312 optics. 



at E, in the direction of the ray H e, produced in 

 the right line HeE. And the ray C d, flowing 

 from the object C, and falling obliquely on the 

 plane surface d k, will be refracted to the eye at 

 H ; which will cause the same object to appear at 

 D, in the direction H D. If the glass be turned 

 round the line c H, as an axis, the object C will 

 keep its place, because the surface b d is not re- 

 moved ; but all the other objects will seem to go 

 round C, because the oblique planes, on which the 

 rays abed fall, will go round by the turning of 

 the glass. 



The Camera Obscura, 



The camera obscura is made by a convex glass 

 C D (Plate 17. fig. 2.), placed in a hole of a win- 

 dow-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 shown in an inverted order, on a white 

 paper, placed at G H in the focus of the glass ; 

 and will afford a most beautiful and perfect piece 

 of perspective or landscape of whatever is before 

 the glass, especially if the sun shine 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 

 forty-five 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 I and K, at the same distance that 



