ESTIMATION OF SIZE. VISUAL IMPRESSIONS. 1 05 



us, it is very difficult to avoid errors, the very causes of 

 which escape us. Thus, the sun and the moon when they 

 are near the horizon, seem to present a much greater diameter 

 than when high in the heavens. The atmosphere causes 

 objects to appear near or distant, according as it is pure, or 

 charged with mist. These illusions are frequent, especially 

 among mountains, and the inexperienced traveller should not 

 count too much upon the exactness of his impressions. 



Bravais points out a very common error in drawing a 

 rough hill-side in relief, or a mountainous horizon. In veri- 

 fying a sketch mathematically, the horizontal distances of 

 the different points in the landscape are found to be suffi- 

 ciently exact, while the height of the summits, or of inequalities 

 of surface, is in double proportion. But we must add, that the 

 design adjusted mathematically seems incorrect in an opposite 

 sense, and does not give the impression of the natural relief. 



If a rainbow is formed over a cascade when the sun is 

 near the horizon, and the circle of this bow is nearly com- 

 plete, we seem to see not a circle, but an ellipse, of which 

 the principal axis is vertical; the same illusion is produced 

 when we look at a halo. 



Visual impressions, separate or mixed. If we look at a 

 print placed at a certain distance, the details of the engraver's 

 work disappears, the dots and the shading are confounded 

 with the white lines which separate them, and the eye per- 

 ceives only a grayish tint more or less distinct; so also, if a 

 red and blue powder be mingled together, the mixture gives 

 us the impression of a violet colour, although every grain of 

 each powder preserves its own proper colour. This is ex- 

 plained as follows. We have already stated that the internal 

 surface of the retina presents a mosaic of extremely small 

 terminal divisions, each one of which acts separately, and 

 transmits to the brain one single impression at a time. If 

 the image of a trace of the burin or of a grain of powder 

 covers one of these divisions, the impression is single (see 

 % 35? P- J 56); but if two lines, one white and the other 

 black, or two grains, one red and the other blue, are so 

 small and so near together, that their images are in juxtaposi- 

 tion on the same division of the retina, the impression is 



