on the use of Oarbon bisulphide in Prisms. 271 
lines were “ wooly” and no definition was i Dr. Draper 
Says: “The bisulphide prism touched with the finger or 
breathed on loses all definition at once. was found how- 
ever, that the dispersive power of the Thollon prism was equal 
to that of about four of the Steinheil flint prisms; and this 
fact, together with the unsatisfactory character of the results 
obtained with the train of prisms as well as with the grating, 
led Dr. Draper to undertake an investigation into the causes of 
this unsteadiness of the bisulphide, with a view to remedying it 
if practicable. 
he same difficulty has been encountered by all experiment- 
ers who have endeavored to use bisulphide prisms. Mr. Ruth- 
erfurd himself found it to be very serious and M. Thollon says 
Such prisms must be protected carefully from temperature- 
variations. It was while using these prisms that Mr. Ruther- 
furd made an important observation. He noticed that “if a 
good prism which with a high power refuses to define the soda 
line (a more stringent test than solar lines) is violently shaken 
and then placed in position, it will for a few minutes define 
beautifully but gradually settle into its former condition.”+ It 
could be seen distinctly reversed. A series of com 
was now undertaken between the train of Steinheil prisms and 
* J. Phys., viii, 73, 1879. ©. R., Ixxxviii, 80, 82, 1879. Loe. cit., p. 130. 
