3o PHYSIOLOGICAL PHYSICS. [Chap. xxiv. 



velocity of light can be calculated. The velocity is 

 said to be 186,000 miles per second, or seven and 

 a half times round the earth per second. This is 

 the velocity in air ; the velocity in other substances, 

 e.g. water, can be estimated by interposing a layer of 

 water in the pathway of the beam and finding the 

 result. The velocity in water is only three-fourths 

 of that in air ; and, in general, the denser the medium 

 the slower the rate. 



Due entirely to the rectilinear propagation of 

 light is the phenomenon that rays transmitted from a 

 luminous object through a small opening in the wall 

 of a dark chamber will form an inverted image of 

 the object on the opposite wall. Thus, in Fig. 126, 



the candle trans- 

 mitting rays 

 through the open- 

 ing o in the cham- 

 ber will form an 

 inverted image. 

 A ray a from the 



ig. 12b. Inverted linage formed by Rays fl QTrio ~ f + V, 

 passing through a small Opening into a nam 



dark Chamber. candle passing in 



a straight line 



will reach a on the wall of the dark chamber, 

 and will have a brightness corresponding to a. Rays 

 from a, owing to the smallness of the aperture, will 

 not illuminate any part other than a. Similarly 

 rays from other parts of the candle passing through 

 the opening will illuminate, each to its own extent, 

 a definite piece of the wall, and thus an image 

 will be formed, inverted, as seen in the figure. The 

 size of the image will depend on the distance of the 

 opposite wall from the wall containing the opening. 

 Thus the inverted image of a landscape may be pro- 

 duced in a darkened room through an opening in the 

 shutter. The smaller the opening the more distinct 



