Physics. 259 
gree of fineness, forming particles distinguishable by the naked 
eye, or particles which are daar far beyond the reach of our 
ighest microscopic pow: 
have no reason to oe that particles may be thus obtained 
whose diameters constitute but a very small fraction of the length 
of a wave of violet light. 
cases when the vapors of the liquids employed are suffi- 
ciently attenuated, no matter what the liquid may be, the visible 
action commences with the formation of a blue cloud I would 
guard myself at the outset against all misconception as to the use 
of this term. The blue cloud to which I here refer is ‘totally 3 invis- 
I had been endeavoring to copes piesa acid gas by light. 
A faint bluish elo ud, due it may be, o not be, to the resi- 
the short diagonal of the prism was vertical, the quantity of light 
ne the eye was greater than when the long diagonal was 
the crystal of tourmaline to revolve round the tube, with its 
perpendicular to eee beam, the quantity of light eit 
reached the eye was in all its pos ositions a 
crystallographic ile was Binal # to the axis ra res — the 
quantity of light transmitted by the erystal w 
rom the illuminated bluish cloud, eae, polazined light was 
dischar ged, the aera oe maximum polarization being at right 
angles to the illuminating beam; the plane of vibration of the 
polarized light, ‘ectiora was that to which the beam was perpen- 
Thin 
and the buch cloud of t 
Colors bei ing most vivid when the line of vision was at ri 
to the experimental tube. The plate of selenite usually employed 
* T assume here that the plane of vibration is perpendicular to the plane of 
Polarization. This j is still lee decided point; but the probabilities are so much 
in ps rites be it is in my opinion so ck prefera erable to have a physical image 
Bye ould i be incorrect, no harm will be 
