196 LETTERS ON NATURAL MAGIC. 



exhibition, indeed, that I ever saw of it was in the case 

 of a painting of a ship upon a sign-board executed in 

 strongly gilt lines. It contained a view of the stern, and 

 side of a ship in the stocks, and, owing to the flatness 

 of the board and the brightness of the lines, the gradual 

 development of the figure from the most violent fore- 

 shortening at great obliquities till it attained its perfect 

 form, was an effect which surprised every person that saw 

 it. 



The only other optical illusion which our limits will 

 permit us to explain, is the very remarkable experiment 

 of what may be truly called breathing light or darkness. 

 Let S be a candle where light falls at an angle of 56 45' 

 upon two glass plates A, B, placed close to each other, 

 and let the reflected rays A C, B D, fall at the same 

 angle upon two similar plates 0, D, but so placed that 

 the plane of reflection from the latter is at right angles 

 to the plane of reflection from the former. An eye placed 



Fig. 29. 



at E, and looking at the same time into the two plates 

 C and D, will see very faint images of the candle S, which 

 by a slight adjustment of the place may be made to dis- 

 appear almost wholly. Allowing the plate C to remain 

 as it is, change the position of D, till its inclination to 

 the ray B D is diminished about 3, or made nearly 

 53 11'. When this is done, the image that had dis- 

 appeared on looking into D will be restored, so that the 



