C. GENERAL PHYSICS. 139 



reflected light to the incident light for two prisms, one of 

 crown and one of flint glass. A comparison with Fresnel's 

 formula, which is identical with Cauchy's, shows that the 

 differences between observation and theory fall quite within 

 the range of errors of observation. 1 A, XLVIIL, 478. 



THE FIXED STARS AS VISIBLE THROUGH MINUTE APERTURES. 



In a recent letter to the English Mechanic, Mr. 'Allen en- 

 deavors to contribute somewhat to the understandine of 

 the phenomena of the black drop, by considering the ques- 

 tion of the visibility of the stars through minute apertures. 

 He states that the stars are not seen ordinarily on a clear 

 night as simply intensely bright and vivid spots on a dark 

 background without rays or bright surrounding burrs, but 

 generally accompanied by three or four or more rays dart- 

 ing out of the bright burr. The question as to whether 

 these phenomena are caused by interference or by the at- 

 mosphere, or whether they are purely subjective, can per- 

 haps be decided by viewing the stars through minute needle- 

 point holes in a thin opaque plate. If through such a mi- 

 nute hole we look at a dull, white, cloudy sky, we see not only 

 the dull hue of the sky, but also a thin, gauze-like covering 

 as of lace hanging between the opaque plate and the sky. 

 When the hole is comparatively large the edges of the field 

 of view only are covered by this gauze veil, while when the 

 hole is exceedingly small the effect is intensified by the 

 closing in of the veil on all sides, so that the field of view is 

 completely filled up with an almost black gauze which can 

 only be seen through darkly. On viewing the stars through 

 such a minute hole, it will be found that the rays ordinarily 

 darting from the star totally disappear, not only the longer 

 rays, but also the shorter ones forming the bright burr, and 

 the star appears like a planet, showing a clearly defined disk 

 with a somewhat fainter light, in the centre of which a mi- 

 nute black point is distinguished. Viewed in this way the 

 star still twinkles, and has still the color and the changes of 

 color as ordinarily seen. If now two needle-holes be made 

 near together, perhaps T 3 ^ of an inch apart, then the two 

 images of the star will be seen apparently touching each 

 other, with a black spot at their point of tangency. 18 A, 

 -KX., 5. 



