I 282 THE POPULAR SCIENCE MONTHLY. 



seen : so the fault is doubled. I shall be able to show this more clearly 

 by experiments. 



The vertical and horizontal lines of the diagram (Fig. 1) are re- 

 flected with equal distinctness upon the screen by the spherical appa- 

 ratus. 



Those among my audience who have a decided form of astigmatisn 

 will, nevertheless, see them differently. Those whose sight is norma 

 will only observe a difference after I have added a cylindrical lens to 

 this apparatus, and thus made it astigmatical (Fig. 2). Ordinary spec- 

 tacle-glasses are worked by a rotating movement on the surface of a 

 sphere ; cylindrical lenses are worked by moving the glass backward 

 and forward upon a cylindrical surface. Such glasses produce an op- 

 tical effect only in one direction. If instead of white lines I make the 

 experiment with colored lines, it will show the mixing of colors pro- 

 duced by astigmatism ; and if I now turn the axis of the lens, you will 

 observe the effect of different forms of astigmatism. I show you 

 a square (Fig. 3) : if I add a cylindrical concave glass, with its axis 

 placed horizontally, the square becomes an oblong. 



In order now to show you how it is possible that the same eye may 

 see an object at too great a distance elongated in a vertical direction, 

 and, on the contrary, one that is too near enlarged in an horizontal 

 direction, I need only place this cylindrical glass before or behind the 

 focus of the apparatus without turning the axis, and you will then see 

 the square, first elongated in a vertical direction (Fig. 4), and then 

 enlarged in an horizontal direction. 



Lastly, I show you a portrait. Imagine to yourself that it repre- 

 sents the person whom the astigmatical painter is painting ; then, by 

 aid of the cylindrical glass, you can form an idea how the painter sees 

 this person. 



If I alter the position of the glass, the portrait assumes the form in 

 which the painter sees his own painting on the canvas. This will 

 explain to you why he paints the portrait still longer than he sees the 

 person. 



With regard to an anomaly of sight, which seems almost foreign 

 to the subject of painting I mean color-blindness I will also say a 

 few words here, as the subject seems to be regarded with particular 

 interest in England. 



What we call color-blindness is a congenital defect of vision, which 

 is characterized by the absence of one of the three primary sensations 

 of color. The primary sensations of color are red, green, and violet, 

 according to Thomas Young and Helmholtz ; or red, green, and blue, 

 according to Maxwell. When, as may easily happen, to this defect is 

 joined a decided talent for painting, drawing alone ought to be at- 

 tempted, because so absolute a defect will soon assert itself. But we 

 meet with slighter degrees of color-blindness, where the perception of 

 red is not entirely wanting, but only considerably diminished ; so 



