THE SPECTRUM-MICROSCOPE. 
75 
with five well-marked absorption bands in the green; yet 
when crystallized along with the perchlorate, the spectrum 
differs in a very interesting manner, and is as fig. 5 b. There 
are four well-marked bands in the green, one moderately dis- 
tinct in the blue, and two or three very faint bands in the 
violet. In the spectrum of the solution these latter are so 
obscure, that I hesitate to decide whether they exist or are 
absent. At all events, all the bands are better defined and 
far more distinct in the spectrum of the compound crystals, 
and yet in other respects there is a remarkable similarity. It 
is, however, very curious to see how the dark bands in one 
always occur where the bright bands exist in the other ; and 
we have here, as in the case of the cobalt solution and cobalt 
glass, an instance of a molecular change, which, besides pro- 
ducing slight variations in the general spectrum, displaces the 
absorption bands, as if, so to speak, the pitch of the natural, 
vibrations of the permanganate were raised by being crystal- 
lized along with the perchlorate of potash. We may also con- 
clude from the spectrum that these compound crystals are 
decomposed when dissolved in water — the two salts do not 
dissolve in combination — and this agrees very well with the fact 
of their containing far less permanganate when re-crystallized. 
The spectra of some natural minerals are very interesting ; 
and some which contain both protoxide and peroxide of iron 
give the spectrum of the protoxide in one direction and of 
the peroxide in another, as if the planes in which the two 
oxides vibrate with greatest facility were perpendicular to 
one another. 
Fortunately, the various modifications of the colouring 
matter of blood yield such well-marked and characteristic 
spectra, that, there are few subjects to which the spectrum- 
microscope can be applied with greater advantage than the 
detection of blood-stains. I have already, in my paper in the 
Quarterly Journal of Science (April, 1865, II., 205), entered 
at so great length into this question, that I need not say much 
about it on the present occasion. The form of apparatus I 
have described enables us, however, to examine the objects in 
a different manner : surface illumination may be used, provided 
a sufficiently bright light be thrown on the object by means 
of a parabolic reflector or bulPs-eye condenser. A speck of 
blood on white paper shows the spectrum very well, provided 
it be fresh, and the colour be neither too dark nor too light, 
and the thickness of the colouring matter neither too great nor 
too little. A mere atom, invisible to the naked eye, which 
would not weigh above the millionth of a grain, is then 
sufficient to show the characteristic absorption bands. They 
are, however, far better seen in a solution. About -y^o of a 
