*76 
POPULAR SCIENCE REVIEW, 
grain of liquid blood, in a cell of of an incli in diameter 
and \ incli long, gives a spectrum as well marked as could be 
desired. In exhibiting the instrument to a number of persons 
at a meeting I have found that no object is more convenient, 
or excites more attention, than one in which a number of cells 
are fixed in a line, side by side, containing a solution of various 
red colouring matters. In one I mount blood, which gives 
two well-marked absorption bands in the green; in another 
magenta, which gives only one distinct band in the green ; 
and in another I place the juice of some red-coloured fruit, 
which shows no well-defined absorption band. Keeping a 
larger cell containing blood on the stage attached to the eye- 
piece, these three objects can be passed one after another in 
front of the object glass, and the total difference between the 
spectrum of blood and that of either fruit-juice or magenta, 
and the perfect identity of the spectra when both are blood, 
can be seen at a glance. By holding coloured glasses, which 
cut off the red, but allow the green rays to pass, we can 
readily show how the presence of any foreign colouring matter, 
which entirely alters the general colour, might not in any 
degree disguise the characteristic part of the spectrum; and 
by changing the cell held on the eye-piece for a tube containing 
an ammoniacal solution of cochineal, it is easy to show that 
though it yields a spectrum with two absorption bands, more 
like those due to blood than I have seen in any other substance, 
they differ so much in relation, size, and position, that there 
is no chance of their being confounded, when compared together 
side by side. 
It would be easy to extend this essay to a most unreasonable 
length by describing the spectra of various other objects. So 
far, too little is known to enable us to form any decided 
opinion with regard to the comparative spectra of the same 
substance in different conditions. Sometimes the spectra of 
the solid material, and of its solution, are quite identical ; 
sometimes the solution shows distinct absorption bands when 
the solid material shows none at all ; and sometimes it is the 
reverse. The presence of different foreign substances may also 
modify the results in a surprising manner, and the only general 
conclusion I can form is, that by comparing the spectra of the 
same material in different conditions, we may detect molecular 
changes which otherwise could not be recognised. 
I probably could not better conclude this communication 
than by saying a few words about what I look upon as the best 
test objects for a spectrum-microscope. For fine definition, I 
know none equal to the pale blue solution of chloride of cobalt 
in a concentrated solution of chloride of calcium. If we can 
see the two lines in the orange, the definition must be very 
