266 ; Scientific Intelligence. 
When this beam was looked at normally through the selenite 
_ and Nicol, the ring system, though not brilliant, was distinct. 
normal, “t ows were opened, the blmds remaining undra 
The resinous fumes slowly diminished, and as they did so the ring 
system became paler. It finally disappeared. Continuing to look 
a the «fume vegies 
e fumes of keia of ammonium substantially the same 
sabi stom obtained as those just described. Sufficient I think 
has been here stated to illustrate the variability of the position of 
the neutral point. The e a - the results will proba 
give new work to the undulatory the 
Before quitting the question of ae ie of the —- 
Son cloudy pepe I will make one or two additional —— _— 
chemi 
con 
was no change in the polariz ic along the normal, for here the 
necessary differences of cloud-texture did not exist. 
urther. By a puff of tobacco smoke or of condensed steam 
blown into the illuminated beam, the brilliancy of the colors may 
be greatly augmented. But with different clouds two different 
effects are pro odueed. For example, let the ring system observed 
in the common air be brought to its maximum strength, and t then 
let an attenuated cloud of chlorid of ammonium be thrown into 
the beam at the point looked at; the rmg system flashes out with 
fine par 
eles of phosphoric acid or of sulphur to rise into the light. With 
the sulphur-tumes the brilliancy of the colors is exceedingly inten- 
sified ; but in none of these cases is there any change in the char 
acter of the polarization. 
ut when a puff of aqueous cloud, or of the fumes of hydro- 
chloric ne, hydriodic acid, or nitric acid is thrown into the beam, 
re is a complete reversal of the selenite tints. Each of these 
clone twists the plane of “gph con 90°, On these and kindr ed 
points experiments are still in pro. } 
* Brewster has prov eutral point for 
skylight with the yer re the navi oe cesta tb ear this recreated by 
the foregoing experiments? 
+ Sir John Herschel has Suggested to me that this change of the polarization 
polarization by refraction. This tho repeatedly oceurred to me while looking 
. ag effects; but it will require much following up before it emerges into 
*. 
