145

Of Optics.

rays E , which come from the top of the object A B, and fall parallelupon the great mirrour at F, are thence reflected converging to its fo-cus, where they form the lower extremity K of the inverted ImageJK, similar to the upper extremity A of the object AB-, and thencepaffing on to the small mirrour L, and falling upon it at h, they arethence reflected in the converging state h O and going on through thehole P of the great mirrour, they would meet somewhere about q,and form there the upper extremity a of the erect Image a b, similar tothe upper extremity A of the object AB: but by paffing through theconvex glast R in their way, they meet and crost sooner, as at a,where that point of the erect image is formed.—The like being under-slood of all thofe rays which flow from the intermediate points of theobject, between A and B, and enter the tube TT; all the intermediatepoints of the image between a and b will be formest: and the rayspaffing on from the image, through the eye-glast S, and through asmall hole e in the end of the leiser tube 11, they enter the ey es,which sees the image ab (by means of the eye-glast) under the largeangle c e d, and magnified in length, under that angle, from c to d.

In the best reflecting telefcopes, the focus of the small mirrour isnever coincident with the focus m of the great one, where the firstimage IK is formest, but a little beyond it (with refpect to the eye)as at n: the confequence of which is, that the rays of the pencils willnot be parallel after reflection from the small mirrour, but converge soas to meet in points about q,e,r where they would form a larger up-right image than a b, if the glase R was not in their way: andthis image might be viewed by means of a single eye-glast properlyplaced between the image and the eye: but then the sielst of viewwould be lest, and consequently not so pleasant; for which reason,the glast R is still retained, to enlarge the scope or area of the fielst.

To find the magnifying power of this telescope, multiply the focalstistance of the great mirrour by the distance of the small mirrourfrom the image next the eye, and multiply the focal distance of thesmall mirrour by the focal distance of the eye-glast : then, divide theproduct of the former multiplication by the product of the latter, andthe quotient will exprest the magnifying power.

I ffiall here set down the dimensions of one of Mr. Sbort's reflectingtelefcopes, as deferibed in Dr. Smith's Optics.

The focal distance of the great mirrour 9.6 inches, its breadth 2.3;the focal distance of the small mirrour 1.5, its breadth 0.6: the breadthof the hole in the great mirrour 0.5; the distance between the smallmirrour and the next eye-glast 14.25 the distance between the two

U eye-