456 SUMMARY OF CUKRENT RESEARCHES RELATING TO 



Optical Illusion observed when Blinking the Eyes. * — L. Bull 

 points out that, if the wheels of a rapidly-moving carriage in the street 

 are observed, the spokes ordinarily invisible owing to their swiftness are 

 yet sometimes clearly seen, especially at the moment of blinking. The 

 phenomenon is more easily observed with a black disk about 30 cm. in 

 diameter on which some twenty white spokes 1 cm. wide have ])een 

 painted. If the disk be turned with a speed of five or six rotations per 

 second, and the observer is a few metres away, the sensation is that of 

 a uniform grey. But if the spectator sharply shuts his eyes without 

 looking off the disk, the spokes comprised in a sector of a size varying 

 according to the individual leave a perfectly distinct impression on the 

 retina, and appear concave in the sense of the rotation. The author 

 describes some experiments undertaken to elucidate the cause, and comes 

 to the conclusion that the phenomenon can only be due to a displacement 

 of the image of the disk on the retina produced l)y a movement of the 

 eye at the moment of blinking. It is possible to verify this photo- 

 graphically, and to establish the fact that when the eyes are closed 

 rapidly the ocular globes are displaced. The movement takes place in 

 a direction which depends on the orientation of the glance with regard 

 to the position of the head, and it gives rise to the same displacement 

 of the image on the retina as a translation of the object in the sense 

 inverse to that of the movement. 



Displacement of the Particles in the Brownian Movement. 

 Phenomenon of the Borders.f — S. Lifchitz has continued his observa- 

 tions on Brownian movement by introducing particles of smoke into the 

 ultramicroscopic condenser of Leitz. The aperture of this condenser 

 was always covered by a plate of glass pierced at its centre by a square 

 hole. The focus of the condenser is at the centre 1 • 5 mm. below the 

 surface of its aperture, and the displacement was observed at this central 

 spot. Now a covered condenser is a resonator of small dimensions, and 

 therefore some acoustic phenomenon was to l^e expected. At the moment 

 of sparking it was observed that the particles on the surface of the aper- 

 ture of the condenser moved perpendicularly to the extremities of the 

 borders. (This is the phenomenon of the borders.) An explosive shock 

 — e.g. a whistle— at the same time brought about a displacement of the 

 particles. The author concluded that in the aperture of a resonator 

 there is produced a movement of air directed towards the centre of the 

 aperture, and that,, by employing a very small hole, one can profit by 

 this phenomenon to construct an ultramicroscopic indicator for very 

 short sonorous waves. The displacement of the particles due to very 

 rapid shocks can be separated from the phenomenon of the borders by 

 the use of holes sufficiently large. 



(61 Miscellaneous. 



How to Use the Microscope.:}: — This little work by C. A. Hall is 

 intended as a guide for the novice, or beginners who have practically no 

 knowledge of the Microscope, and it deals only with instruments of the 



* Comptes Rendus, cliv. (1912) pp. 1251-4 (2 figs.). 



t Comptes Rendus, cliv. (1912) pp. 1218-20. 



X London : A. and C. Black (1912) 88 pp. (20 photo pis. and 2-5 text figs.). 



