C. GENERAL PHYSICS. 77 



direction of the star is then twenty seconds from the apparent 

 direction. The disputed question amounts to this : in order 

 that a ray of light from a star may suffer no refraction on en- 

 tering a lens, must the surface of the latter be perpendicular 

 to the true or to the apparent direction of the star ? This 

 question Professor Airy has sought to answer by mounting 

 a zenith telescope, of which the entire tube between the eye- 

 piece and objective was filled with water, and observing the 

 zenith distance of the star y Draconis at different times of 

 the year. 



The result was that the apparent position of the star fluc- 

 tuated exactly as if there had been no water in the telescope, 

 thus showing that the thickness of the object-glass was with- 

 out influence on the amount of the aberration. Applied to 

 the question we have suggested, this proves that to have no 

 refraction the surface must be perpendicular to the apparent 

 and not to the true direction of a star. The result is expected 

 to throw some light on the various questions of the ethereal 

 medium, and especially of its density in transparent bodies, 

 and of its motion with such bodies. 



TIME AND DURATION OF VISUAL IMPRESSIONS. 



In an article by M. Baxt, of St. Petersburg, on the time 

 requisite for a visual impression to arrive at the conscious- 

 ness, and upon the duration of the period of consciousness 

 caused by a visual impression of definite duration, he remarks 

 that, from the experiments of Helmholtz and Exner, it has 

 been shown that, if a number of ordinary letter-press letters 

 be exhibited to the eye on a white ground, sometimes one, 

 sometimes two or more of them, will be distinguished from 

 the row according to the duration of the impression and that 

 of the positive after image. He proceeded on the same prin- 

 ciple as Helmholtz, and with apparatus similar to that em- 

 ployed by him, which consisted of two disks, that could be 

 caused to revolve at known speed, but the posterior of which 

 rotated twelve times quicker than the anterior. As the re- 

 sult of a series of experiments by means of this apparatus, it 

 was shown, first, that the consciousness of a given excitation 

 is only realized or perfected by degrees ; second, that, under 

 particular circumstances of his experiments, a period of one 

 twentieth of a second must elapse between the occurrence of 



