ZOOLOGY AND BOTANY. MICROSCOPY, ETC 411 



is the calculation of w, which is found to have the same numerical value 

 as for the mean kinetic energy of gaseous molecules. This leads up 

 to the calculation of Avogadro's constant, or the number of gaseous 

 molecules contained in a molecule-gram of any gas at 0° and under a 

 pressure of one atmosphere. This constant is found to have a probable 

 value of 7(f. r » x 10' 22 . 



Stereoscopic Illusion.*—! 1 . Gr. Baily describes a stereoscopic obser- 

 vation which lends itself to quantitative expression and may possibly 

 prove to be capable of practical application. It is well known that, if 

 a finger be held vertically in front of the eyes and a distant object 

 be looked at, two images of the finger will be seen, quite transparent 

 if the two images do not overlap, and opaque only at overlapping 

 parts. In place of the finger use a thiu rod at a distance of some six 

 or more feet, and focus on another vertical rod at a distance of 30 feet 

 or more so that the second rod is seen between the two images of the 

 first. The apparent position of the second rod will be found to be 

 distinctly nearer than its real position. Thus, let A a A 2 (fig. 48) be 

 the observer's eyes, B the first rod, and C the second. Then each eye 



Fig. 48. 



sees C without hindrance, and the observer sees a transparent image of 

 B on each side. C then appears at C. To observe this, B is preferably 

 not strongly lighted, or the tendency to focus on B may be too great. 

 C must be well away from its background, so as to stand alone with the 

 background out of focus. The ground on which C stands must be 

 hidden so as to prevent any other knowledge of its position. For if the 

 actual situation on the ground is seen, the visible proof of its position 

 will mentally outweigh the indication given by the stereoscopic effects. 

 The apparent position of C may be determined by setting up a similar 

 rod D near to the line A C sufficiently on one side to be visible to both 

 eyes. D is then moved backwards and forwards until it is judged to be 

 the same distance from the observer as C appears to be. The diagram 

 has been drawn to correspond with the following actual figures, the 

 vertical scale being exaggerated to twenty times the horizontal : A x A 2 , 

 2| inches ; B, 1 inch broad ; A C, 100 feet ; and C D, 1 foot. The 

 author gives a full account of a number of out-of-door experiments 

 tried by him. These experiments seemed to show that the displacement 

 C C bears a definite relation to B C. The observation of the phe- 



* Proc. Roy. Soc. Edinburgh, xxx. (1909-10) pp. 551-5 (2 figs). 



