IN studying the spectra of coloured solutions and solid substances by means of the spectrum microscope, it is most important to employ prisms having a suitable amount of dispersion. It would be a very great mistake to suppose that the result is better with a very wide dispersion. This, of course, makes the spectrum larger, but very greatly impairs the definition of the absorption-bands. Everyone who has had experience with an ordinary microscope must be well aware that a particular magnifying power is best for each particular class of object or kind of structure, and that in some cases nearly all the important characters would be lost by employing too high a power; but at the same time too low a power would be equally disadvantageous in other respects. This analogy holds good in the case of the dispersion of prisms. The power ought to be regulated by the character of the absorption-bands. If they are dark, narrow, well-defined, and lie close together, as in the case of partially opaque crystalline blw-pipe beads of borax containing deposited crystals of oxide of lanthanum with oxide of didymium, a somewhat powerful dispersion is not only admissible, but quite necessary to separate some of the bands. If, however, they are broad and faint like those seen in the spectra of many of the colouring matters found in animals and plants, a powerful dispersion spreads them over such a wide space, and makes the shading off so gradual, that the eye can scarcely appreciate the extra amount of absorption; whereas, when a lower dispersive power is used, a well-marked absorption-band can easily be seen. This is more especially the case with impure mixtures. I have found that when it was requisite to examine a mixed, somewhat turbid, coloured solution to detect, if possible, the presence of some substance which, when alone, gave a spectrum with distinct absorption-bands, no trace could be recognised by means of a prism of high dispersive power; but it could be detected without any difficulty with a lower. In carrying on practical investiga-tions it is far more important to be able to succeed in such a case than to exhibit on a large and more imposing scale the spectra of a few substances which give dark and well-defined bands. There can be no doubt that it is a great advantage to have a number of prisms of different dispersive power, so that in all cases the most suitable may be used; but at the same time some observers might not wish to have more than one, and thus it becomes important to decide what amount of dispersion is the best for the generality of objects—is sufficiently great to divide narrow, closely-placed bands, and yet not so great as to prevent our seeing broad and fairter. No magnifying power whatever is applied to the spectrum itself in the instrument now under consideration.