Refrangibility of Light. 159 



reflected upon oiled paper or polished glass 

 properly placed, upon which the artist sketches 

 his draught. With regard to the contours, or 

 outlines, which this picture gives, nothing can 

 be more exact ; but, so far as respects the shading 

 and colouring, the artist can expect but little as- 

 sistance from it : for, as the sun is every moment 

 altering its situation, so is the landscape every 

 moment varying its shade; and so swift is this 

 succession of new shades, that, while the painter 

 is copying one part of a shade, the other part is 

 lost, and a new shade is thrown upon some other 

 object. 



If such a glass, that is, double convex, is so 

 fitted to a hole in a dark lantern, that little 

 pictures, painted in transparent colours on pieces 

 of glass, may be passed successively along be- 

 tween the gloss and the candle in the lantern, we 

 shall thus have a magic lantern. The pictures, 

 striking the glass very divergent, will be refracted 

 very divergent also, and will be painted upon the 

 wall of the chamber in all their colours, as large 

 as we please to make them ; for the farther the 

 wall is from the glass, the more room will the 

 rays have to diverge. As these figures would be 

 painted on the wall reversed, if the picture were 

 held upright, it is necessary to turn them upside 

 down, when we would exhibit the shadows on the 

 wall erect. The same kind of contrivance is 

 now employed, with great success, to elucidate 

 the principal phenomena of astronomy. 



