14-3

Os Optics .

be easily drawn with a black-lead pencil; and then copied on aclean fheet, and coloured by art, as the objects themfelves are by nature.—In this machine, it is ufual to place a plane glass, unpolifhed, in thehorizontal situation IK, which glass receives the images of the' outwardobjects; and their outlines may be traced upon it by a black-leadpencil.

N. B. The tube in which the convex glass CD is fixed, must bemade to draw out, or pufh in, so as to adjust the distance of that glassfrom the plane mirrour, in proportion to the distance of the outwardobjects; which the operator does, until he sees their images distinctlypainted on the horizontal glass at IK.

The forming a horizontal image, as IK, of an upright object AB,depends upon the angles of incidence of the rays upon the plane mir-rour E F, being equal to their angles of reflection from it. For, if aperpendicular be fuppofed to be drawn to the furface of the plane mir-rour at e, where the ray A a Ce falls upon it, that ray will be reflectedupwards in an equal angle with the other side of the perpendicular, inthe line edi, Again, if a perpendicular be drawn to the mirrour fromthe point f, where the ray Abf falls upon it, that ray will be reflectedin an equal angle from the other side of the perpendicular, in the linefh I. And if a perpendicular be drawn from the point g, where theray Ac g falis upon the mirrour, that ray will be reflected in an equalangle from the other side of the perpendicular, in the line g i I. So thatall the rays of the pencil abc, flowing from the upper extremity ofthe object A B, and paffing through the convex glass CD, to the planemirrour E F, will be reflected from the mirrour, and meet at I, wherethey will form the extremity I of the image IK, similar to the ex-tremity A of the object AB. The like is to be understood of thepencil q r s, flowing from the lower extremity of the object A B, andmeeting at K (aster reflection from the plane mirrour) the rays formthe extremity K of the image, similar to the extremity B of the ob-ject : and so of all the pencils that flow from the intermediate points ofthe object to the mirrour, through the convex glass.

The opera - If a convex glass, of a fhort focal distance, be placed near the planeglafi. mirrour, in the end of a fhort tube, and a convex glass be placed in ahole in the side of the tube, so as the image may be formed between thelast-mentioned convex glass, and the plane mirrour; the image beingviewed through this glass, will appear magnified.—In this manner,the opera-glajfes are constructed ; with which a gentleman may look atany lady at a distance in the company, and the lady know nothingof it.

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