268 



FERGUSON'S LECTURES. 



LECT. 

 V!T. 



same object to appear at D, in the direction H f m.-- 

 If the glass be turned round the line g I H, as an axis? 

 the object C will keep its place, because the surface 

 b I d is not removed ; but all the other objects will seem 

 to go round C, because the oblique planes, on which 

 the rays a b, c d fall, will go round by the turning of the 

 glass. 

 The came- The camera obscura is made by a convex glass C D, 



ra obscura. 



y 



placed in a hole of a window-shutter. Then, if 

 the room be darkened so that no light can enter 

 but what comes through the glass, the pictures of 

 all the objects (as fields, trees, buildings, men, 

 cattle, &c.) on the outside, will be shewn in an 

 inverted order, on a white paper placed at G // in the 

 focus of the glass ; and will afford a most beautiful and 

 and perfect piece of perspective or landscape of what- 

 ever is before the glass ; especially if the sun shines 

 upon the objects. 



If the convex glass C D be placed in a tube in the 

 side of a square box, within which is the plane mirror 

 E F, reclining backwards in an angle of 45 degrees from 

 the perpendicular k q, the pencils of rays flowing from 

 the outward objects, and passing through the convex 

 glass to the plane mirror, will be reflected upwards 

 from it, and meet in points, as / and K (at the same 

 distance that they would have met at // and G, if the 

 mirror had not been in the way) and will form the afore- 

 said images on an oiled paper stretched horizontally in 

 the direction IK; on which paper, the outlines of 

 the images may be easily drawn with a black-lead pencil , 



