334          ON  THE CHANGE  OF KEFRANGIBILITY  OF  LIGHT.
110.    There is another  mode  of observation which I have occasionally found convenient when the object was to determine whether  a  substance  exhibited  so  much  as  a  low   degree   of sensibility.    In this method the sun's light was reflected horizontally through a large lens, and then transmitted through a small lens placed in the condensed beam.    The small lens was covered by a small vessel with parallel sides of glass, containing a blue ammoniacal solution of copper, or else by a deep blue glass combined with a weak solution of nitrate or sulphate of copper.    The  object of the  latter  solution  was  to  absorb  the extreme red  which is transmitted by a blue glass.    The light coming through  the lens was then analysed by a prisrn, being received directly into the eye, or else allowed to fall on a white object which had been previously ascertained not to change the refrangibility of the light incident upon it.    I found clean white earthenware to  serve  very well  for  such  an   object,  but  each observer ought to test  for  himself the  substance   he  employs. When a test object, such as white earthenware, is  used, it is placed at the focus of the lens, and the spot of blue light formed upon  it  is  analysed  by  a prism  to  see  if  the   absorption  is sufficient.    When the visible rays are considered to  have been sufficiently absorbed, the object to be observed is placed at the focus of the lens, and the spot of light formed upon it is viewed through a prism.    The spectrum then seen is compared with that given by the test object.    This method of observation is rather easier than that of a linear spectrum, and is at least as delicate if the  object  be  merely to determine  whether a substance is sensitive or not, but on the whole it is not near so useful.    It may sometimes be used with advantage in the case of translucent bodies.
111.    An  extremely pale solution of nitrate or sulphate of copper is sufficient to absorb the extreme red transmitted by a deep  blue  glass.    This  is   not  the  case   with   the   ammoniacal solution, which does not absorb the extreme red till it is of a pretty deep  blue.    Its absorbing power is greatest, not at the extreme red, but about the orange, as   may be   seen   by using candle-light, which is richer in red rays than daylight.
112.    Another  method  of  observation   which   is   sometimes useful, consists in employing a large lens and absorbing medium,