52 W. Gibbs— Miscellaneous Optical Notices. 
paper. On the other hand, however, the on quantity of sul- 
phur present P wood. the fire from spreading, a drop placed 
preserved two during a onan and hot summer. The viscid, or 
rather oily, nature of the solution serves to prevent, to a great 
extent, the formation of ascending and desouheatin ing currents from 
slight ‘changes of temperature, and when the prisms are well 
shaken before use the definition remains perfect for a long time. 
In my 2 Say the prisms rest upon a plate of glass instead 
of upon one of me 
§ 3. 
On an pagent gts of apparatus for the study of the ab- 
sorption of light wn colored liquids, 
In his aiiaten of the spectra of ae fluids, Mr. Glad-. 
stone employed a hollow wedge of glass, the two refracting sur- 
faces of which made with each other an acute angle. The 
time retain the oe of the method, I have devised what 
may be termed a double wedge. Two hollow wedges, of aie 
or metal, are placed eee in such a manner that 
construction of the apparatus will be readily un- & fl 
derstood from the diagram. In» it with a oe RK) 
ored aqueous solution one = the hollow prisms, 0: 
wedges, is to be filled with distilled water, Ae 
other with the aqueous sokutiti to be examined. The incident 
beam of sunlight is then allowed to fall perpendicularly upot 
