir Isaac Newton, but has reeeived considerable improvements sincehis time; and is now generally constructed in the following mannet,which was first proposed by Dr. Gregory.

At the bottom of the great tube TTTT is placed the large con-cave mirrour DUVF, whoie principal focus is at m ; and in its middleis a round hole P, opposite to which is placed the fmall mirrour L,concave toward the great one; and so fixed to a strong wire M, thatit may be moved farther from the great mirrour, or nearer to it, bymeans of a long screw on the outside of the tube, keeping its axisstill in the fame YinePmn with that of the great one.—Now, sincein viewing a very remote object, we can fcarce fee a point of it butwhat is at least as broad as the great mirrour, we may consider the raysof each pencil, which flow from every point of the object, to beparallel to each other, and to cover the whoie reflecting furface DUVF.But to avoid confusion in the figure, we (hall only draw two rays of apencil flowing from each extremity of the object into the great tube,and trace their progress, through all theil- reflections and refractions, tothe ey cf at the end of the fmall tube 11, which is joined to thegreat one.

Let us then fuppofe the object AB to be at such a distance, that therays <7 may flow from its lower extremity B, and the rays E from itsupper extremity A. Then, the rays C falling parallel upon the greatmirrour at D, will be thence reflected converging, in the directionDG -, and by Crossing at I in the principal focus of the mirrour, theywill form the upper extremity I of the inverted image IK , similar tothe lower extremity B of the object AB: and pafling on to the con-cave mirrour L (whofe focus is at n) they will fall upon it at g, andbe thence reflected converging, in the direction g N, becaufe g m islonger than g n ; and pafling through the hole P in the large mirrour,they would meet fomewhere about r, and form the lower extremity b .of the erect image a b , similar to the lower extremity B of the objectA B. But by pafling through the plano-convex glafs R in their way,they form that extremity of the image at b. In like manner, the- . , rays