SYSTEMATIC ANALYSIS OF APPARATUS. 69 



186. EACH KAY OF LIGHT has color 'and direction. 



therefore a single organ is required that shall produce 

 two sensations. 



187. A SINGLE RAY will produce a very feeble effect, 

 therefore the organ to receive it must be so constructed 

 that several rays from the same point shall act together 

 and thus increase the effect. 



188. SEVERAL RAYS RADIATING FROM THE SAME 

 POINT have different directions, but if the organ be prop- 

 erly constructed, the effect of several acting upon the 

 same point in the organ will be the same as if one only 

 acted with the same power as all of them. (See PI. 8.) 



189. THE SAME MEANS NECESSARY TO CAUSE THE 

 RAYS FROM ONE POINT OF AN OBJECT TO ACT UPON ONE 



POINT IN THE ORGAN, WILL BE SUFFICIENT to Cause the 



rays from thousands, yes, millions of adjacent points, to 

 act upon adjacent points in the organ, and it will only 

 be necessary to furnish it with the corresponding num- 



FIG. 74. 



Fig. 74 repre- 

 sents a perpen- 

 dicular section of 

 the eye, from 

 front to rear. 1, 

 white of eye, ex- 

 tending round, ^ 

 and forming a 

 strong spherical 

 box, the opening 

 in front filled by 

 2, a window ; 7, 

 the pupil, an 

 opening in 6, the 



iris, muscles that contract or enlarge 7 ; 9, a space filled by a transparent 

 fluid ; 11, lens ; 13, transparent substance, filling all back of 11 to 8, a 

 multitude of nerves spread out to receive light coming back through 7 ; 

 15, the bundle of nerves 8, continuing back to brain. Make a hole in pa- 

 per, place two candles before it, and the light will shine through in two 

 directions ; place a lens behind the hole, and at a proper distance the light 

 will be brought to two points. 



1S6. What has ? 1ST. Why are more than needed ? 188. What directions 

 hava ? 189. For what will ? Describe FJg. 74. 



