Chap. 3.] 'The cylindrical Mirror. 197 



between the two rooms, or in the fcreen, through 

 which the rays pafs, by which the image is formed. 

 The fpectator then, when he cafts his eyes upon the 

 partition of the fcreen, will, ia certain fituations, re- 

 ceive the rays coming through this fmall aperture. 

 He will fee the image formed in the air ; he will have 

 no idea, if not previously acquainted with optics, of 

 the nature of the deception j and may either be amuf- 

 ed, according to the inclination of his friends, with 

 tempting fruit, or be terrified at the fight of a ghaftly 

 apparition. 



Since it is the property of a concave mirror to caufe 

 thofe rays which proceed in a parallel direction to its 

 furface to converge to a focus, and fmce the folar jays, 

 from the immenfe diftance of that body, may be con- 

 fidered as parallel, concave mirrors prove very ufeful 

 burning glaffes, and the focus of parallel rays, or prin- 

 cipal focus," is their focus or burning point. 



CYLINDRICAL MIRRORS, fuch as that reprefented 

 in figure 3, are employed more for the purpofe of 

 amuiement than of philofophy. They are called mix- 

 ed mirrors, becaufe they produce at the fame inftant 

 the effects of plain and of convex mirrors. Suppofe, 

 for inftance, G F (fig. 4.) to be the height of fuch a 

 mirror, and A E an object placed before, or rather be- 

 low it ; all the rays, which proceed from the points 

 A, B, C, D, E, falling on the furface G F of the mirror, 

 and reflected to the eye at O, will reprefent the images 

 of thefe different points at a, b, , d y <?, as they would 

 be reprefented in a plane mirror \ and with refpect to 

 thefe the dimenfions of the object will not be altered 

 in the correfponding image. But fmce the mirror is 

 alfo curved, if we luppofe the fpace ^, /,jy (fig. 5.) 

 to reprefent a part of its circumference, the rays A q, , 

 L r, M s, N /, O AT, P z, F y, being reflected to the 

 O 3 eye 



